Plant Breeding Reviews (Janick/Plant Breeding Reviews, Volume 17) || Hybrid Rice: Genetics, Breeding, and Seed Production

Plant Breeding Reviews, Volume 17 Edited by Jules Janick

© 2000 John Wiley & Sons, Inc. ISBN: 978-0-471-33373-9

16 J. LI AND L. YUAN

V. Breeding for Two-line System Hybrid RiceA. ConsiderationsB. Development of T(P)GMS LinesC. China's ProgressD. Breeding for Two-line System Rice Hybrids Using Chemical Emasculators

VI. Wide Compatibility and Utilization of Intersubspecific HeterosisA. Classification in RiceB. Phenomenon of Wide CompatibilityC. Genetics of Wide Compatibility TraitsD. Development of WCVsE. Utilization of Intersubspecific Heterosis

VII. Hybrid Rice Seed ProductionA. China's SuccessB. Key TechniquesC: Specifics for CMS Line MultiplicationD. Purification of Parental Lines

VIII. Future ProspectsA. Breeding of Diverse Parental LinesB. Molecular BreedingC. Apomixis BreedingD. Hybrid Seed ProductionE. Socioeconomic Impact

Literature Cited

LIST OF ABBREVIATIONS

ABAACCADHADVAFLPA lineAVGB lineBTCGRCHAeMSCSPDESDWEATEIEMSFASS

abscisic acidl-amino-eyclopane-l-earboxylie acidalcohol dehydrogenasealkali digestion valueamplified fragment length polymorphisma cytoplasmic male sterile lineaminoethoxy vinylglycinea maintainer line in the three-line hybrid systemBoro-Taichung 65 (type male sterile cytoplasm or line)crop growth ratechemical hybridizing agentcytoplasmic male sterilitycritical sterility pointdiethyl sulfateDong-pu wild riceeffective accumulated temperatureethyleneimineethyl methane sulfonatefertility alteration sensitive stage

2. HYBRID RICE: GENETICS, BREEDING. AND SEED PRODUCTION 17

GGAGCAHPGMRHLIAAI-KIIPIRRIIRTPLWMHNARSNEUNMSPCRPGMS

QTLRFLPR lineSCAs lineSTSTGMS

T(P)GMS

WAWC

WCGWCV

Gambiaka-type male sterile cytoplasm or linegibberellic acidgeneral combining abilityHubei Photoperiod Sensitive Genic Male Sterile RiceHong-Lian type male sterile cytoplasm or lineindoleacetic acidiodine-potassium iodineIndonesia Paddy riceInternational Rice Research Instituteinternational rice testing programLong-An wild ricemaleic hydrazidenational agricultural research serviceN-ethy1-N-nitrosoureanuclear male sterilitypolymerase chain reactionphotoperiod sensitive genic male sterile line in the twoline hybrid systemquantitative trait locirestriction fragment length polymorphismrestorer linespecific combining abilityphotoperiod or temperature sensitive male sterile linesequence tagged sitestemperature sensitive or thermo-sensitive genic malesterile line in the two-line hybrid systemtemperature sensitive or photoperiod-sensitive genic malesterile line in the two-line hybrid systemwild~abortive,a male sterile cytoplasm or linewide compatibility, which can produce F1 hybrids withnormal male fertility both to most of indica and to most ofjaponica rice cultivarswide compatibility genecultivar which has wide compatibility

I. INTRODUCTION

The commercial production of hybrid rice in China represents one of themost successful breeding efforts of the twentieth century. Heterosisbreeding in rice has been reviewed by Chang et al. (1973), Davis andRutger (1976), Virmani and Edwards (1983), Kim and Rutger (1988), and

18 J. LI AND 1. YUAN

Virmani (1994a, 1996). This review emphasizes hybrid rice breeding andseed production in China. It includes the three-line, two-line, and oneline breeding approaches (see Sections IV, V, VIII).

Documentation on heterosis in rice (Oryza sativa L.) has a long history.Jones (1926) first indicated its existence and it was subsequently reportedby Ramiah (1935), Idasumi (1936), Kadam et al. (1937), Capinpin andSingh (1938), Ramiah and Rangaswamy (1941), Brown (1953), Oka (1957),Sen and Mitra (1958), Pillai (1961), Namboodri (1963), Rao (1965), Purohit (1972), Saini and Kumar (1973), Sivasubranian and Menon (1973),Saini et al. (1974), Singh et al. (1977), Singh and Singh (1977, 1979),Singh et al. (1980, 1984), Yoshida and Fujimaki (1985), respectively.

Producing commercial F1 hybrid seed by hand emasculation isimpractical in rice. Thus, development of male sterile lines is essentialin order to exploit rice heterosis. Some male sterile lines from the japonica subspecies were developed in the 1960s, including 'Fujisaka 5 A'(Katsuo and Mizushima 1958; Watanabe et al. 1968) and 'Taichung 65A' (Shinjyo and Omura 1966). Erickson, the first U.S. researcher of ricecytoplasmic male sterility, determined that both 'Bir-Co' and O. glaberrima contained the cytoplasm that facilitated male sterility, based oncrosses with the California japonica rice cuItivars 'Calrose', 'Caloro', and'Colusa' (Erickson 1969; Carnahan et al. 1972). The male sterile cytoplasm in 'Taichung Native l' also resulted in 'Pankhari 203A' (Athwaland Virmani 1972). However, these male sterile lines have never beenput into large-scale commercial production.

A. China's Achievements

China was the first country to produce hybrid rice for commercial use.Research on male sterile rice was initiated in 1964 (Yuan 1966). However,rice heterosis was not successfully exploited until after the discovery ofthe wild abortive (WA) male sterile cytoplasm in the wild species (0. rufipogon Griff or O. sativa f. Spontanea) at Hainan Island in 1970 (Li 1977).The first set of genetic tools (a male sterile or A line, a maintainer or Bline, and a restoring or R line) for the three-line system of hybrid rice production was developed in 1973 (Yuan and Virmani 1988).

With the establishment of the three-line technology for hybrid riceseed production, the first hybrid rice combinations were put into commercial production in China in 1976. Since then, the area under hybridrice production has increased from 2.1 million ha in 1977 to 10.9 million ha in 1987 and to 15.3 million ha in 1997. Hybrid rice normally hasa yield advantage of 20-30% over non-hybrid rice cultivars (Lin andYuan 1980; Shen 1980). From 1976 to 1997 hybrid rice enabled China

2. HYBRID RICE: GENETICS, BREEDING, AND SEED PRODUCTION 19

to increase rice production by more than 312 million t to feed its everincreasing population. Recently, hybrid rice has yielded about 6.6 t/hacompared with 5 t/ha for conventional cultivars. In 1994 hybrids weregrown on 15.7 million ha, 50% of the total rice area and 57% of China'stotal rice production. Record yields of 11.2 t/ha from a single hybrid cropon a: large scale (1,000 hal and 17.1 t/ha in a small plot (0.1 hal have beenreported (Bai and Luo 1996). The double cropping record for hybrid riceis 23.3 t/ha. Furthermore, hybrid rice requires about 4% less labor, and2% less draft animal services while yielding 19% more than conventional modern cultivars (Lin 1994).

High hybrid seed yield has been important for hybrid rice production.Recent average seed yield in China has been 2.4 t/ha. To further reducecosts, many new cytoplasmic male sterile (CMS) lines with high outcrossing efficiency have been developed, thus raising hybrid rice seedproduction. The current land area ratio among A line multiplication, F1

seed production, and F1 commercial cultivation is 1:50:5000. The highest recorded F1 hybrid rice seed production yield was 7.4 t/ha on asmall plot (0.2 hal by Zixing Seed Company in Hunan Province in 1993(Yuan 1996; Mao et al. 1998).

B. Hybrid Rice Technology Outside China

China successfully commercialized hybrid rice technology in the 1970sand obtained the first patent on this technology in the United States in1989 (Yuan 1989). As a result of China's success in hybrid rice production, the International Rice Research Institute ORRI) revived its hybridrice work in 1979 (Lin and Yuan 1980; Int. Rice Res. Inst. 1980; Yuanand Virmani 1988). Many other countries initiated research on hybridrice during the period from the 1970s to 1990s, including Japan(Murayama 1973; Murayama et al. 1974; Kato et al. 1994), the UnitedStates (Rutger and Shinjyo 1980; Mackill and Rutger 1994), India(Mohanty and Mohapatra 1973; Maurya and Singh 1978; Mallick et al.1978; Panawar et al. 1983; Devarathinam 1984; Parmasivian 1986; Anandakumar and Sreerangasamy 1986; Prakash and Mahadevappa 1987;Virmani 1993; Siddiq 1994; Barwale 1994; Siddiq et al. 1994; Rangaswamy et al. 1994), Thailand (Chitrakon et al. 1986; Chitrakon 1987),Korea (Kim and Heu 1979; Koh 1987; Moon 1988; Choi 1991; Moon etal. 1994), Vietnam (Nguyen et al. 1985, 1994; Pham et al. 1991; Yin1993; Nguyen 1994; Li 1995), Indonesia (Suprihatno 1986; Subandi etal. 1987; Suherman 1989; Suprihatno et al. 1994), the Philippines (Laraet al. 1994), Myanmar (K. L. Zhou, pers. commun.), Brazil (Neves et al.1994), Egypt (Maximos and Aidy 1994), Colombia (Munoz 1992,1994),


Malaysia (Mohamad et al. 1987; Osman et al. 1988; Guok 1994), Iran(Dorosti 1997; Sattari 1997), Pakistan (Cheema and Awan 1985; Cheemaet al. 1988; Ali and Khan 1998), Mexico (Armenta-Soto 1988), Bangladesh (Julfiquar 1998), Sri Lanka (Rothschild 1998), as well as international research institutes (Virmani et al. 1991; Virmani 1994b; Taillebois1991, 1994) and private companies such as RiceTec, Inc. in the UnitedStates, and Mahyco Seed Company, Pioneer Overseas Corporation, andHybrid Rice International in India. Hybrid rice technology has alsoattracted the attention ofthe FAO, which started its hybrid rice programfollowing the recommendations of the 16th Session of the InternationalRice Commission (IRC) held at the International Rice Research Institutein 1985 (Trinh 1992, 1993, 1994; McWilliam et al. 1995). Technicalsupport for hybrid rice technology has been provided to countries suchas India, Vietnam, and Bangladesh from the International Rice ResearchInstitute and China. The China National Hybrid Rice Research & Development Center (the former Hunan Hybrid Rice Research Center) has heldsix international courses on hybrid rice production technology andtrained more than 150 rice scientists from various countries includingIndia, Vietnam, Thailand, and Colombia (CNHRRDC 1997).

India's hybrid rice project was started in the late 1980s, and its potential for the development and commercialization of hybrid rice is encouraging. Since 1991 India's research network has involved 12 researchcenters. Over 400 hybrids were developed and evaluated between 1990and 1994. The best 35 hybrids exceeded the yield of the best check byover 1 t/ha. Several hybrid cultivars released to farmers, including'APRH1' (IR58025A x Vajram), 'APRH2' (IR58025A x MTU9992), 'DRR-l'(IR58025A x IR40750), 'DRR-2', 'DRR-3', 'MGR-1' (IR62829A x IR10198)and 'KRH1' (IR58025A x IR9761), have performed admirably (Table2.1). Other noteworthy hybrids include 'CoRH1', which was developedin Tamil Nadu (Rangaswamy et al. 1994); 'CNHR 3' (IR62829A x Ajaya),which was released for dry season cultivation in West Bengal, India; thesalt-tolerant hybrid 'TNRH16' (IR58025A x C20R), which recorded agrain yield of 5t/ha, 20% over the check; and 'C043' (Ali et al. 1998). Pioneer Overseas began breeding hybrid rice in Hyderabad, India in 1988and released 'PHB31' in 1993. Other private sectors such as E.I.D. ParryLtd. are also involved in the development and commercialization ofhybrid rice technology. India's current hybrid seed yield is about 1.5-2.0t/ha for its standardized hybrid seed production package. A total of 1,300t of hybrid rice seed was produced for 60,000 ha of the cultivated areaunder hybrid rice in 1996 (Ahmed et al. 1997a,b; Ahmed 1997). Indiaaims to have two million ha of hybrid rice by the beginning of the21st century (Trinh 1993). The present challenge facing India is the

N ,...,

Tab

le2.

1.R

elea

sed

rice

hy

bri

ds

by

Ind

ian

publ

icse

ctor

s.S

ourc

e:A

hm

edet

al.

1997

a.

Gro

wth

Yie

ldin

farm

tria

l(t

1ha)

Adv

anta

ged

ura

tio

nov

erch

eck

Ch

eck

Yea

rH

ybri

dP

aren

tage

(day

s)H

yb

rid

Ch

eck

(%)

cult

ivar

1994

APH

R-1

IR58

025A

xV

AJR

AM

13

0-1

35

7.14

5.27

35.4

Ch

aita

ny

a

1994

AP

HR

-2IR

6282

9Ax

MT

U99

921

20

-12

57.

525.

2144

.2C

hai

tan

ya

1994

MG

R-1

IR62

829A

xIR

1019

81

10

-11

56.

085.

2316

.2IR

50

1994

KR

H-1

IR58

025A

xIR

9761

12

0-1

25

6.0

24.

5831

.4M

anga

la

1995

CN

RH

-3IR

6282

9Ax

AJA

YA

R1

25

-13

07.

495.

4537

.4K

hit

ish

1996

DR

RH

-1IR

5802

5Ax

IR40

750

12

5-1

30

7.30

5.50

32.7

Tel

iaH

amsa

1996

KR

H-2

IR58

025A

xK

MR

3R1

30

-13

57

.40

6.10

21.3

Jaya


successful transfer of technology for hybrid rice seed production inorder to achieve practical results for farmers.

In 1983 Vietnam started research on hybrid rice at Hau Giang in theMekong River Delta (Nguyen et al. 1995). Several rice hybrids from theInternational Rice Research Institute showed 18-45% yield advantageover Vietnam's best local inbreds (Table 2.2) at Cuu Long Delta RiceResearch Institute (CLRRI). The Chinese rice hybrids are highly adaptable to the northern mountainous area near China. Some rice hybridssuch as 'Shan-You 63', 'Shan-You Gui 99', 'Shan-You Guang 12' and 'BoYou 64' were introduced directly from China to northern Vietnam yielding 6.5-8.5 t/ha, 13-14% higher than the local check 'CR203'. Somefarmers obtained up to 10 t/ha in Dien Chou (Nhge An Province) and atPhu Xuyen (Ha Tay Province). Some Chinese hybrids yielded up to 14.0t/ha at Dien Bien (Lai Chau Province), 12.0 t/ha in Hoa An (Cao BangProvince), and 12.6 t/ha in Van Quan (Lang Son Province). The areaunder hybrid rice production in the Red River Delta of Vietnam reached40,000 ha in 1993 and 86,000 ha in 1996 (Hoan et al. 1998). But ricehybrids from China are not adapted to the tropical conditions in theMekong River Delta where IRRI-bred rice hybrids and parental lines cangrow well. By the turn of the century, Vietnam plans to cover about 0.5million ha with hybrid rice (Pingali et al. 1997).

Japan has studied hybrid rice since the 1950s and the Ministry of Agriculture, Forestry, and Fisheries initiated a hybrid rice program in 1983.The first three-line rice hybrid 'Hokuriku-ko 1', developed in 1985, outyielded the check inbred by about 20% (Yasuki et al. 1997). Zen-Noh(the National Federation of Agricultural Cooperative Association) andseveral private companies such as RAMM Hybrid International Co-

Table 2.2. Yield performance in Vietnam of some experimental rice hybrids fromIRRI. Source: Nguyen et al. 1995.

Year Season Hybrid Yield (t/ha) % of Check Check

1989/90 Dry 54752A x IR64R 7.5 131* OMBOIR54752A x IR64R 7.2 125* aMBOIR54752A x OM80R 6.7 118* OM80

1990 Wet 25A x IR29723R 7.6 143* MTL58IR62829A x IR29723R 6.7 126* MTL58

1990/91 Dry 29A x IR29723R 6.1 123* MTL61IR58025A x IR29723R 6.0 122* MTL61

1992 Wet 25A x IR52287R 6.7 131* IR64

1992/93 Dry 25A x IR3235BR 6.8 145* IR64

*Significantly higher than check at 5% level.


operation, Kirin Brewer Co., Ltd., and Sumitomo Chemical Co. are alsodeveloping and testing rice hybrids (Kato et al. 1994).

The Philippines released the first rice hybrid 'IR64616H', registeredas 'PSB Rc26H' and named 'Magat' hybrid, in 1994. Another hybrid'IR68284H' showed standard heterosis of 16-27% across seasons. Morehybrids from PhilRice, the International Rice Research Institute, andCargill are now being evaluated in test nurseries (de Leon et al. 1998).

Due to the increasing world population and its requirement for morefood, especially in developing countries, the FAO considers the use ofhybrid rice technology to be essential for the next 10 years. To meet thisgoal, the FAO is organizing a task force for Latin America and Caribbeancountries. Similarly, FAO is providing financial support to some Southeastern Asian countries, including India, Vietnam, Myanmar, andBangladesh. It is expected that hybrid rice will be important in fightingworld hunger for the next several decades.

C. "Bottlenecks" and Potential Solutions

The current Chinese hybri<;l rice cultivars are primarily from the threeline hybrid rice system, but the yield level of these hybrids have reacheda plateau since the 1980s (Yuan 1994d, 1997a; Fig. 2.1). An additional

20 8000

18 Yield-. 7000~

16 .......................................d

= 6000Q .':= 14

! // .................. '212 5000

~~ Area ~~ 10 4000 "0t=ll 13~ 8 >:; 3000

6:: 6

20004

2 1000

0 0

~ ~ ;c; ~ ~ ;C; ~ ~ ~ (0 (0 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~--J --J --J --J --J co~ CD CD CD CD co CD CD CD (0

~(0 (0 (0 (0

(Jl 0> --l CD <D 0 I\J (,) ~ (Jl 0> --J CD <D 0 I\J (,) ~ (Jl

Year

Fig. 2.1. The leveling out of yield and planting area of hybrid rice in China (1976-1995).


threat is that more than 85% of all A lines belong to the "WA" type. Thissingle cyto-sterility system may be vulnerable to the development ofdestructive pests or diseases. For attaining a higher yield potential fromrice heterosis, Yuan (1987) put forward the following three breedingapproaches for rice heterosis breeding: (1) three-line method or CMS system; (2) two-line method or T(P)GMS system (the thermo-sensitive orphotoperiod-sensitive genic male sterility); and (3) one-line method orapomictic system. The goal is to enhance heterosis, and improve eachof the breeding approaches at the following three levels: (1) intercultivar hybrids; (2) intersubspecific hybrids; and (3) distant hybrids (interspecific or intergeneric hybrids). These strategies will be detailed in thefollowing sections.

II. HETEROSIS IN RICE

A. Concept of Heterosis

In 1776 Koelreuter published his work on plant hybridization after noting an excessive luxuriance in his Nicotiana hybrids. A hundred yearslater Darwin (1877) described the hybrid vigor of plants in his book "TheEffects of Cross and Self Fertilization in the Vegetable Kingdom." Hestated:

... the first and most important conclusion which may be drawn from theobservations given in this volume, is that cross-fertilization is generallybeneficial and self-fertilization injurious.

At almost the same time (1865) Mendel observed hybrid vigor in hispea hybrids. The term "heterosis" was first coined in a lecture at Gottingen, Germany by Shull in 1914. It referred to "the increased vigor,size, fruitfulness, speed of development, resistance to disease and toinsect pests, or to climatic rigors of any kind, manifested by crossbredorganisms as compared with corresponding inbreds, as the specificresults of unlikeness in the constitutions of the uniting parental gametes"(Shull 1952; Zirkle 1952).

Heterosis was first exploited in the 1930s with the large-scale production of hybrid corn, which provided an important impetus for othercrops (Pingali et al. 1997). However, unlike in the easily emasculatedmaize, the inability to emasculate the seed parent had been the primarybarrier for the utilization of heterosis in many cross-pollinated and selfpollinated species. The onion research conducted by Jones and Clarke(1943) provided a solution to this problem. They identified male steril-


ity in the onion cultivar 'Italian Red' (Jones and Emsweller 1936) in1925, developed the CMS system for hybrid onion production, andrevealed the genetic mechanism of eMS in onion (Janick 1989). Thebreeding strategy for hybrid onion was used by rice scientists in developing three-line hybrid rice.

B. Performance of Heterosis

Heterosis is apparent in many morphological and physiological traits.For rice, three main categories of heterosis can be observed.

1. Vegetative Heterosis. Normally F1 rice hybrids have higher growth rateand greater vegetative vigor.

Early and Higher Tillering Capacity. When cultivated as a single crop,the rice hybrids 'Nan You 2' and 'Nan You 6' started to develop tillers12 days after seeding, 6-8 days earlier than their male parental lines. Atthe Hunan Teacher's College, the largest number of tillers per ha of 'NanYou 2' reached 4.24 million, 0.29-1.25 million more than its parentallines 'Er-Jiu-Nan lA' and 'IR24', and the check conventional cultivar'Guang Xuan 3' (Yuan and Chen 1988). The growth rate and biomass ofthe hybrid were greater than those of the parental lines under both highand moderate temperatures.

Wider and Deeper Root Distribution and Higher Nutritional Absorption. Root number per plant of 'Nan-You 3' was 121.3% higher than forthe conventional rice cultivar 'Guang-Liu-Ai 4', for the same seeding rate(Li et al. 1982). Yichun Agricultural Research Institute in JiangxiProvince, China also reported that the root system of the hybrid 'ShanYou 2' reached 22 cm average length (the longest being 30 cm) and 24cm average width (the widest being 34 cm) at maturity, compared with5-9 cm oflength and 9-10 cm of the width at the same stage in the conventional cultivar 'Yi Chun Ai l' (Yuan and Chen 1988). The root system of the hybrids was also larger than that of their parents (Bai and Xiao1988; Lu et al. 1988).

Taller, More Sturdy Culm and Higher Lodging Resistance. 27 of 29 hybrids had positive heterosis for plant height. Guangxi Academy of Agricultural Sciences in China found that wall thickness between the 1st tothe 6th internode of the hybrid 'Shan-You 2' was much greater than thatof 'Bao-Xuan 3'. Thus, the hybrid rice has higher lodging resistance eventhough its plants are taller than their parents (Yuan and Chen 1988).


Greater Leaf Area. The leaf area per plant of the hybrid 'Nan You 2' atheading and maturity were 6914 cm2 and 4124 cm2

, respectively, compared with 4354 cm2 and 2285 cm2 for the male parent 'IR24' (Li et al.1982). Significantly positive heterosis and heterobeltiosis for flag leafarea was also described in most hybrids (Singh 1997).

Superior Physiological Performance. The rice hybrid 'Nan-You 2' hadhigher photosynthetic efficiency but lower respiration and photorespiration intensity (Lin and Yuan 1980). Greater capacity for synthesis ofchlorophyll and faster quenching rate of the chlorophyll fluorescence ofthe seedling leaves, and higher photosynthetic rate of the flag leaves atthe primary heading stage were also observed in hybrid rice as comparedto rice inbreds (Li, Wang, and Liu 1990).

2. Reproductive Heterosis and Growth Duration. Hybrid rice generallyhas higher rice yield. This is due to a larger panicle, more spikelets orlonger growth duration.

Larger Panicles, More Spikelets and Higher 1,OOO-grain Weight. Thepanicle-spikelet structure of China's most popular rice cultivars orhybrids of the last 30 years was studied. The yield increase by31.3-98.5% of semidwarfrice cultivars in the 1960s, compared with thetaller cultivars of the 1950s, was primarily due to an increase of paniclenumber by 67.5-77.7%. There was little difference in number ofspikelets and grain weight. In contrast, the yield increase of hybrid riceby 11.2-32.1 % in the 1970s, compared to the semidwarf rice of the1960s, came from increase in spikelet number per panicle by 18.0-30.9%and in the 1,000-grain weight by 9.2-12.0% (Table 2.3, Chinese Academy of Agricultural Sciences and Hunan Hybrid Rice Research Center

Table 2.3. The panicle-spikelet structure of hybrid rice and conventional rice in asingle crop in China. Source: Chinese Academy of Agricultural Sciences & HunanHybrid Rice Research Center 1991.

Effective panicle Number of 1,000-grainType of number spikelets weight Yield

Year cultivar (million/ha) per panicle (g) (kg/ha)

1962-1963 Tall 1.62-2.31 83.6-113.1 25.0-26.6 3465-5580

1964-1965 Semidwarf 2.88-3.87 85.8-113.5 23.5-25.1 6780-7320

1976-1979 Hybrid 2.37-2.99 112.3-133.9 26.0-28.1 7530-9675


1991). Zeng et al. (1979) also found the 1,000-grain weight of 23 ricehybrids to be superior to the better parent, and the 1,000-grain weightof 31 of 34 rice hybrids was higher than the mid-parent value.

Higher Yield. Various reports (Sun and Cheng 1994) indicated that heterosis for rice yield ranged from 2-157%. Manuel and Palanisamy (1989)also reported that all nine traits measured in 15 hybrids showed heterosis, with the highest yield heterosis being 46%. China's Jiangxi Academy of Agricultural Sciences found 28 out of 29 rice hybrids to exhibitsuperior yield to their better parent or heterobeltiosis. The yield gain of18 hybrids was statistically significant. All 29 hybrids exhibited heterosis over the local check cultivars. The average yield heterosis was35.5% (Yuan and Chen 1988).

Longer Growth Duration. The growth duration of rice hybrids are highlycorrelated with the ecotype of their parental lines. Most data indicatenegative heterosis in days to flowering (Namboodri 1963; Dhulappanavar and Mensikai 1967; Purohit 1972; Chang et al. 1973; Mallick etal. 1978; Singh et al. 1980; Lin and Yuan 1980; Fujimaki and Yoshida1984). The inter-subspecific hybrids have longer growth duration thanthe intercultivar hybrids. Song et al. (1990) reported that the growthduration of seven out of nine indica-japonica rice hybrids was 15-28days longer than 'Shan-You 63', a late hybrid rice check.

3. Heterosis for Resistance to Adverse Environmental Conditions. Ricehybrids have exhibited good resistance to some diseases, insect pests,drought (Yab and Chang 1976; Tian et al. 1980), low temperature, poorsoil fertility, high salt content (Akbar and Yabuno 1975), deep water(Singh 1983), and other adverse conditions (Lin and Yuan 1980). Therefore, hybrid rice can be grown between 500 N and 18°N and, in SouthChina, at altitudes up to 1500 III (Chen 1985). Researchers at the HunanAgricultural College tested the resistance to rice blast of 224 ricehybrids with their parental lines. Gfthe 224 hybrids, 102 showed.dominance for resistance and 15 showed incomplete dominance (Yuan andChen 1988). It was also reported that all 140 hybrids under three nitrogen levels (0, 60 and 120 kg/hal showed yield heterosis in both dry andwet seasons (Young and Virmani 1990). In the IRTP nurseries, including locations in India, Malaysia, the Philippines, and Vietnam during1980-1986, the average standard heterosis in the tested rice hybrids was108-117% under different environmental conditions (Sun and Cheng1994).


C. Genetic Basis of Heterosis

Bruce (1910) explained heterosis as the combined action of favorabledominant or partially dominant factors. Gustafsson (1946), Hull (1945),Castle (1946) and others emphasized interallelic action as the basis ofheterosis (Hayes 1952). For practical hybrid rice breeding, the explanation of "gene interaction" was proposed. It is assumed that rice heterosis arises from the overall effects of three types of gene interactions:allelic gene interaction, non-allelic gene interaction, and the interactionbetween the nuclear and cytoplasmic gene(s) (Yuan and Chen 1988).

1. Interaction of Allelic Nuclear Genes

Dominance Effects. The dominance hypothesis was first suggested byDavenport (1908). Based on the "dominant complementary" hypothesis,Jones (1917) explained heterosis as the integration of beneficial dominant genes from both parents of F1 hybrids, and the inhibition of harmful recessive genes by the dominant beneficial genes. In a recentmolecular analysis of rice heterosis using RFLP markers, for 82% of 37significant QTL the heterozygotes were superior to the respectivehomozygotes. There was no correlation between most traits and overallgenome heterozygosity. Some recombinant inbred lines in the Fa population had phenotypic values superior to the F1 for all of the traits evaluated. Moreover, this molecular study did not show evident digenicepistasis and suggested that dominance complementation, instead ofoverdominance, is the major genetic basis of heterosis in rice (Xiao et al.1995). Yield heterosis ofIR58025A and IR62829A hybrids resulted fromthe complementation of traits between parents (Vijayakumar et al. 1997).But this hypothesis does not take into account non-allelic gene interaction or that quantitative traits such as yield are governed by polygeneswith additive effect, i.e. there is neither dominance nor recessiveness.

Over-dominance Effects. Shull (1908) proposed over-dominance as thebasis of heterosis. This hypothesis stated that the heterozygote was superior to the two homozygotes for the same gene. Therefore, an F1 individual having the greatest number of heterozygous alleles will be mostvigorous compared to the two parents. Brewbaker (1964) explained overdominance as the effects of (1) supplementary allelic action; (2) alternative pathways; (3) optimal amount; and (4) hybrid substance. A recentstudy of the molecular basis for heterosis using QTL analysis for sevenagronomic traits of maize also suggested that over-dominance played arole in the heterosis observed (Stuber et al. 1992). Although this hypoth-


esis was preferred, especially before the 1970s, it does not explain whysome traits in rice hybrids are inferior to their parental lines. Manyresearchers no longer think that over-dominance makes large contributions to heterosis (Crow 1997, 1999).

2. Interaction of Non-allelic Nuclear Genes. In maize breeding, significant amounts of epistasis may exist in certain specific combinations, butthe magnitude is small (Sprague 1983). Yu et al. (1997) reported thatthere was little correlation between marker heterozygosity and traitexpression, but digenic interactions frequently existed in the F3 progenyderived from 'Zhen-Shan 97 x Minhui 63'. This suggested that epistasissignificantly affected the performance of heterosis in rice.

3. Interaction between Nuclear Gene(s) and Cytoplasmic Gene(s). Inplants, all three genetic sources-nuclear, mitochondrial, and chloroplast genomes-are involved in heterosis (Gillham 1978; Kirk andTilney-Bassett 1978; Srivastava 1983) and, in some cases, the cytoplasmic contributions are critical (Wagner 1969; Srivastava 1972). The reciprocal F1 crosses in some rice hybrids have shown different levels ofheterosis. Furthermore, the same nuclear genome in different cytoplasmic backgrounds has shown different heterosis levels. The cytoplasm,therefore, must play some role in rice heterosis. A study on the effect ofeight rice cytoplasms on 12 traits indicated that all eight cytoplasms negatively affected most traits such as plant height, panicle length, numberof kernels, number of effective tillers, number of panicles, seed settingpercentage, 1,OOO-grain weight, grain weight per plant, yield, and heading date (Sheng 1987).

D. Prediction of Heterosis

Heterosis is a complicated phenomenon that is influenced by both genotype and environment. There is no single method that can accuratelypredict heterosis; however, the following genetical and biochemicalmethods have been suggested.

1. Genetic Diversity. Genetic diversity can be estimated using the following three methods: (1) geographic origin, (2) multivariate analysisusing Mahalanobis D2 statistics (Mahalanobis 1936; Ram and Panwar1970; Vairavan et al. 1973; Maurya and Singh 1977; Rao et al. 1981; Julfiquar et al. 1985; Vaidyanath and Reddy 1985), and (3) isozyme andRFLP polymorphism (Schwartz and Laughner 1969). Genetic diversityor distance has been reported to be highly correlated with the level of


heterosis (Maurya and Singh 1978; Xu and Wang 1981; Li and Ang1988; Zhang et al. 1987). For 43 rice hybrids, yield potential and heterosis had significant positive correlation with genetic distance for 15indica-indica crosses and 6 japonica-japonica crosses, but no correlationfor 22 indica-japonica crosses. The genetic distance method, therefore,seems to be predictive for intra-subspecific heterosis, but not for intersubspecific heterosis (Xiao et al. 1996a). Other workers have foundeither no direct correlation between heterosis and genetic distance (Cress1977; Khalique et al. 1977; Peng et al. 1991; Xie 1993) or that it was onlya weak indicator of heterosis (Liu et al. 1997b; Liu and Wu 1998).

2. Combining Ability. Crosses with great heterosis are more likely obtained when at least one of the parents has high GCA effects (Peng andVirmani 1990). However, prediction of heterosis on the basis ofGCA maynot always hold true (Srivastava and Seshu 1983; Kumar and Saini 1981).

3. Isozymes. There have been two main isozyme methods used to predict heterosis.

Esterase Isozyme Complementary Band. Esterase isozyme is a comparatively dependable biochemical indication for predicting heterosis. IfanF1 hybrid has the specific band(s) from both parental lines, i.e. dominantcomplementary band, there will be heterosis for this combination (Xiaoand Liu 1981; Shi et al. 1988a,b). But it still seems difficult to predictthe existence of heterosis because the existence of the complementaryband(s) is not always coincident with the performance of heterosis.Therefore, some researchers do not agree that heterosis can be predictedby the complementary isozyme band(s) (Peng et al. 1988).

Isozyme Difference Index. The isozyme difference index uses the number of isozymes that show difference in band(s) among the F1 and itsparental lines. A difference in at most six isozymes and at least twoisozymes in the F1 hybrids was reported from 12 rice hybrids and eightisozymes. The F1 hybrid would show heterosis if its isozyme differenceindex was more than four (Zhu and Zhang 1987). This method is weakand requires further evidence.

4. Mitochondrial Complementation. This method was first proposed byMcDaniel and Sarkissian (1966). The concept is that heterosis might beestimated based on the oxidization activity level of mitochondria in theF1 and both parental lines at the seedling stage. However, mitochondriaare derived from the female parent only.


A modification of the mitochondrial complementation method is thehomogenate complementation method. The homogenate complementation value is positively correlated with the heterosis. Homogenate complementation measures the phenomenon of the oxidation activity of theF1 being higher than that of the mean value of the two parental lines. Yellowish young plants from both parental lines were provided homogenatemixed at the ratio of 1:1. It was speculated that the complementation ofthe two homogenates was due to the interaction between the mitochondria and the supernatant of both parental lines (Yang 1991a).

5. ATP Content. ATP content of parental lines in yellowish rice seedlingtips is correlated with heterosis of rice. The ATP content of the parentallines of 15 rice hybrids with heterosis was more than 2.0 x 10-6 mM/gof homogenate. The prediction of heterosis was higher than 90% (Yanget al. 1990; Yang 1991b). This method uses a small amount of tissue andtakes only a short time, but it requires further study.

6. Enzyme Activity. Some researchers reported that the activities ofsome enzymes (such as esterase, nitrate reductase, and superoxide dismutase) are correlated with the level of heterosis. To improve the accuracy of the prediction, other enzymes should be tested (Liang 1991;Liang et al. 1991; Xiao et al. 1991; He 1990).

7. Performance of Parents. In general, high-yielding parents produce alarger proportion of high-yielding hybrids than do low-yielding parents(Virmani 1994a).

E. Approaches for Utilization of Heterosis

1. Approaches

Utilization ofIntercultivar Heterosis. Most current rice hybrids are intercultivar crosses, which can yield 20-30% more than improved semidwarf conventional rice inbreds. China's intercultivar hybrid yieldshave been plateaued at this level for years, due to the narrowing parentalgermplasm diversity (Luo and Yuan 1990).

Utilization of In tersubspecific Heterosis. In the 1950s the idea was proposed to develop rice inbreds by means of indica-japonica crossing (Yang1959). Some high-yielding indica-japonica inbreds were released inChina, such as 'Ai-Jing 23', 'Er-Wan 5' and 'Liao Jing 5', 'Milyang' system


rice cultivars in Korea, and 'Chogoku 91' in Japan. 'C57', the first restorerline for China's three-line system japonica hybrid rice, used restorergene(s) transferred from indica to 'Jing-Ying 35', a japonica cultivar.

Although indica-japonica rice hybrids have strong heterosis, normally yielding 30-50% more than the intercultivar rice hybrids, fourmain obstacles exist: low seed set, excessive plant height, excessivegrowth duration, and unfilled kernels (Wang et al. 1991). Discovery of"wide compatibility" (WC) gene(s) by Ikehashi (1982) provides a mucheasier use of the intersubspecific indica-japonica heterosis. More detailswill be discussed in Section VI.

Intersubspecific heterosis from crosses between an indica and a japonica cultivar cannot be exploited directly, owing to the genetic divergencebeing too large and poor adaptability to tropical conditions (Yang 1990b;Virmani 1994b). Another concern is that eating and cooking qualities oftypical indica-japonica crosses segregate, and therefore are not acceptable to most rice consumers in China (]. S. Zou, pers. commun.). Yuan(1991a,b) suggested the alternative ofusingjavanica as a parent. Geneticdivergence is larger between javanica and indica or between javanicaand japonica types than for intercultivar crosses. Both indica-javanicaand japonica-javanica hybrids have shown stronger heterosis than theintercultivar rice hybrids. These crosses have fewer problems than thetypical indica-japonica intersubspecific crosses.

Utilization ofDistant Heterosis. Many agricultural scientists have triedto transfer target genes or traits to rice from maize, sorghum, bamboo,and other distant plant species. For example, marker-assisted selectioncan now be used to transfer the desired gene(s) from wild rice or otherdistant species to cultivated rice (Yuan 1996; Xiao et al. 1996b; Tanksleyand McCouch 1997). Once inbred rice has been improved using biotechnology, genetic sources for stronger heterosis may be found, especiallyusing NMS (nuclear male sterility), which is not limited by cytoplasmicfunction.

2. Methodology

Three-line System. Currently most commercial rice hybrids are threeline system rice hybrids with intercultivar heterosis. Breeders are nowtrying to transfer wide compatibility gene(s) to the parental A, B, or Rlines. Some intersubspecific rice hybrids using the three-line systemhave been successfully developed in China. Because the breeding andseed production procedures are complicated, labor-intensive and costly,


in the long run, the three-line system will be replaced by less complicated systems.

Two-line System. There are two techniques that have been used: chemical emasculation and the T(P)GMS system. Chemical emasculation ofplants was reported as early as the 1950s and China began using chemical emasculation to produce rice hybrids in the 1970s. Some highyielding rice hybrids such as 'Gan-Hua 2' were successfully released.The technique will be presented in more detail in Section V. TheT(P)GMS system refers to the thermo-photoperiod sensitive genic malesterility. T(P)GMS lines can be used for male sterile line multiplicationand F1 seed production under different temperature or daylengthregimes. This system is genetically controlled by nuclear gene(s) andthus there is no negative effect from the cytoplasm and no risk of unilateral cytoplasmic breakdown. There is more opportunity to developelite rice hybrids using the T(P)GMS system than using the three-linesystem. Omission of the B line used to maintain male sterility for thethree-line system reduces the seed cost. Also, it is easier to combineT(P)GMS gene(s) with the WC gene(s).

One-line System (or Apomictic System): Apomixis is asexual reproduction via seeds. It results in no deterioration of heterosis with year-afteryear seed production because no genetic segregation occurs. The idea forheterosis fixation by apomixis was proposed in the 1930s, but the onlyprominent example has been for Buffelgrass (Bashaw 1980a,b). Tobypass the need for hybrid rice seed production each year, Zhao (1977)and Yuan (1987) proposed utilization of apomixis in rice. Apomixisbreeding for fixing rice heterosis will be described in Section VIII.

III. MALE STERILITY IN RICE

Cytoplasmic male sterility in rice has been reported by many scientists(Nagai 1926a,b; Ishikawa 1927; Miyazawa 1932; Takezaki 1932; Ramanujam 1935; Hara 1946; Jones 1952; Weerarathe 1954; Sampath andMohanty 1954; Katsuo and Mizushima 1958; Yuan 1966; Athwal and Virmani 1972; Hoff and Chandrapanya 1973; Razzaque 1975; Trees and Rutger 1978; Mahadevappa and Coffman 1980; Rutger and Shinjyo 1980). Thefirst CMS line was developed by Shinjyo and Omura (1966) in Japanusing 'Chinsurah Boro II' cytoplasm. The CMS line 'Er-Jiu-Nan 1 A' wasthe first to be put into commercial production in China in the early 1970s.


A. Morphology, Cytology, and Histology of Male Sterile Lines

1. Morphological Features. The male sterile rice A line [male sterilecytoplasm (S), recessive nuclear gene (fj)] appears morphologically similar before heading to its maintainer B line [normal cytoplasm (N), recessive nuclear gene (fj)]. After heading, sterility can be recognized fromvarious morphological features involving the anthers and floweringbehaviors (Table 2.4). Most of the morphological features of the T(P)GMSlines or gametophytic male sterile lines are almost the same as those ofthe sporophytic male sterile lines.

2. Cytological Features. Laser and Lerstern (1972) summarized the cytological studies conducted between 1925 and 1972 that analyzed thepollen abortion resulting in male sterility in crops. Rice male sterile linesare classified by the stages of pollen abortion: pollen-free abortion, uninucleate stage abortion, binucleate stage abortion, and the trinucleatestage abortion type.

Table 2.4. The morphological differences between sporophytic A and B lines. Source:Yuan 1985; Sun and Cheng 1994.

Morphologicalfeature

Plant height

Tillering capacity

Heading date

Panicle

Flowering behavior

Anther shape

Anther dehiscence

Pollens

Fertility

Maintainer line

Taller than the A line

Lower than the A line

Earlier than the A line

Normal heading

Concentrated floweringtime and shorterglume opening time

Plump, golden in color

Dehiscent

Round and dark-brownwhen stained with I-KI

Normal

Sporophytic indica male sterile line

Shorter than the maintainer line

Higher, and longer tillering stage

3-5 days later than the B line

Shorter neck, panicle basal partenclosed in the leaf sheath for thedwarf sporophytic indica type

Diffused and longer flowering time

Empty, slender, thin, milky-whiteor yellowish in color

Indehiscent

I. Irregular in shape and unstainedwith I-KI; or II. Round andunstained;or III. Round and lightbrown in color

Self incompatible


Pollen-free Type Abortion. In this type, pollen abortion occurs before theuninucleate stage. This type has mainly three patterns of abnormalpollen development: sporogenous cells; pollen mother cells; and abnormality after tetraspore formation (Hunan Teachers' College 1972).

Uninucleate Stage Abortion. In the uninucleate stage of 'V20A' the percentage of pollen abortion is 96.7% (Rao and Xie 1983). Pollen abortionof WA-type male sterile lines normally occurs at the uninucleate stage;the pollen grain has a dissolved nucleus or nucleoli to some degree, inaddition to a collapsed thin cell wall, a vague germination aperture, anda very small or condensed protoplast content (Sun Yat SenUniv. 1976).For the two-line system, some pollen grains of the T(P)GMS lines abortat the uninucleate stage with withered pollen grains, very little cytoplasm, and disappearance of the nucleus (Wu and Wang 1990). Thepollen grains that have aborted by the uninucleate stage are irregular inshape (often triangular under the microscope). Uninucleate-type abortion is also referred to as typical abortion.

Binucleate Stage Abortion. In this type the reproductive nuclei andnutritive nuclei of most pollen grains start to collapse only at the binucleate stage, such as in the 'Hong-Lian' type A lines. Chiang et al. (1981)reported that 80.3% of the pollen grains of'Hong-Lian' male sterile linesaborted at the binucleate stage, as compared to 12.8% that aborted at theuninucleate stage. For this abortion type, part of the pollen mother cellsvacuolize and damaged nuclei are without distinguished cell walls.Some cells form protoplasmic masses. Two or three pollen mother cellsconnect at their nucleoli in irregular ways. The aborted pollen grains aremostly spherical, hence, the term spherical abortion.

Trinucleate Stage Abortion. BT-type 'Taichung 65A' has no distinguishable abnormality in pollen development before the trinucleatestage. At the binucleate or trinucleate stage, size of the nucleoli isreduced in only a few cells with some nuclear membrane collapse (SunYat Sen Univ. 1976).

At the anaphase of the reproductive karyokinesis, the chromatin grainsdisappear at the later stage in 'Nong-Jin 2 A'. In 'Fu-You lA', manymicronucleoli are scattered in the cytoplasm through nuleolar buddingand then disappear. In some cases, reproductive nuclei form two spermsof different size, and some nutritive nuclei of equal size (Teng 1982).Some starch has already been produced and the pollen grains stainbrown using I-KI solution, but a lighter brown than for normal pollengrains; hence, this is called stained pollen abortion.


Pollen abortion can occur at any time from sporogenous cells to thetrinucleate stage. Different abortion types occur, not only in different malesterile lines, but also in different flowers of the same rice plant, or evendifferent anthers of the same floret (Pan et al. 1982). The four abortiontypes mentioned above were classified for practical purposes as: restorermaintainer relationship (WA type-uninucleate abortive; HL type-binucleate abortive; BT type-trinucleate abortive) and cytoplasm-nucleusrelationship (sporophytic male sterile system-uninucleate abortive;gametophytic system-binucleate and trinucleate abortive).

3. Histological Features. The following histological abnormalities werefound in the stamens of male sterile rice plants:

Abnormality of the Anther Wall. The anther pulling force of some ricemale sterile lines is not strong enough to open a dehiscence cavity, asoccurs in the normal rice plant. In some cases the dehiscence cavity isnot formed or is formed on only one side of the anther. Consequently,anthers fail to dehisce. For example, although there is a strong pullingforce in the anthers of some male sterile lines such as in WA-type 'NanTai 13 A', no dehiscence cavity is formed on either side of the anther,thus the pulling force cannot open the anther wall to bring about dehiscence (Chou 1978; Pan and He 1981a,b).

Abnormality of the Intercellular and Tapetal Cells. The tapetal cellsencircle sporogeneous cells and provide nourishment to the reproductive cells. Abnormal development or damage of intercellular or tapetalcells often results in the pollen abortion of rice male sterile lines (Rao1988). Excessive proliferation of tapetal cells in HL-type 'Hua-Ai 15 A'causes tapetal periplasmodia to form, pushing the pollen mother cellsto the center of the anther, which results in the dissolution of the pollenmother cells. In the case of the WA-type 'Hua-Ai 15A,' fibrocytes of thedermal layer become deformed, thus damaging the tapetal cells andcausing pollen abortion (Xu 1979). In 'Er-Jiu-Nan 1 A' and some otherWA-type A lines, vacuolization of the intercellular cells and abnormalincrease of the radial thickness of the middle lamella cells by the uninucleate stage push the tapetal cells towards the center of anther cells.These cells are distinctively thin with many vacuoles and light-coloredcytoplasm. By the binucleate stage, the pollen cells are completelyaborted with complete vacuolization and withering of the intercellularcells. At this point the secondary tapetal walls in the intercellular cellscan be easily observed under the microscope (Pan 1979; Pan and He1981a,b). The tapetal and endothecial abnormalities are also the main


histological causes of male sterility in 'V20A' and 'Pragathi A' (Yogeesha and Mahadevappa 1995).

In some three-line system and two-line system male sterile lines, dissolution of tapetal cells is abnormally delayed. For example, the tapetalcells of 'Guang-Xuan 3 A' remain intact even at the trinucleate stage,before pollen abortion occurs (Guangxi Teachers' College 1975). TheT(P)GMS line, 'Nong-Ken 58s', under daylength of 14 h, maintains anintact structure of tapetal cells with rich cytoplasm, nuclei, and a smallnumber of vacuoles, plastids and other organelles. Even though metabolism in tapetal cells is active and there is no appearance of pollen dissolution, the grains still start to abort (Li et al. 1993). However, for thesame line, 'Nong-Ken 58s' under long daylengths, the intercellular cellsand the inner tangential wall of the tapetal cells started to dissolve atmeiosis. At the end of the uninucleate stage, the intercellular cells arealmost completely collapsed, and the cytoplasm forms a cytoplasmicmass due to the dissolution of the tapetal cell wall (Wang and Tong1992). This abnormal tapetum development and effect on the male sterility are also observed in other T(P)GMS lines (Zhang et al. 1994a).

Abnormality of Filaments and Connective Vascular Bundles. The filaments and the connective vascular bundles are the channels that transport water and nutrients in anthers. Their development directly affectsthe quantity of nutrients available to an anther. Filament vessel degeneration has been observed in the wild abortive and the pollen-freeabortive types of male sterile lines. The degree of degeneration is correlated with the pollen abortion percentage (Pan 1979).

For HL-type 'Hua-Ai 15 A', abnormal development occurs in the connective vascular bundles and the tracheary cells. Vessels of the connective vascular bundles develop poorly, with an enlarged annularspace. In some cases the vessels and vessel cavities are damaged. Thejunction at which vessel cells meet becomes disconnected and the cellsbecome fibrillous with loose connections, disorderly arrangement, anddegenerated function. Poor development of connective vascular bundlesoccurs at the uninucleate stage and binucleate stage, with poor differentiation of phloem and xylem in the WA-type male sterile lines. Thecells wrinkle and shrink. The vascular bundles are not highly visibleowing to the degenerated and disordered cells. The extent of development of the vascular bundles is negatively related to the degree of pollenabortion (Sun Yat Sen Univ. 1976). There are different abnormalities ofthe vascular bundles in different rice male sterile lines, and even of malesterile lines with the same nuclear background but different cytoplasmicsources. Vascular bundles of the fertile anthers of 'Nong-Ken 58s' are


similar to those of the normal rice cultivars, yet abnormal developmentof the vascular bundles and thin-wall cells of the sterile anthers of'Nong-Ken 58s' exist at some stages (Wang and Tong 1992).

4. Classification of Male Sterility. More than 600 rice male sterile lineshave been developed in China. Classification systems were proposed inthe late 1970s and 1980s (Zhu 1979; Li 1980a; Wan et al. 1988). At present, the taxonomic system (Table 2.5) suggested by Wan et al. (1988) isoften used. It is a five-step taxonomic key based on (1) the inheritanceof male sterility (sporophytic male sterility or gametophytic male sterility); (2) the pollen abortion stage [uninucleate (typical abortion), binucleate, (spherical abortion), or trinucleate (stained abortion)]; (3) therestorer-maintainer relationship; (4) nucleus substitution type, such aswild-cultivar, cultivated-wild; and (5) cultivar differentiation based onthe cytoplasmic source.

B. Physiological and Biochemical Basis

1. Transportation and Metabolism. Abnormality of vascular bundlesrestricts transportation and metabolism of nutrients, which was revealedby an experiment using 32P. In this experiment, 32p was transported topanicles in large amounts in 'Nong-Ken 58s' under conditions favoringfertility, in the sequence: panicle> flag leaf> the leaf beneath the flagleaf. Under conditions favoring sterility, the 32p amount was small in thepanicles, and the sequence above reversed compared to conditions forfertility (He et al. 1992). The three-line system male sterile lines had similar mechanism for transportation and metabolism to 'Nong-Ken 58s'.Researchers at Sun Yat Sen University reported that absorptivity of 32p,14C and 35S in the WA-type male sterile lines 'Er-Jiu-Ai A', 'Zhen-Shan97A' and in BT-type 'Taichung 65 A' was weaker in their anthers, panicle branches, and the vascular bundle system of the glumes in comparison to the corresponding B lines. Interestingly, the same absorptivityoccurred in the ovules of A and B lines. It is concluded that there is ametabolism barrier in anthers of the male sterile lines, whereas themetabolism is normal in their ovules. For 'Nong-Ken 58s', a markeddecline of photochemical activity of the chloroplasts, such as lower PSII photochemical activity and less chlorophyll b in the chloroplasts,induced by long daylength, may reduce the available photosyntheticproducts and result in male sterility (Tang et al. 1994).

ATP content is highly related to the fertility performance ofT(P)GMSlines. For example, the ATP content of 'Er-Yi lOSs' is much lower inmale sterile plants at the early uninuclear stage than for fertile plants,

Tab

le2.

5.T

he

tax

on

om

icsy

stem

for

the

clas

sifi

cati

onof

the

thre

e-li

nesy

stem

rice

mal

est

eril

ity.

Sou

rce:

Wan

etal

.19

88.

w co

Fer

tili

tyin

her

itan

ce

Sp

oro

ph

yti

cp

oll

enab

orti

on

Gam

etop

hyti

c

poll

en

abor

tion

Po

llen

abo

rtio

nst

age

Un

inu

clea

te(t

ypic

al)

abo

rtio

n

Bin

ucle

ate

(sph

eric

al)

abo

rtio

n

Tri

nu

clea

te(s

tain

ed)

abo

rtio

n

Res

tore

rm

ain

tain

erty

pes

DW

WA

LW

IP G HL

BT

Nuc

leus

subs

titu

tion

wil

dx

cult

ivat

ed

wil

dx

cult

ivat

ed

cult

ivat

edx

wil

d

indi

cax

japo

nica

japo

nica

xin

dica

indi

cax

indi

ca

japo

nica

xja

poni

ca

wil

dx

cult

ivat

ed

indi

cax

indi

ca

cult

ivat

edx

wil

d

indi

cax

indi

ca

wil

dx

cult

ivat

ed

indi

cax

japo

nica

indi

cax

indi

ca

indi

cax

japo

nica

Cyt

opla

smic

sour

ce

Don

g-pu

wil

dri

ce(D

W),

Tia

n-D

ong

long

Aw

nw

ild

rice

Hai

-Nan

wil

dab

orti

veri

ce(W

A),

Liu

-Z

ho

ure

d-a

wn

edw

ild

rice

Cha

o-Y

ang

1,L

ian-

Tan

g-Z

hao

She

ng-Q

i,N

an-G

uang

-Zha

n

Gui

-Hua

-Hua

ng

Dis

si

Zha

o-T

ong-

Bei

-Zi-

Gu

Lon

g-A

nw

ild

rice

,G

uang

-Xi

wil

dri

ce

Ind

on

esia

Pad

dy

rice

6,

Gu

Y-1

2

Jin-

Nan

-Te

43

Gam

biak

a

Hai

-Nan

red

-aw

nw

ild

rice

Tia

n-Ji

-Du

Jing

-Qua

n-N

uo

Ch

insu

rah

Bor

oII


only 1/2-1/7 of that of fertile plants and 1/9-1/10 of that of conventionalinbred rice (Deng et al. 1990). It was also found that ATP in the anthersof 'An-Nang s-l' and 'Heng-Nong s-l' under high temperature wasdecreased, and the respiration rate of the floscules and anthers declinedgradually (Chen et al. 1994).

2. Protein and Amino Acid Content. The protein content in anthers of'Er-Jiu-Nan lA' is lower than that of 'Er-Jiu-Nan 1 B' and the restorer line'IR661' (Shanghai Plant Physiology Research Institute 1977). The content of free histones in male sterile lines is also much lower than in theircorresponding B lines from meiosis stage to the trinucleate stage (Dai etal. 1978). Xu et al. (1992) indicated that 'Zhen-Shan 97B' produced fivebands of soluble chloroplast proteins and six bands of the water-solublecomponents, but 'Zhen-Shan 97A' did not. In T(P)GMS system, studiesdemonstrated that, besides the change of soluble protein content atdevelopmental stages, there were different patterns of protein bands between 'Nong-Ken 58s' and 'Nong-Ken 58' or between some TGMS linesand their ancestral lines, and that some specific protein bands forT(P)GMS lines were present (Shu et al. 1989; Wang, Xiao and Liu 1990;Bai and Tan 1990; Peng and Wang 1991; Huang, Tang, and Mao 1994;Wang et al. 1997b). Study of specific proteins can promote understanding of the mechanism for male sterility. However, currently there is noconvincing evidence of a relationship between specific polypeptides andthe male fertility performance of rice. In fact, some results from proteinstudies are contradictory.

For the three-line system, the higher amino acid content of A than ofB or R lines indicates that synthesis of protein is slower than proteindegradation, or that a barrier exists for protein synthesis in the male sterile lines. In some cases, the relative amounts of amino acids differ amongthe rice male sterile lines. For example, one study demonstrated that proline content was lower in male sterile plants compared to their maintainer lines, while the asparagine content was higher in the male sterilelines than in their respective maintainer lines (Shanghai Plant Physiology Research Institute 1977). Out of 17 amino acids examined, the relative amounts of proline and alanine were most related to pollenabortion in 'Nong-Ken 58s' and 'V20A' compared to 'Nong-Ken 58' and'V20B' (Xiao et al. 1987). The tendency for declining proline content alsooccurred for chemically emasculated male sterile and CMS lines. Proline content in B or R line anthers is three to six times more than in thesterile anthers (Raj and Siddiq 1986; Yu et al. 1991). It seems thatreduced proline content hinders carbohydrate metabolism and decreases


the content of other amino acids. Nutritional disorders are a generalcause of pollen abortion in male sterile rice lines.

3. Activity of Enzymes. Activity of peroxidase in rice plants of pollenfree sterile lines is stronger than in fertile plants. Peroxidase activity in'V20A' is stronger than that in the fertile anthers at the early uninucleate stage, but it decreases with pollen abortion and reaches the lowestat the binucleate stage. The increase of peroxidase activity in 'Nong-Ken58s' is similar to that in 'V20A', but the lowest activity of peroxidase inthe sterile anthers of 'Nong-Ken 58s', under long daylength, is at the trinucleate stage, only 37.5% of the activity at the uninucleate stage (Chenand Xiao 1987). The V-max value of ribulose-1,5-bisphosphate carboxylase of male sterile lines is higher than the restorer line or F1 hybrids(Wei et al. 1994). The activity of other enzymes such as ADH (alcoholdehydrogenase), phosphorylase, RuBPcase, catalase, superoxide dismutase, glycolic acid oxidase, S-adenomethionine decarboxylase, andarginine decarboxylase may also affect the fertility in 'Nong-Ken 58s'(Zhu and Yang 1992).

4. Growth Regulator. Decrease ofIAA by oxidases has been observed tohinder metabolism during sporophytic development in anthers andbring about pollen abortion (Huang et al. 1984; Yang, Zhu, and Tang1990). GA and ABA also influence male sterility of 'Nong-Ken 58s'(Yang, Zhu, and Tang 1990; Nakajima et al. 1991; Zhang and Zhou1992). The study on 'Norin PL12' shows that the presence of GA4 / 7 isclosely related to the expression of the TGMS gene and subsequentdevelopment of pollen and anthers (Honda et al. 1997). A significantnegative correlation was observed between the ethylene release rate ofyoung panicles and the corresponding pollen fertility in 'Nong-Ken 58s'.Application of AVG (aminoethoxy vinylglycine) causes the fertility of'Nong-Ken 58s' under long daylength, but the fertility level decreasessharply in 'Nong-Ken 58s' under short daylength treated with ACe (Li,Luo, and Qu 1996). Other studies revealed that ethylene biosynthesiswas correlated with the performance of fertility in T(P)GMS lines (Luoet al. 1990).

5. Products of Cytoplasmic and Nuclear Genes. The zymograms ofrestriction enzymes applied to mtDNA can be strikingly different betweenrice male sterile lines and maintainer lines. The gene structures for subunit I and subunit II of cytochrome in mitochondria differ between A andB lines, but no difference is found in a Hind III zymogram of ctDNA of

42 ]. LI AND L. YUAN

both lines (Kadowaki et al. 1986; Liu et al. 1988; Sakamoto et al. 1990).In contrast, a special dsRNA (double-stranded RNA) was found with 18kb molecular weight in mitochondrial nucleic acid of the BT-type CMSlines and of 'Nong-Ken 58s', but not of their maintainer lines and 'NongKen 58' (Zhang and Wang 1990; Wang et al. 1990). Some scientistsbelieve the mitochondrial DNA modifications may support the hypothesis of the mitochondrial inheritance of eMS in rice (Mignouna et al.1987). But, the relationship between the differences between mtDNA orctDNA and male sterility requires further study. As for the products ofmtDNA and ctDNA translation in rice male sterile lines, there is evidence for large differences in polypeptides with male sterility, a resultof genes from the mitochondria, chloroplast, and nucleus (Liu 1986; Xuet al. 1992).

There have been few reports on the relationship between male sterility and nuclear gene products in rice. That a specific protein causes malesterility in rice has not been confirmed. The abnormal transcripts of theatp6 gene produced in the antisense direction may be involved in cytoplasmic male sterility (Kadowaki et al. 1990; Akagi et al. 1994). CMSanthers were also found to have lower insoluble polysaccharides, proteins, and RNA content than their maintainer lines (Yogeesha andMahadevappa 1995).

Generally the metabolic level of rice male sterile lines are lower thanfor maintainer lines, especially in the production of starch, proteins, andchange of enzyme activity. Male sterility in the three-line system isrelated to mitochondria and chloroplasts, so mtDNA, ctDNA, and theirmetabolism have been closely studied.

6. Other Biochemical Factors. Several other biochemical factors havebeen implicated in male sterility. These include a weaker oxygen scavenger system, high content of H20 2and 02; lower efficiency of oxidativephosphorylation, a high level of lipid peroxidation in the anthers of CMSlines or T(P)GMS lines, and a higher level of aspartic acid in sterileanthers (Raj and Siddiq 1986; Chen and Liang 1991, 1992; Liang andChen 1993).

c. Genetic Basis

Despite a great deal of research on the genetic basis of male sterility inrice, the interaction between the nuclear and cytoplasmic gene(s)remains unclear. Several hypotheses have been proposed to explain themechanism of male sterility.


1. Nuclear Male Sterility (NMS). In some cases, nuclear male sterilityis conditioned only by one recessive gene and female fertility is unimpaired (Kaul 1988). If male sterility is conditioned by recessive gene(s),a maintainer cannot be identified. In contrast, restorer cannot be developed if male sterility is governed by dominant gene(s).

2. Cytoplasmic-Nuclear Male Sterility. In this case, male sterility iscontrolled by an interaction of cytoplasmic and nuclear gene(s). Only inthis type can a maintainer (B) line and a restorer (R) line be found anddeveloped to attain a complete three-line system. The cytoplasms controlling male sterility in rice are described in Table 2.6.

3. Other Hypotheses

.Cytoplasmic Male Sterility. The male sterility is controlled by the cytoplasmic gene(s) alone, so it is impossible to find a restorer for the cytoplasmic male sterile lines (Edwardson 1956). Some researchers do notaccept this explanation of male sterility.

"Relationship" Theory. Male sterility is considered to be a quantitativetrait, resulting from segregation in the F1 generation and continuousdistribution of male sterility in the F2 generation. Moreover, male sterility is influenced by environmental factors such as temperature. Malesterility is assumed to be generated by the lack of coordination of thegenetic factors between the two parental lines (Pei 1980).

4. Genes Controlling Male Sterility in Rice

Genes Controlling Male Sterility in CMS Lines. Male sterility of WA-typehas been hypothesized to be controlled by a single recessive gene (Wang1980), by two recessive genes (Gao 1981), or by multiple minor genes (Fuand Wang 1988). It is commonly thought that male sterility of the BTtype male sterile lines is controlled by one pair of recessive genes (Shinjyo 1984; Sheng 1994).

Genes Controlling the Male Sterility in T(P}GMS Lines. The segregationratios of the F2 and Bel show that the photoperiod-sensitive genic malesterility in PGMS lines such as 'Nong-Ken 58s' is controlled by a singlegene (Lu and Wang 1986; Jin et al. 1987; Lei and Li 1989; Zhang et al.1990; Zhang and Zhu 1991; Lin et al. 1996) or by two major recessivegenes (Sheng 1992; Shao et al. 1993; Shao and Tang 1993; Wan and Ma1996; Yang 1997). Zhang et al. (1994b) studied a PGMS line, '31001s',

*'-T

able

2.6.

Cyt

opla

smic

sour

ces

for

ind

uci

ng

mal

est

eril

ity

inri

ce.

Sou

rce:

Kin

osh

ita

1997

.*'-

Cyt

opla

smN

ame

Mai

nta

iner

Res

tore

rR

efer

ence

cms-

boC

hin

sura

hbo

roII

cyto

plas

mT

aich

un

g65

etc.

RfI

-aR

fI-a

Shi

njyo

1969

,19

75em

s-id

Lea

dri

cecy

top

lasm

Fu

jisa

ka

5R

f2R

f2W

atan

abe

1971

ems-

TA

TA

820

(Tad

ukan

)cy

topl

asm

No

rin

8K

itam

ura

1962

b,19

62c

cms-

CW

Chi

nese

wil

dri

cecy

topl

asm

Fu

jisa

ka

5K

atsu

o&

Miz

ush

ima

1958

ems-

WA

Wil

dab

orti

vecy

topl

asm

IR24

etc.

Rf3

Rf3

,L

in&

Yua

n19

80R

f4R

f4K

adow

aki

etal

.19

88cm

s-H

LR

edaw

ned

wil

dcy

topl

asm

IR54

753A

etc.

Lin

&Y

uan

1980

ems-

okA

kebo

nocy

top

lasm

Lie

n-T

ong-

Tao

Rfa

kR

fak

Yab

uno

1977

(em

s-jp

jS

akam

oto

etal

.19

90em

s-A

RC

AR

C13

829-

16cy

topl

asm

IR54

755

IR42

etc.

Vir

man

iet

al.

1989

ems-

GA

MG

ambi

aca

cyto

pla

smC

hao

Yan

g1

etc.

IR58

etc.

Lin

&Y

uan

1980

;K

adow

aki

etal

.19

88;

Vir

man

iet

al.

1989

cms-

spM

S5

77A

cyto

pla

smIR

42et

c.K

adow

aki

etal

.19

88;

Vir

man

iet

al.

1989

cms-

UR

89U

R89

Fcy

topl

asm

Tai

chu

ng

65R

fI-b

Rfl

-bK

adow

aki

etal

.19

88;

Shi

njyo

1990

ems-

UR

I02

UR

102F

cyto

plas

mT

aich

un

g65

Rfl

-cR

fl-C

Kad

owak

iet

al.

1988

;S

hinj

yo19

90

cms-

UR

104

UR

104F

cyto

plas

mT

aich

un

g65

Rfl

-dR

fI-d

Kad

owak

iet

al.

1988

;S

hinj

yo19

90

cms-

UR

I06

UR

106F

cyto

plas

mT

aich

un

g65

Rfl

-eR

fI-e

Kad

owak

iet

al.

1988

;S

hinj

yo19

90

cms-

UR

27U

R27

Fcy

topl

asm

Tai

chu

ng

65S

hinj

yo19

90

cms-

5425

754

257

cyto

plas

mL

ing

etal

.19

89

cms-

Khi

abor

oK

hiab

oro

cyto

pla

smA

kib

are

Nag

amin

eet

al.

1995

cms-

IRO

ryza

per

enn

isA

ccIR

64D

alm

acio

etal

.19

92,

1995

6670

7A10

4823

cyto

plas

m

2. HYBRID RICE: GENETICS. BREEDING. AND SEED PRODUCTION 45

using molecular markers. Two chromosomal regions each containing aphotoperiod-sensitive genic male sterility locus designated as pmsl (onchromosome 7) and pms2 (on chromosome 3) were detected. The effectof pmsl is 2-3 times larger than that of pms-2, and dominance is nearlycomplete at both loci, but pmsl is not the locus relevant to the fertilitydifference between 'Nang-Ken 58s' and 'Nong-Ken 58' (Wang et al.1997a). A recessive PGMS gene in 'Nong-Ken 58s', designated as msPh ,

has been also reported to be linked with gh-l and st-2 on chromosome5 (Qian et al. 1995). This was confirmed by Lin et al. (1996). Using primary trisomies analysis, it was found that one gene for the male sterility in 'Nong-Ken 58s' was linked with d-l on chromosome 5, with arecombination value near 28.41 (Zhang et al. 1990). In breeding practice,continuous distribution of photoperiod-sensitive male sterility in theprogeny of the primary generations is indicative of the effect of modifying genes on photoperiod-sensitive male sterility (Mei et al. 1990; Xueand Deng 1991).

TGMS genes of '5460s' and 'H89-1' were designated as tmsl andtms2, respectively (Sun et al. 1989; Maruyama et al. 1991a; Kinoshita1992). The gene tms-l of '5460s' was identified in China. TGMS1.2,located about 6.7 eM from the TGMS gene tms-l, is on chromosome 8(Wang et al. 1995a, 1996). However, another study indicated that twomajor recessive genes controlled the thermosensitive male sterility in'5460s' (Wan and Ma 1996). The male sterility of two other ChineseTGMS source materials, 'An-Nong s-1' and 'Heng-Nong s-1', was speculated to be controlled by two unmapped recessive genes (Zhou et al.1991a; Jiang et al. 1993; Wu and Yin 1992; Wan and Ma 1997) or onerecessive gene on chromosome 8 in 'An-Nong s-1' (B. Wang, pers. commun.). The pollen fertility and spikelet fertility of 'Norin PL12' or 'H89l' and 'IR32364TGMS' are controlled by a single recessive gene.Complementation tests revealed that these two genes were different.The TGMS gene in 'Norin PL12' has been designated as tms-2 and theTGMS gene in 'IR32364TGMS' has been designated as tms-3(t)(Maruyama et al. 1991a; Borkakati and Virmani 1996). RFLP analysisrevealed that the tms-2 was located between R463A and R1440 on chromosome 7 (Yamaguchi et al. 1997). A recent study indicated that fourRAPD markers were linked with tms3(t) (Subudhi et al. 1997). Thestudy used bulked segregant analysis of the F2 population between'IR32364TGMS' and 'IR68'. The gene tms3(t) was mapped to the shortarm of chromosome 6. The TGMS gene for the Indian TGMS line 'SA2'has been designated as tms4. It was reported that the 0.7-kb ampliconofOPA 12 and 1.9-kb amplicon ofOPS 1 were specific to the TGMS traitof 'SA2' (Reddy et al. 1998b). Gene mapping for reverse TGMS lines is


under way in China (B. Wang, pers. commun.). Besides the majorgenets), TGMS lines could be controlled by some modifying genes sinceindividual F2 progeny from the same population shows different fertility levels (Reddy et al. 1998b).

Allelic relationship analysis revealed that the genes controlling malesterility in 'Nong-Ken 58s' are allelic to those of its derivative lines'7001s', 'N5088s', and 'M105-9s' (Yang 1997). The genes controlling themale sterility of'Pei-Ai 64s', 'An-Nong s-1', 'Heng-Nong s-1', and '5460s'are non-allelic, but the genes of 'Xin-Guang s' and 'Pei-Ai 64s' are allelic(Luo et al. 1996). The allelic relationships among most current ChineseT(P)GMS lines are indicated in Fig. 2.2 (Sun and Cheng 1994).

Genes Controlling the Male Sterility in NMS Lines. The male sterility ofone rice mutant is found to be controlled by a single recessive gene, ms9, in linkage group 6 with two marker genes, Ur (undulate rachis) andCl (Clustered spikelet), the order being Cl-ms-9-Ur (Sato and Shinjyo1991). Suh et al. (1989) reported four NMS genes: ms-ir36(t) fromIR36ms; ms-m67(tj from 'Milyang 67ms'; ms-m77(tj from 'Milyang77ms', and ms-m55(tj from 'Milyang 55ms'. Later Suh et al. (1991)revealed that the gene ms-m67{t) was linked with lax (lax panicle), eg(extra glume), d-l0 (dwarf-l 0) and A (anthocyanin activator) in linkagegroup III, with recombination values of 0,13.7,23.6 and 34.0%, respectively. The map position of ms-m67(t) is eg-ms-m67{tJ-d-l0-A, and it iscompletely linked with lax. The NMS gene of ms-m77{t) is linked to mp1 (multiple pistil-l), with a recombination frequency of 14.9%. For malesterile rice mutants derived by chemical or irradiation induction, different recessive genes are involved (Ko and Yamagata 1987; Fujimakiand Hiraiwa 1986). The genes responsible for nuclear male sterility,including environment-conditioned nuclear male sterility, are listed inTable 2.7.

IV. BREEDING FOR THREE-LINE SYSTEM HYBRID RICE

The genetic basis of the three-line hybrid rice breeding system are a malesterile line (a CMS or A line), a maintainer line (B line), and a restorerline (R line).

A. Breeding Procedure

The procedure for the three-line system hybrid rice breeding can bedivided into two phases, parental line development and heterosis

8912

s

japo

nic

a-ty

pe...

....

~

~I

~-Nongs-D

indi

ca-t

ype

W61

54s I

Pei

-Ai6

4s

...~----

311ilS~

Er-

Yi-

l05s

/~

WD

1s

M10

5s7001

s

...

alle

lic;

//

non-

alle

lic;

c::::::

:::::>

indi

caso

urce

T(P

)GM

So

japo

nica

sou

rce

T(P

)GM

S

Fig.

2.2.

The

alle

lic

rela

tion

ship

amon

gT

(P)G

MS

line

sde

velo

ped

inC

hina

.S

ourc

e:S

unan

dC

heng

19

94

.

*'" '1

..I::-

Tab

le2.

7.G

enet

icm

ale

ster

ilit

yin

rice

.S

ourc

e:K

inos

hita

1997

.O

:l

Gen

eN

ame

Sou

rce

Der

ivat

ion

Ch

rom

oso

me

Ref

eren

ce

msl

{stl

mal

est

eril

e-l

Fu

ku

kam

esp

on

tan

eou

s6

Har

a19

46

GR

OU

PA

(ms2

-ms6

)S

hib

uy

a19

73

ms2

mal

est

eril

e-2

md

-str

ain

spo

nta

neo

us

ms3

mal

est

eril

e-3

Buf

umoc

hisp

on

tan

eou

sm

s4

mal

est

eril

e-4

Fuj

imin

ori

spo

nta

neo

us

ms5

mal

est

eril

e-5

Oto

risp

on

tan

eou

sm

s6

mal

est

eril

e-6

Buf

umoc

hisp

on

tan

eou

s

GR

OU

PB

(ms7

-ms1

7)K

o&

Yam

agat

a19

87,1

989

ms7

mal

est

eril

e-7

K1:

Kos

hihi

kari

EI3

Sat

o&

Sh

injy

o19

91m

sBm

ale

ster

ile-

8K

2:K

oshi

hika

riEI

7m

s9

mal

est

eril

e-9a

RT

I-3a

T65

spo

nta

neo

us

6m

s9

mal

est

eril

e-9b

E-l

:Ets

unan

77EI

mslO

mal

est

eril

e-l0

T-1

:Toy

onis

hiki

EI9

msll

mal

est

eril

e-ll

T-2

:Toy

onis

hiki

EI

ms1

2m

ale

ster

ile-

12T

-3:T

oyon

ishi

kiEI

ms1

3m

ale

ster

ile-

13S

-32:

Sas

anis

hiki

EIm

s1

4m

ale

ster

ile-

14S

-40:

Sas

anis

hiki

y-ra

y5

ms1

5m

ale

ster

ile-

15S

-55

:Sas

anis

hiki

EIm

s1

6m

ale

ster

ile-

16S

-59:

Sas

anis

hiki

EI

ms1

7m

ale

ster

ile-

17S

-81

:Sas

anis

hiki

y-ra

y2

GR

OU

PC

(ms

18

-ms

45

)F

uji

mak

iet

al.

1977

;

ms1

8m

ale

ster

ile-

ISM

SZ7:

Nih

onm

asar

iE

IH

arai

wa

&T

anak

a19

80;

ms1

9m

ale

ster

ile-

19M

SZ

8:N

ihon

mas

ari

y-ra

yF

uji

mak

i&

Har

aiw

a19

86;

ms2

0m

ale

ster

ile-

20a

MS

15:N

ihon

mas

ari

EIT

amar

u19

94m

s2

0m

ale

ster

ile-

20b

MS

Z9:

Nih

onm

asar

iy-

ray

ms2

1-m

s2

3m

ale

ster

ile

21-2

3M

S:N

ihon

mas

ari

y-ra

ym

s24

mal

est

eril

e-24

aM

S4:

Nih

onm

asar

iy-

ray

ms2

4m

ale

ster

ile-

24b

MS

9:N

ihon

mas

ari

y-ra

ym

s2

5-m

s3

0m

ale

ster

ile

25-3

0M

S5-

10:N

ihon

mas

ari

y-ra

ym

s3

1-m

s4

5m

ale

ster

ile

31-4

5M

S11

-25

:Nih

onm

asar

iE1

GR

OU

PD

(ms4

6-m

s63)

Tre

es&

Rut

ger

1978

;

ms4

6m

ale

ster

ile-

46M

201

stre

pto

my

cin

Mes

eet

al.

1984

;H

u&

ms4

7,

ms4

8m

ale

ster

ile-

47,

48M

101

y-ra

yR

utge

r19

92m

s4

9m

ale

ster

ile-

49M

201

EM

Sm

s5

0m

ale

ster

ile-

50a

Cal

ady

spo

nta

neo

us

ms5

0m

ale

ster

ile-

50b

Ear

iros

esp

on

tan

eou

sm

s51

mal

est

eril

e-51

M20

1E

MS

ms5

2m

ale

ster

ile-

52M

1D1

y-ra

ym

s5

3m

ale

ster

ile-

53a

M1D

1y-

ray

ms5

3m

ale

ster

ile-

53b

M2D

1E

MS

ms5

4-m

s5

6m

ale

ster

ile

54-5

5M

2D1

EM

Sm

s5

7m

ale

ster

ile-

57M

1D1

y-ra

ym

s5

8m

ale

ster

ile-

58M

2D1

EM

Sm

s5

9m

ale

ster

ile-

59a

M10

1y-

ray

ms5

9m

ale

ster

ile-

59b

M20

1E

MS

ms6

0m

ale

ster

ile-

50a

M1D

1y-

ray

ms6

0m

ale

ster

ile-

6Db

M2D

1E

MS

ms6

0m

ale

ster

ile-

50C

alro

se76

anth

ercu

ltu

rem

s61

mal

est

eril

e-51

M20

1E1

ms6

2m

ale

ster

ile-

52M

1D1

y-ra

ym

s6

3m

ale

ster

ile-

53C

alor

osp

on

tan

eou

s

GR

OU

PE:

(msI

R36

-msm

77(t

))

msIR

36

ms-

1R36

/1R

36

E1S

ingh

&1k

ehas

hi19

81;

msIR

36

5495

ms/

Lin

e549

5sp

on

tan

eou

sS

uh

etal

.19

89,

1991

~ CDm

sIR

36

5683

ms/

Lin

e568

3sp

on

tan

eou

s

CJ1

Tab

le2

.7.

(Co

nti

nu

ed

)0

Gen

eN

ame

Sou

rce

Der

ivat

ion

Chr

omos

ome

Ref

eren

ce

msI

R3

6M

ily

ang

54

ms/

spo

nta

neo

us

Mil

yan

g54

msm

55

(t)

Mil

yan

g5

5m

s/sp

on

tan

eou

sM

ily

ang

55m

sm6

7(t

)M

ilya

ng6

7m

s/sp

on

tan

eou

sM

ily

ang

67m

sm7

7(t

)M

ily

ang

77

ms/

spo

nta

neo

us

Mil

yan

g77

GR

OU

PF

(ms1

(t)-

ms2

(t)

msl(

t)C

o40

xV

aiga

iF

1

ms2

(t)

Co4

0x

Vai

gai

F1

om

so

pen

hu

llm

ale

ster

ile

tmsl

5460

:IR

54

GR

OU

PG

(pm

s1,

pm

s2

)

pm

sl

Nan

g-K

en58

s

tms2

tms3

(t)

pm

s2

pm

s(t

)

Rei

mei

IR32

364

Nan

g-K

en58

s

M20

1

anth

ercu

ltu

rean

ther

cult

ure

spo

nta

neo

us

spo

nta

neo

us

y-ra

yy-

ray

spo

nta

neo

us

spo

nta

neo

us

EM

S

8 6 7 3

Kau

l19

86

Tak

eda

1987

Su

net

al.

1989

;Y

ang

etal

.19

92;

Wan

get

al.

19

95

a,1

99

6M

aruy

ama

etal

.19

91a

Bor

kaka

ti&

Vir

man

i19

93;

Su

bu

dh

iet

al.

1995

,1

99

6,1

99

7

Zh

u&

Yu

1989

;W

ang

etal

.19

91;

Sha

o&

Tan

g19

92,1

995;

Zha

nget

al.

1993

aO

ard

etal

.19

91;

Oar

d&

Hu

1995


evaluation (Fig. 2.3). This breeding procedure is flexible, and some routine steps may be omitted or repeated, depending on the performanceof parental lines or of the hybrids (Yuan 1985).

1. Development of Parental Lines

Source Nursery. This nursery is for plant source materials to be used inbreeding the three parental lines. All germplasm except A, B, and R linesshould be grown each with 10-20 plants and one seedling per hill or pot.The A and B lines should be planted in isolated plots. To attain bettersynchronization of flowering time, the A lines should be seeded at regular intervals for testcrossing.

Testcross Nursery. This nursery is to determine the fertility of the F1

hybrids and to screen for Rand B lines. Ten to 20 plants are usually

Source Nursery

Male sterilc plantsMatcmalmaterials x

tPaternal materials

MSline x

~Restorer line

Combining Ability EvaluationNursery

Farmer's Field Evaluation

Replicated Trial

,..-----=;..--~Regional Trial I

~ /Release to farmers

Fig. 2.3. The breeding procedures for hybrid rice. Source: Yuan 1985.


grown in one row for each testcross with the check cultivar betweenevery 10-20 F1 hybrids. If the F1 is found to be male sterile, it can beused for the development of an A line by successive backcrossing. Whenthe F1 shows both normal fertility and good performance overall fordesired traits, the male parent can be used to develop R lines throughthe re-testcrossing. Crosses for which the male parents have both poorrestoring ability and poor maintaining ability are generally discarded.

Re-testcross Nursery. This nursery is to confirm the restoring abilityand to evaluate heterosis. More than 100 plants are grown and comparedwith leading commercial hybrids or cultivars.

Backcross Nursery. The objectives in this nursery are to develop A andB lines. The male sterile plants should be grown with the recurrentmale plants in pairs. The goal is to achieve stable male sterility and uniform agronomic traits in a population of about 1,000 plants. When thisis achieved, the male sterile line and its corresponding male parent (Bline) are ready for commercial evaluation.

2. Heterosis Evaluation

Combining AbilityEvaluation Nursery. Already developed A (or R) linesare tested with existing R (or A) lines. Each combination is planted witha single seedling per hill, about 500 plants per plot with replicates andthe standard commercial cultivar or leading rice hybrid as the check.

Replicated Trials. The promising selected hybrids are evaluated in replicated trials. The best-performing for agronomic traits will be selected forthe regional trial. Three or four replications with a check cultivar orhybrid and 20 m 2 of plot size in each replication are needed. Usually oneor two years is used for the replicated trial.

Regional Trials. This trial is to evaluate the adaptability of rice hybridsto different regions and their yield potential. Evaluation in farmers'fields and study of seed production techniques may be conducted simultaneously.

B. Development of A and B Lines

1. Breeding Objectives. The development of A and B lines is supremelyimportant in the three-line system. In addition to yield and productquality of the F1 hybrid, these lines affect seed yield of the hybrid. Elite


A lines require: (1) stable performance of male sterility: with no riskof recovering the self fertility after generations of backcrossing or due todifferent ecological conditions; (2) easy restorability for male fertility:with ease of identifying or developing R lines for which the (A x R) F1

has normal seed set with minimal influence by environmental conditions; (3) high outcrossing potential: with little enclosure of thepanicle within the flag leaf sheath, daily blooming time essentially synchronized with the normal Band R lines, a long floret opening duration,a large floret opening angle, a high stigma exsertion proportion, shortand narrow flag leaf, and thus a high yield potential for F1 seed production and for A line multiplication; (4) good grain quality and resistance to diseases and insect pests; and (5) good combining ability.

The B line is a nuclear "twin" of the A line but with a different cytoplasm. Characters such as superior combining ability, outcrossingcharacteristics, and grain quality are also the breeding objectives fordeveloping B lines (Yuan and Chen 1988).

2. Observation of Male Sterility in Rice. Male sterility can be the resultof pollen sterility, indehiscent anthers, anther abortion, pistilloidy of theanthers, or other causes (Rutger and Shinjyo 1980). Constant observationof the expressed level of male sterility is obligatory for both hybrid ricebreeding and for hybrid seed production. Male sterility is observableonly after heading using the following three methods (Yuan 1985):

Visual Inspection. For sporophytic male sterile lines, an obvious panicle enclosure in the flag leaf sheath is a good indication of male sterility. During anthesis or shortly thereafter, color and plumpness of antherscan be visually observed to determine completeness of the male sterility. Brightly yellow and plump anthers indicate male fertility. The flowering panicles can be shaken to check for anther dehiscence. If pollengrains come from the anthers, the male sterility is incomplete.

Bagging Panicles. The most accurate estimation of the male sterility isthe seed set two-plus weeks after bagging the panicles with glassinepaper bags, after heading starts but prior to any anther dehiscence. Insome cases the male sterility will be overestimated, because increasedtemperature in the glassine bag under the hot sun may increase malesterility.

Microscopic Observations. Anthers should be sampled from differentparts of the rice panicle, smashed into fine pieces in I-KI solution usingtweezers, and observed for male sterility under a light microscope. The


fertile (well-stained) pollen should not exceed 1% of the pollen grainsfor sporophytic male sterile lines.

3. Sources of A Lines. There are two main ways of creating male sterilerice plants: (1) mutation (chemical or irradiation) or natural outcrossingthat maintains nuclear male sterility, and (2) distant hybridization. In thecase of natural outcrossing, it is generally difficult to find a maintainerline. The second method is distant hybridization. From more than 660CMS lines, there are about 64 cytoplasmic sources, among which 22 sterile cytoplasms were from wild rice, 38 from indica, one from O. glaberrima, and three from japonica (Sheng 1994). Upon identification of amale sterile plant, confirmation of its genetic basis is required. If, forexample, this male sterile plant is testcrossed with a fertile plant of thesame or a different cultivar, the Fls are fertile, and the Fzs have a segregation ratio of 3 fertile to 1 sterile, then the male sterility of this malesterile plant is controlled by a single recessive gene.

4. Breeding Approaches for A and B Lines

Nuclear Substitution. Nuclear substitution has proven effective in developing a stable male sterile line from an ancestral male sterile plant eventhough it is difficult to identify a maintainer in some cases. For mostwild-cultivated, indica-japonica, indica-indica, and japonica-japonicacrosses in China, the source male parental lines are backcrossed recurrently for several generations until segregation of male sterility andother agronomic traits is no longer observed. Male sterility is difficultto stabilize for some of the original male parental lines. When thisoccurs, a different rice cultivar should be selected as the recurrentparental line.

Interspecific Crossing. The origin of O. glaberrima seems to be moreprimitive than that of O. sativa. A stable male sterile line may be developed by crossing and backcrossing with O. glaberrima as the female andO. sativa as the male. On the contrary, for O. sativa cytoplasm combinedwith an O. glaberrima nucleus, the substitution line does not set seedeven though the pollen grains stain normally with I-KI solution (Sano1985). Erickson (1969) obtained 100% male sterility in the FI , BC I andBCz by means of nuclear substitution between three rice cultivars fromO. glaberrima and three japonica cultivars. Similarly, 0.25% selfingoccurred in BC l of the backcross between'Sakotira', a rice cultivar fromWest Africa, and an indica cultivar AC 5636 (Swaminathan et al. 1972).It seems difficult to develop a restorer for most A lines developed frominterspecific crosses.


Crossing Between Wild and Cultivated Rice. It is easiest to developmale sterile lines from wild-cultivated crosses. China's first and mostpopular WA-type A lines, 'Er-Jiu-Nan 1 A' (Fig. 2.4), 'V20A', and 'ZhenShan 97A', were all developed from backcrosses with a wild abortiveplant at Ya-Xian on Hainan Island. This made possible the breakthroughin the commercialization of hybrid rice technology in China. In addition,Hong-Lian (HL) type A lines were developed from backcrosses withHainan red-awn wild rice as the female parent. 'IR66707A', which hadthe cytoplasm of O. perennis 'ACC 104823' and the nuclear backgroundof 'IR64', was found to be stable with almost complete (93-100%) malesterility in crosses with 10 restorers having the WA cytoplasm (Dalmacio et al. 1995). Primitive types such as wild rice are normally employedas the female to develop male sterile lines (Yang and Lu 1989).

Crossing Between indica and japonica Rice. Many A lines were developed from indica-japonica crosses. Based on their cytoplasmic sources,

male female MonthlYear

wild abortive X 6044 03/1971plant /

F X Er-Jiu-Nan t 12/1971

F/X .®Er-Jiu-Nan I 06/1972

/ ~®DCIFI X Er-Jiu-Nan I 10/1972

/ ~®DC2Ft X Er-Jiu-Nan I 02/1973

BC3/X *®Er-Jiu-Nan 1 06/1973

BC4/X *®Er-Jiu-Nan I 09/1973

Er-Jiu-Nan 1A Er-Jiu-Nan 1B

Fig. 2.4. The breeding procedure of Er-Jiu-Nan lA. Source: Lin and Yuan 1980.


these A lines can be classified into five types: (1) Bora type. Includedare 'Chinsurah Bora II' x 'Taichung 65' (Shinjyo and Omura 1966), 'LeadRice' x 'Taichung 65' (Watanabe et al. 1968) and 'Chun 190' x 'HongMao-Ying' (Li 1980); (2) Yunnan upland indica type. These include 'ErShan-Da-Bai-Gu' x 'Hong-Mao-Ying', 'Er-Shan-Da-Bai-Gu' x 'Ke-Qing 3'(Li 1980b); (3) Southeast Asian indica type. These include 'IR24' x 'XiuLing', 'Tetep' x 'Norin 8' (Yuan and Chen 1988); (4) Chinese indicalandrace type. This type includes 'Tian-Ji-du' x 'Fujisawa 5', 'Lian-TangZao' x 'Li-Ming'; and (5) Late indica or indica waxy rice type. It is represented by 'Jing-Quan-Nuo' x 'Nan-Tai-Jing'.

Most japonica A lines were developed from indica-japonica crosses.The basic principles are summarized as follows:

It seems easier to develop A lines from indica-japonica crosses withindica rice as female and japonica type as male in the backcross breeding. It is better to select the more primitive type of indica rice as the cytoplasmic source and the more advanced japonica cultivar as the nuclearbackground.

It is more effective in A line breeding to test the performance of malesterility in the reciprocal crosses. If there is much difference in male sterility of the reciprocal crosses, the probability of successful development ofan A line will be increased because the male sterility in some indicajaponica crosses may originate from the lack of coordination between thenucleus of the two parental lines, and the fertility will be restored alongwith the nuclear substitution by several generations of backcrossing.

The pollen abortion type of the male sterile plants of indica-japonicacrosses belongs to the trinucleate abortion type. This differs from sterility of the WA-type A lines. This type may have some portion of thepollen stained with I-KI solution, so a more dependable procedure is todetermine the selfing seed set by bagging panicles.

Crosses Between Geographically Distantly Related Rice Cultivars orCultivars of Different Ecotypes. In Yunnan province of China differentjaponica ecotypes are grown at different elevations. The Yunnan Agricultural University developed the Dian 4-type and Dian 6-type A linesfrom the cross between two japonica cultivars: 'Zhao-Tong-Bei-Zi-Gu'from high elevation and 'Ke-Qing 3' from low elevation.

Scientists at the Hunan Academy of Agricultural Sciences have determined the frequency of male sterility for different types of crosses. Malesterility is 100% in wild-japonica crosses, 85% in wild-indica crosses,and 4.0% in indica-japonica crosses. The order of frequency of producing male sterile plant(s) is: wild-cultivated> indica-japonica > geographically distant hybridization> mutation induced by physical orchemical factors> intercultivar crosses. Breeding efficiency can beimproved for the development of indica A line through wild-cultivated


crosses and japonica A line through indica-japonica crosses. Experiencein China indicates that a closer relation between the cytoplasmic andnuclear donor parents produces gametophytic male sterility, making itharder to obtain a stable CMS line. The more distantly related the cytoplasm and substituted nucleus, the more likely the male sterile lines andtheir maintainer lines will be obtained (Zhang 1985).

Male Sterility by Mutation. A few natural mutants for male sterility inrice were identified by intensive screening, as exemplified by the malesterile rice material "424" (Yuan and Chen 1988). Male sterile mutantswere induced by irradiation (X ray, yray, neutron, and laser) or chemical mutagens such as EMS, DES, and NED. The Mei-Xian AgriculturalResearch Institute of Guangdong Province in China identified 30 malesterile plants from 5908 F3 segregants from 'Zhen-Zhu-Ai 11', 'GuangNong 1', and 'Guang-Xuan 3', each of which had been exposed to25,000-30,000 R CooD y ray. Natural and induced mutants for male sterility are generally thought to be nuclear mutations, making it difficult todevelop a maintainer or restorer.

Backcross Breeding of Male Sterility. Transfer of male sterility into newlines is needed to increase the diversity of the A and B lines. Therestorer-maintainer relationship and pollen abortion of A lines by backcross breeding are basically the same as the source A lines. Stable A linescan be developed by backcrossing for four or more generations. Thefirst step is to select a new rice cultivar with the desired trait(s) and thentestcross it with the source A line and observe the F1 to estimate themaintenance of male sterility. The second step is to backcross and selectsegregants with complete male sterility, good blooming characteristics,high stigma exsertion rate, and most of the agronomic traits of the maleparent. Crossing and backcrossing with the male parent are continued,with selection for male sterility at each step.

Transfer Breeding ofMaintainer Lines. Most rice cultivars with superiortarget trait(s) have minimal, if any, sterility-maintaining ability. Thus itis necessary to develop new maintainer lines through transfer breeding.For transfer breeding of maintainer lines (Fig. 2.5), the source maintainerline used as the female parent should be closely related to the target cultivar of the intended new maintainer line. If it possesses the same agronomic traits and similar pedigree, there will be less segregation, so fewergenerations will be needed to stabilize maintaining ability. Agronomictraits similar to the target cultivar (i.e. the male parent) should beselected during the backcross with the plant containing the male sterilecytoplasm.


New cultivar or line

(SFFor NFF)x

+F1

(NFt)

B line(Nft)

X New cultivar or line+ (SFF or NFF)BC1F1

(NFF, NFt)

r-- .....,1,-<8> --:J.

f f ,Sft X NFF Sft X NFt Sft X Nft

i I +SFt t t Sft

(fertile) Sft SFt (sterile)(sterile) (fertile)

(segregation)

Backcrossing

Crossing

Testcrossing

Selfing

tSFt

(fertile)

(segregation)

X

•NFt

tSft

(sterile)

Nft

SFt

(fertile)

tJNffl(sterile)

The maintainer line fromthe new cultivar or line

S =sterile cytoplasm; N =fertile cytoplasm; f =sterile nuclear gene; F =fertile nuclear gene

Backcrossing

Backcrossing

Selfing

SFF(NFF)

~X SFF(NFF)

•NFF, NFt

...------*<8>t ,Testcrossing Sft X NFF Sft X NFt

I

Fig. 2.5. The procedure of transfer breeding for a maintainer line in rice. Source: Li etal. 1982.

Other Methods. Protoplast fusion was used to transfer cytoplasmic malesterility from two CMS lines (MTC-5A and MTC-9A) into a fertile japoniea cultivar, 'Sasanishiki'. The CMS, which can be restored by 'MTClOR' with a single dominant gene Rf-l, was successfully expressed in thecybrid plants. The gene was stably transmitted to their progenies throughat least eight generations (Akagi et al. 1995).

China has developed many A and B lines, but only a few, such as 'ZhenShan 97A' and 'V20A', have been in large-scale commercial productionfor years (Table 2.8). These much-used A lines have stable sterility, easyrestoration, and good combining ability, but poor grain quality or poorstress resistance (Zhou 1994). Outside China only a few eMS lines suchas 'IR58025A', 'IR62829A', 'IR64608A', 'PMSIA', 'PMSBA', and 'PMSIOA',have demonstrated commercial potential on a large scale (Virmani 1994b).

Tab

le2.

8.T

hech

arac

teri

stic

so

fse

vera

lm

ajor

ind

ica

eMS

line

san

dth

eir

area

of

pro

du

ctio

n.

Sou

rce:

Zh

ou

1994

.

Acc

um

ula

ted

%to

tal

Ty

pe

of

Rep

rese

nta-

area

(mil

.ha

lar

eau

nd

ercy

topl

asm

eMS

line

Maj

orch

arac

teri

stic

sti

vehy

brid

(s)

(19

88

-92

)h

yb

rid

rice

WA

-typ

eE

r-Ji

u-N

anlA

Goo

dco

mbi

ning

abil

ity.

stab

leN

an-Y

ou2

0.0

0.0

ster

ilit

y,p

oo

rgr

ain

qual

ity

Nan

-You

6(L

arge

area

and

resi

stan

cein

1970

s)

Zh

en-S

han

97A

Goo

dco

mbi

ning

abil

ity.

stab

leS

han-

You

6343

.665

.9st

eril

ity.

poor

grai

nq

ual

ity

and

mod

erat

ere

sist

ance

V20

AG

ood

com

bini

ngab

ilit

y.st

able

V64

12

.018

.1st

eril

ity.

poor

grai

nqu

alit

yV

6an

dm

oder

ate

resi

stan

ce

TeA

Goo

dou

tcro

ssin

gra

tean

dco

mbi

n-T

e-Y

ou63

0.2

0.3

ing

abil

ity,

inco

mpl

ete

ster

ilit

y

Bo-

Bai

AV

ery

high

outc

ross

ing

rate

,go

odgr

ain

Eo-

You

642.

203.

3(o

rE

oA

)qu

alit

y,b

ut

inco

mp

lete

ster

ilit

y

DA

-typ

eX

ie-Q

ing-

Zao

AG

ood

grai

nqu

alit

y,b

ut

poor

rest

orin

gX

ie-Y

ou63

1.6

2.4

abil

ity,

inco

mpl

ete

ster

ilit

y.X

ie-Y

ou64

Fjs

sens

tive

tolo

wte

mp

erat

ure

D-t

ype

DS

han

AG

ood

com

bini

ngab

ilit

yan

dre

stor

a-D

-Sha

n-Y

ou63

5.1

7.6

bili

ty,

bu

tpo

orgr

ain

qual

ity

and

inco

mpl

ete

ster

ilit

y

ID-t

ype

II-3

2A

Goo

dco

mbi

ning

abil

ity.

high

out-

II-Y

ou63

0.2

0.4

cros

sing

rate

,in

com

ple

test

eril

ity

II-Y

ou64

Yo

u-l

AG

ood

com

bini

ngab

ilit

y.hi

ghou

t-Y

ouI

630.

10.

1(o

rU

-lA

)cr

ossi

ngra

te.

inco

mpl

ete

ster

ilit

y.Y

ouI

64::.n

Fjs

sens

itiv

eto

low

tem

per

atu

rec.o


5. Main Features ofWA-type A Lines. The WA-type A lines are currentlyemployed over the largest area of three-line hybrid rice production inChina. This type is the most stable and hence has been introduced to several other countries, such as Vietnam. All WA-type A lines have beendeveloped from the wild abortive (WA) source rice plant. Discovered in1970, it has very strong tillering, slender culm, narrow leaf and sheath,long purple awns, black seeds, and a long seed dormancy duration. Itsanthers are slender and slightly yellow. They usually do not dehisce, butsome dehiscence and seed set are observed when the average temperature is over 30°C for several continuous days. This source WA rice plantwas a heterozygote, i.e., there was segregation in the F1 when it wascrossed with indica or japonica cultivars. It is quite probable that thiswild abortive rice originated from natural crossing between Hainan redawned wild rice (as the female) and the local late cultivated rice cultivar (as the male).

The Hunan Rice Heterosis Utilization Research Cooperative Group inChina reported that of 731 rice cultivars, 624 showed good maintainingability and 18 had partial maintaining ability to the wild abortive rice.All 345 japonica cultivars had maintaining ability (Li et al. 1982). Hundreds ofWA-type A lines (such as 'V20A', 'Zhen-Shan 97A', and 'V41A')were developed by crossing to rice cultivars with good maintaining ability known from testcross nursery, and backcrossing to selected progenythat expressed complete male sterility, normal blooming traits, and agronomic traits similar to the male parent. In general, four to five backcrossgenerations are needed to develop a new WA-type indica A line. Malesterility in the crosses of (wild abortive rice x japonica) can be stabilizedmuch faster than for the crosses of (wild abortive rice x indica). The percentage of male sterile plants in the backcrossing F1 can reach 100% forthe crosses of (wild abortive rice x japonica).

Male Sterility. The arrow-shaped and slender anthers of the WA-type Alines have a milky white color. Most pollen grains abort at the uninucleate stage and some at meiosis and the binucleate stage, so the pollengrains have irregular shape and cannot be stained with I-KI. The malesterility of the WA-type A lines is stable with little influence from temperature and other environmental conditions.

Heterosis Utilization. WA-type A lines are mostly developed from semidwarf Chinese rice cultivars, so their complementation with other ecotypes and geographically distant genotypes results in strong heterosis forgrowth vigor, yield potential, grain quality, and resistance to adverseconditions. However, the WA-type A lines need further improvement for


resistance to diseases and insect pests, as most WA-type A lines haveweak or no resistance to diseases such as rice blast and leaf sheathblight. For hybrid rice, the effect on disease resistance from the femaleparent is normally larger than from the male parent, indicating somecytoplasmic effect. Therefore, the introduction of various resistancegenes to the WA-type A and B lines seems important. In addition, thereare striking differences in the restorability of different WA-type A lines.The male sterility of 'Zhen-Shan 97A' is most easily restored. There isa tendency of the WA-type A lines with easier restoration of male sterility to have wider adaptability to variations in environmental conditionssuch as high or low temperatures.

C. Development of R Lines

1. Breeding Objectives

Strong Restoring Ability. Good R lines should have the ability to restorenormal pollen and seed set in the Fl' The F1 hybrid should have goodadaptation to low or high temperature, good pollen shedding, and highseed set (>80%) even under adverse environmental conditions.

Good Combining Ability and Other Traits. In addition to good generalcombining ability and specific combining ability, a reliable restorer lineshould have resistance to diseases, adaptability, and high grain quality.

Good Outcrossing Characteristics. For superior yield of hybrid rice seedproduction, a restorer line should have a large quantity of pollen, longblooming duration, good pollen shedding, strong tillering capacity,slightly longer growth duration, and be taller than A lines.

2. Source and Distribution of Restorer Gene(s). The Ping-Xiang Agricultural Research Institute of Jiangxi Province in China found all 16 wildrice species (0. rufipogon) collected from the Hainan Island completelyrestored the male fertility of the wild abortive rice. Similarly, 'Boro II'restored the male fertility of the BT-type A lines. This indicates that thenuclear genome of the rice germplasm that provides the male sterilecytoplasm is an important source of the restorer gene(s).

Restorer gene(s) can be screened by testcross. Cultivars closely relatedto the germplasm that provides the male sterile cytoplasm may haverestorer gene(s). Most restorer lines of the WA-type A lines, for example, are cultivars from IRRI-bred materials or with origin close to wildrice, or late-season indica cultivars found at low latitudes. Mutation


induction by chemicals or by irradiation may create new restorer linewith lower plant height, earlier maturity, or stronger restoring ability.

The Zhejiang Academy of Agricultural Sciences in China screened1,500 rice cultivars for restorer lines during 1978-1982. They found55.5% of rice cultivars from South Asian countries at low latitudes hadrestoring ability for the WA-type A lines, and 20.5% were indica cultivars from Southern China and Korea. All early-season indica rice cultivars from China's Yangtze Valley and all japonica cUltivars fromNorthern China, Japan, and Korea lacked restoring ability for the WAtype A lines.

The current popular restorer lines for the WA-type A lines, such as'IR24' and 'IR26', have 'Peta', which has strong restoring ability, in theirpedigree. New R lines can be developed through transfer breeding, examples being 'Milyang 46' from the cross [(Tongil x IR24) x (IR1317 xIR24)] and 'IRlll0-78' from the cross (Peta x Taichung Native 1).

The evolution sequence was considered to be from wild rice to lateindica, to early indica, to late japonica, and to early japonica. It appearsthat the closer rice cultivars are to wild rice, the more likely they are tohave the restorer gene(s) for the male sterile WA-type A lines.

3. Breeding Methods. China has released hundreds of R lines since the1970s, but only about nine are currently used in large-scale commercialproduction as shown in Table 2.9 (Xie et al. 1996). R line breeding isessential to further enhance yield levels of hybrid rice.

Table 2.9. China's main commercialized indica restorer lines in 1994. Source: Xie etal. 1996.

Number % of totalof hybrid Area area under

R line combinations (1,000 hal hybrid rice

Ming-Hui 63 8 6,130 47.7

Ce64 4 1,757 13.7

Milyang 46 4 904 7.0

Gui 99 2 490 3.8

Ming-Hui 77 3 477 3.7

Duo-Xi 1 1 311 2.4

Ce 49 4 216 1.7

CDR22 1 215 1.7

903 1 205 1.6

Total 28 10,705 82.0


Testcross and Selection for R Lines. In preliminary testcrossing nurseries, at least 10 F1 plants are required for the initial evaluation of seedset, plant type, other agronomic traits, and resistance to adverse conditions. In the re-testcross nurseries, at least 50-100 plants should begrown of each re-testcross F1 to evaluate the yield and other agronomictraits. Only a few rice hybrids are selected from the re-testcrossing nursery. These hybrids should be grown on production nurseries to evaluate the productivity of the hybrid and consumer acceptance.

Cross Breeding for R Lines. Testcrossing is insufficient in R line breeding for improving resistance, early maturity, and acceptable grain quality. Cross breeding is required. In the single cross breeding method, therestorer gene(s) are transferred to a new rice cultivar through singlecrossing. The segregating progeny that have restoring ability are selectedfor the improved traits using the pedigree method. The single crossingcan consist of the following combinations:

RxR: The frequency of restoring gene(s) will be high, so only the agronomic traits needing improvement require selection. The restoring ability will need to be tested only in the later generations. The new R lineswill have the complementary restoring gene(s) and improved traits.

BxR or RxB: Because the targeted traits segregate with and without therestoring ability, the selection of improved traits in combination withrestoring ability is a tedious effort. The smallest number of plants neededfor testcrossing can be determined using the following formula, m ~ loga/log P where m =the smallest number of plants for testcrossing, P =theprobability for restorer gene(s), a = the probability for losing the restorergene(s). This formula was proposed by Wang (1983b; Table 2.10).

AxR: The iso-cytoplasmic R lines developed with this method can easily coordinate nuclear and cytoplasmic contents. The genetic diversitywill be decreased by selecting new R lines from the cross between the

Table 2.10. The smallest number of plants for testcrossing in generations withprobability of 95% and 99% for including the restoring gene(s). Source: Wang 1983b.

Number of plants for testcrossing

Number of F2 F:1 F F" F(j4

restoringgene(s) 99% 95% 99% 95% 99% 95% 99% 95% 99% 95%

1 16 11 10 7 8 6 8 5 6 5

2 71 47 29 20 22 15 19 13 17 12

3 286 191 85 56 53 35 43 28 38 25


A and R line, so the heterosis of rice hybrids using the new R lines maydecrease. This lowered heterosis can be overcome by increasing thegenetic diversity between the R and A lines.

Multiple cross breeding methods can combine advantages, such asgood resistance, good grain quality, and early maturity, from each parent into a new restorer line. The early maturing R line 'Z6-Zai-Zao' wasdeveloped from the cross [(IRZ6 x Zai-Ye-Qing 8) x Zao-Hui 1]. Thebackcross method for developing new R lines with the Rf-l restorergene was also demonstrated by Fujii et al. (1991).

4. Identification of Fertility Restorer Genes. Fertility restoration for theWA-type A lines was reported to be controlled by a single dominant gene(Shinjyo 1969; Wang 1980), but later researchers found that two R genescontrolled restoration (Gao 1981; Zhou et al. 1983; Yang and Lu 1984;Sohu and Phu11995; Shen et al. 1996a; Ganesan and Rangaswamy 1997;Kumari et al. 1998). There are two pairs in 'IRZ4'; R1 derives from 'Cina',a Chinese late indica, and R2 derives from 'SL017' (Li and Yuan 1985).For five restorer lines ('IRZ4', 'IRZ9Z73', 'IR5474Z', 'IR9761' and'ACR11353') there are four restoring gene loci (Ramalingam et al. 1995).There are reports of the restoration of male sterility governed by threeor four genes (Huang et al. 1987) or multiple genes (Pei 1980; Fu andWang 1988; Yang and Chen 1990). A recessive restoration gene "r" wasalso identified (Wang 1983a; Lei 1983).

In contrast, the inheritance of fertility restoration in BT, Dian 1, andHong-Lian types is gametophytic. The genotypes of the BT system areS(rr) for BT-C (MS line), N(rr) for maintainer, S(RR) for restorer BT-A,and N(RR) for restorer BT-x (Shinjyo 1984). The fertility of Dian 1 type'Hua-Jing 14A', 'Tu-Dao 4A', and BT type 'Nong-Jin ZA' is controlled bya recessive gene (rr) and the male sterility genes in Dian 1 and BT typesare allelic. The fertility of Hong-Lian type 'Hua-Ai 15A' was alsoreported to be controlled by major gene(s) and possibly also some mod- .Hying factors (Hu and Li 1985).

Rf-l for BT-type eMS lines (cms-ba) is located on chromosome 10 andlinked with pgl and fl (pgl-Rf-l-fl) (Shinjyo 1975). Yoshimura andcoworkers located the Rf gene for cms-ba on chromosome 7 using thetranslocation method (Virmani 1996). The Rf-2 gene for cms-ld cytoplasm was recently located on chromosome 2, using primary trisomic andlinkage tester lines (Shinjyo and Sato 1994). Virmani and Shinjyo (1988)reported that at the Rf-llocus there are at least fOUf multi-alleles: Rf-l a(Rf1), Rf-1 b , Rj-l c and Rf-1 d. Rf-l, Rf-2, and Rj-ak derive from 'ChinsurahBoro II', 'Fujisaka 5', and '0. glaberrima W0440' (Shinjyo 1969; Shinjyo


and Watanabe 1977; Yabuno 1977). Two independent dominant genescontrolling fertility restoration were reported, with the stronger gene RfWA -1 and the weaker gene Rf-WA -2 being located on chromosome 7 andchromosome 10, respectively. 'IR26', 'IR36', 'IR53', and 'IR9761-19-1' allpossessed the same restorer genes, whereas 'IR42' and 'IR2797-105-2-23' each had different restorer genes (Raj and Virmani 1988; Bharaj et al.1991,1995). Rf-1 and Rf-2 are generally considered as independent genes,but Li and Zhu (1988) reported that the two restoration genes of 'IR24'and 'IR26' were linked with the recombination being 38.26% (Fz) or37.56% (BC l ). For Rf-3, six RAPD markers were found to be associatedwith this gene. Three markers, i.e. OPK05-800, OPUI0-I100 and OPW01350, were mapped on chromosome 1. Using RFLP technology, three markers (RG532, RG140, and RG458) were also found to be closely linked withRf-3. At the RG532 locus, different alleles were found to restore the malesterility between the two eMS lines, 'Zhen-Shan 97A' and 'IR58025A'(Zhang et al. 1997a). Interval mapping technique showed eight QTLs tobe associated with fertility restoration; the two major genes Rfi-3 and Rfi4 were located on chromosome 3 and 4, respectively, and were responsible for 49.6% and 35.4% of the phenotypic variation (Li et al. 1996b).A partial restoration gene Ifr(t) was also identified in 'T65T' (Sano et al.1992; Teng and Shen 1994a). Restorer genes have been reported from timeto time to be modified by other genes (Govinda and Siddiq 1984). Forexample, an inhibitor gene identified in 'IR17492A' modified the activity of a restoration gene (Govinda and Virmani 1988). The identifiedrestorer genes are described in Table 2.11.

D. Development of Elite Hybrid Combinations

The key for breeding of elite rice hybrids is the development and selection of the parental lines. The following principles for the selection ofparental lines have proven to be helpful:

1. High Genetic Diversity. Within limits, higher genetic diversity willresult in increased heterosis. The estimated genetic diversity can bebased on the pedigree relationships, geographical sources, and ecotypes(Xu and Wang 1980; Singh et al. 1984; Subramanian and Rathinam1984; Yuan and Chen 1988).

2. Complementary Traits. The current Chinese rice hybrids have complementation for agronomic traits, resistance to adverse conditions, andgrain quality derived from each parent.

O'l

O'l

Tab

le2

.11

.F

erti

lity

rest

orin

gge

nes

inri

ce.

Sou

rce:

Kin

oshi

ta19

97.

Gen

eC

yto

pla

smO

rigi

nC

hro

mo

som

eR

efer

ence

Rfl

cms-

bo

Ch

insu

rah

bo

raII

10S

hinj

yo19

75,

1990

;S

ato

eta1

.(R

fl-a

,b,c

,d)

1985

;V

irm

ani

&S

hin

jyo

1988

;F

uk

uta

eta1

.19

92;

Yu

eta1

.19

95;

Aka

giet

a1.

1996

;Y

okoz

eki

eta1

.19

96;

Ich

ikaw

aet

a1.

1997

Rf2

cms-

IdF

uk

uy

ama

2W

atan

abe

1971

;S

hin

jyo

&

Sat

o19

94R

f3(R

2,R

f2)

cms-

WA

IR24

etc.

1fo

rR

f3an

dR

f4:

Lu

&Z

han

g19

86;

Bha

raj

eta1

.1

99

1,1

99

5;

Rf4

cms-

WA

IR24

etc.

7T

eng

&S

hen

19

94

b;

Zh

ang

(Rl,

Rfl

,et

a1.

1996

RfW

A1

)R

fWA

2cm

s-W

AIR

361

0B

hara

jet

a1.

1995

Rf2

,3,4

,5cm

s-W

AD

Hfr

om

(QT

L)

ZY

Q/J

X2,

3,4,

5Z

hu

eta1

.19

96

Rf5

(t}

cms-

WA

Fer

t.R

ever

tant

from

Rf(

t)II

32A

(WA

-typ

e)1

Sh

enet

a1.

1993

a,1

99

6a,

bR

fa,b

,ccm

s-b

oH

406

Mae

kaw

a19

82R

fa',

b',c

',d '

cms-

bo

Hl0

3M

aeka

wa

1982

Rfa

k(R

fjp)

ems-

ok

Ake

bono

Yab

uno

1977

Ifr

cms-

bo

part

iall

yst

eril

em

uta

nt

San

a&

Eig

uchi

1991

;fr

omT

aich

un

g65

CM

SS

ana

eta1

.19

92


3. High Yielding Ability. Both parental lines should have elite agronomictraits, since in most cases the performance of F1 hybrids was correlatedwith the mean value of the two parental lines for many traits, such asthe number of spikelets per panicle, 1,000-grain weight, growth duration, plant height, and the number of productive panicles.

4. Good Combining Ability. A rice cultivar with excellent performanceitself may not be a good parental line in hybrid rice breeding. The important factor is the combining ability, including general combining ability(GCA) and specific combining ability (SCA). China has developed hundreds of A lines, but only a few, such as 'V20A', 'Zhen-Shan 97 A', and'Xie-Qing-Zao A', are used in large-scale commercial production, owingto their superior combining ability, as shown in Tables 2.12 and 2.13(Xie et al. 1996).

E. Breeding for Rice Hybrids with Resistance to Insect Pestsand Diseases

1. Resistance to Diseases and Insect Pests. The Hunan Agricultural College evaluated the resistance to rice blast in 224 rice hybrids. Theyfound four types of inheritance for rice blast resistance in the Fls: 102hybrids showed dominant resistance; 31 had recessive resistance; 15 hadintermediate resistance between the A line and the R line; and the resthad a different resistance type in the F1 than in the parents. The resistance to bacterial blight performed similarly to the rice blast resistancein the Fl'

Table 2.12. China's main commercialized indica male sterile lines in 1994. Source:Xie et al. 1996.

Number % of totalof hybrid Area area under

A line combinations (1,000 hal hybrid rice

Zhen-Shan 97A 12 7,221 56.2V20A 8 1,307 10.2Xie-Qing-Zao A 7 838 6.5BoA 5 724 5.6Gang-type A 3 722 5.6D-type A 2 399 3.1II-32 A 2 331 2.6Long-Te-Pu A 2 298 2.3others 5 256 2.0Total 46 12,096 94.1


Table 2.13. China's main commercialized indica hybridrice combinations in 1994. Source: Xie et aL 1996.

Area % of totalRice hybrid (1,000 ha) area

Shan-You 63 4,455 34.7

Shan-You 64 772 6.1

V64 515 4.0

Gang-You 12 471 3.7

Shan-You Gui 99 451 3.5

D-You 63 360 2.8

Bo-You 64 323 2.5

Shan-You 46 323 2.5

Shan-You Duo-Xi 1 311 2.4

II-You 63 297 2.3

V46 291 2.3

Te-You 63 258 2.0

V77 258 2.0

Xie-You 46 255 2.0

Gang-You 22 215 1.7

Xie-You 63 207 1.6

Bo-You 903 205 1.6

Total 9,967 77.7

It was proposed that the R line was important to breed resistance tobrown planthopper. For example, the rice hybrid ('V20A' x 'IR26') andits R line, 'IR26', were resistant to brown planthopper, whereas 'Nan-You2' (,Er-Jiu-Nan lA' x 'IR24') and 'IR24' had no resistance to brown planthopper (both 'Er-Jiu-Nan lA' and 'V20A' had no resistance to brownplanthopper). But, it should be noted that there is a cumulative effect forresistance to brown planthopper and, hence, introduction of resistancegene(s) into male sterile lines can improve the resistance in hybrid riceto brown planthopper and other insect pests or diseases (Yuan and Chen1988).

2. Breeding Techniques

Selection of Source Materials. Most IRRI-bred indica cultivars havestrong resistance to diseases and insect pests, such as rice blast, bacterial blight, and planthopper. Some cultivars introduced from the Inter-

2. HYBRID RICE: GENETICS, BREEDING. AND SEED PRODUCTION 69

national Rice Research Institute, Sri Lanka, and India and some Chineselandraces also have good resistance to brown planthopper.

Testcross Method. If the F1 between an A line and germplasm with goodresistance also shows good maintaining ability of male sterility, successive backcrossing to this germplasm can be used to develop a highlyresistant A and B line. 'Zhen-Shan 97A' and its B line are examplesdeveloped using this method. If the F1 shows normal fertility, the testedgermplasm is also a potential R line. For example, 'IR9761-19-1', withgood resistance to rice blast and bacterial blight, was testcrossed withthe WA-type A lines and found to have very good restoring ability andcombining ability but was segregating. Some segregants were selected forpaired testcrossing in two generations. Thus, the restorer 'Ce64-7' wasbred with strong resistance to rice blast, bacterial blight, brown planthopper, and leaf hopper.

Cross Breeding Method. Cross breeding can combine strong resistancewith other important traits from different rice lines into a new R line.

Single Cross Breeding. 'Ming-Hui 63', a restorer line highly resistantto rice blast, was developed from a single cross between 'Gui 630', alarger-grain type restorer, and 'IR30' which has multiple resistance.

Multiple Cross Breeding. A multi-resistant and early-maturing R line,'26-Zhai-Zao', was developed from the multiple crossing between 'IR26'(with multiple resistance), 'Zhai-Ye-Qing 8' (highly resistant to riceblast), and 'Zao-Hui l' (good resistance and large panicle size).

Recurrent Backcross Breeding. When R or B lines have good combining ability but little resistance to diseases and insect pests, these R or Blines can be used as recurrent parents in backcrosses with a donor toobtain new B or R lines with the advantages of both parents (Yuan andChen 1988).

Breeding for resistance to insect pests and diseases is more extensivelycovered by Virmani (1994a).

F. Breeding for Rice Hybrids with High Grain Quality

Since the 1980s, China has been attempting to develop rice hybridswith superior grain quality. For example, aromatic rice hybrids 'XiangYou 63', 'Xing-Xiang-You 77', and 'Xing-Xiang-You 80' were developedwith good quality and high yield potential (Zhou and Liao 1995, 1997;Chen et al. 1997a). A survey of 500 households in China showed that forspecial occasions, such as entertaining guests and celebrating festivals,25.4% preferred inbreds, 39.8% preferred hybrids, and 34.8% reported


having no preference. This result indicated that the cooking and eatingquality of hybrid rice is acceptable to Chinese consumers (Virmani1994a). Of 47 rice hybrids tested, most of the indica hybrid rice cultivars had high amylose content, hard gel consistency, intermediate gelatinization temperature and kernel elongation. Most japonica rice hybridshad low amylose content, soft gel consistency, and low gelatinizationtemperature (Tang 1987). The protein content of most of 30 rice hybridswas between the two parents, near to the mid-parent value and showedpositive incomplete dominance. But, 20% of 30 hybrids were superiorto their better parent (Liu, Sun, and Cai 1990). A hybrid 'L301A x R29'was developed in 1985 with first-grade grain quality (long grain, alkalidigestion value of 2.0, 23% amylose content, 70% of milled rice yield,57% of head rice yield, and 0-1 chalkiness) (Yuan and Virmani 1988).Other CMS lines such as 'Xing-Xiang A', 'Jin-23 A', and 'Di-Gu A' havebeen developed with excellent grain quality and good combining abilityfor grain quality traits (Zhou 1994). Even though the quality of hybrid riceis generally better than for early-season conventional cultivars and nearto that of late-season or single-cropping conventional cultivars, most ofthe Chinese rice hybrids in the 1970s and early 1980s had poor grain quality. Improvement of grain quality is essential for further commercialization of hybrid rice in most developed countries. In Iran, development ofhybrid rice technology has been hindered by segregation of the gel consistency and gelatinization temperature of hybrid rice (Dorosti 1997).

Major grain quality traits in hybrid rice are (1) milling and head ricerecovery; (2) size, shape, and appearance; and (3) cooking and eating characteristics (Khush et al. 1988). The rice grains of the F1 rice hybrid areactually F2 seeds. Therefore, both parental lines should have similargood-quality traits for the hybrid to have good quality. In breeding practice, the following principles should be observed (Yuan and Chen 1988):

1. Selecting A and R lines with Reduced Chalkiness and Elite Appearance. The milling-quality traits are controlled by both seed genotype andmaternal genotype. Hybrids with higher head rice recovery can beobtained if the parents are selected carefully. If either parent has a highertendency for grain breakage, the F1 hybrids will normally give lower headrice recovery than the better parent. White centers and white bellies arecontrolled by a single recessive or dominant gene (United States Dept. Agr.1963; Chalam and Venkateswarlu 1965; Nagai 1958) or by polygenes(Nakatat and Jackson 1973; Somoto and Hamamura 1973; Somrith et al.1979). 'V20A' and 'Zhen-Shan 97A' are early-season indica in the YangtzeValley of China. Both have a large percentage of undesirable chalky grainsand chalky area. Most Chinese-bred R lines for japonica hybrid rice have


this chalkiness due to indica-japonica crosses in their pedigree. Toimprove the rice quality concerning grain chalkiness, both parental linesshould be free from chalkiness (Khush et al. 1988; Yuan and Chen 1988).For example, the chalky area of the indica hybrid ('L301A' x 'IR29') is6.5% because the chalky area of both parents is less than 100/0.

Grain width and length/width ratio were affected not only by maternal additive x environment, but also by direct additive x environmenteffects (Shi et al. 1998; Chen et al. 1998). Grain length is governed by asingle gene, or two or three genes, or polygenes (Ramiah et al. 1931; Bollich 1957; Ramiah and Parthasarathy 1933; Mitra 1962; Chang 1974;Somrith et al. 1979). Grain width and weight are controlled by polygenes(Ramiah and Parthasarathy 1933; Nakatat and Jackson 1973; Chang 1974;Lin 1978). Because the grain size and shape are also determined by thesize of lemma and palea, which are governed by the genetic composition of the female parent, the female parent should have the desirablegrain size and shape. The length and shape of F1 grains generally arebetween those of the parents. Therefore, to develop medium-grainhybrids, parents having long and short grain may be used, but to producelong-grain hybrids, both parents must have long, slender grains. Parentswith similar endosperm appearance should be selected to avoid segregation for physical appearance among the grains.

2. Selecting A and R Lines with Elite Cooking and Eating Quality. Shiet al. (1998) reported that endosperm additive and dominance effectsaccounted for 74.6% of total genetic effects for amylose content, followed by cytoplasmic and maternal effects. Amylose content is alsoreported to show both dosage and maternal effects, and monogenicinheritance (Kumar and Khush 1986, 1987, 1988; Kumar et al. 1987,1994). Cooking and eating quality traits such as amylose content, tenderness, and cohesiveness of cooked rice for hybrids are between thoseof the parents (Bollich and Webb 1973; Ghosh and Govindaswamy 1972;McKenzie and Rutger 1983; Seetharaman 1959; Stansel 1966). In hybridgrains the heterogeneity for amylose content, gelatinization temperature,and gel consistency does not reduce cooking and eating qualities (Khushet al. 1988). In practice, if one parental line has high amylose content,the other parental line should have medium or low amylose content foran indica rice hybrid with an amylose content of about 220/0. Parentswith intermediate amylose content and gelatinization temperature andlow intrapopulation variation should be crossed to obtain hybrids thathave a uniform texture and cooking time (Yuan and Chen 1988).

The aromatic trait, whose principal component is 2-acetyl-l-pyroline,was reported to be controlled by a single recessive gene (Sood and


Siddiq 1978; Bollich et al. 1992), two genes (Ramiah and Rao 1953;Zhou 1994), or two or three complementary genes (Tomar and Nanda1983; Reddy and Sathyanarayanaiah 1981). For good post-cooking aromaand grain elongation, both parents must perform well for both of thesetraits (Bong and Singh 1993; Zhou 1994).

V. BREEDING FOR TWO-LINE SYSTEM HYBRID RICE

For simplification of the procedure for hybrid rice seed production,two-line system hybrid rice has been extensively studied to eventuallyreplace the existing three-line system. The two-line system shouldinclude the T(P)GMS system and the chemical-emasculation system. Inthis section, the two-line system is referred to as the T(P)GMS system,unless otherwise indicated. The application of the chemical emasculation techniques will also be discussed in this section.

A. Considerations

1. Advantages. There are four principal advantages to the two-linebreeding.

Simplicity and Effectiveness. Since a maintainer line for the three-linesystem hybrid rice is not needed, multiplication of T(P)GMS lines ismuch easier and does not require synchronization of both parental linesin the multiplication plots as in the three-line system. In China, the average seed yield of T(P)GMS lines is 3 to 5 t/ha as compared to 2 t/ha forA line multiplication. Moreover, nuclear genes for male sterility ofT(P)GMS lines are much easier to transfer than CMS genes, becausethey are unaffected by cytoplasmic genets).

Removal of the Restriction ofRestorer Genes. In the three-line breedingsystem, the F1 hybrids between a male sterile line and most rice cultivars of the same subspecies show male sterility or partial fertility (seedset less than 30-50 0jo). Only a small percentage of rice cultivars arerestorers of A lines. Therefore, the potential to develop superior ricehybrids by using newly-developed rice cultivars is limited in the threeline system. On the contrary, the male sterility ofT(P)GMS lines is controlled by recessive nuclear genets), so the Fls between a T(P)GMS lineand cultivars of the same subspecies show normal fertility. The male


sterility of 'W6154s' and 'An-Nong s-l' can be restored to normal fertility by 97.6% and 99.3% of indica cultivars, respectively, and that of'Nong-Ken 58s' can be restored by more than 96.6% of japonica cultivars (He and Yuan 1993). Thus the potential for successful developmentof elite hybrids is greatly increased.

Easier Utilization of Intersubspecific Heterosis in indica-japonicaCrosses. It is easier to introduce wide compatibility gene(s) into T(P)GMSlines than into A lines. Therefore the combination of T(P)GMS gene(s)and WC (wide compatibility) genes will make intersubspecific heterosis breeding more feasible.

Overcoming Negative Effects of the Male Sterile Cytoplasm. Yield potential may be fully tapped owing to the absence of negative effects fromthe male sterile cytoplasm of the three-line system (Wang and Tang1990; Young and Virmani 1990). For example, a two-line system hybrid'Liang-You-Pei-Te' (Pei'ai 64s x Teqing) yielded more than 7.5 t/ha(maximum yield 10.4 t/ha) in Hunan, China during the late season of1991-94 and 9.0 t/ha in single crop plus an average yield of 2.3-3.0 t/haof the ratooning crop. The highest yield recorded for this hybrid was 17.1t/ha in Yongsheng County, Yunnan Province of China (Bai and Luo1996).

2. Disadvantages. The dependency of male sterility on temperaturerequires much attention from breeders and agronomists concerning F1

seed production and multiplication of the T(P)GMS lines. Consistentseed production requires that the climatic data of the seed productionregion should be analyzed in great detail. The scenario for the seed production should be set forth on the basis of local climatic conditions.More cautious and stricter evaluation is required for the commercialization of newly bred T(P)GMS lines. Once a T(P)GMS line is registered,its core seed production procedure should be followed for each generation to keep the CSP at the same level as when first registered in orderto minimize the risk of unsuccessful F1 seed production.

B. Development of T(P)GMS Lines

Six usable T(P)GMS lines have been released in China (Yuan 1997a,Table 2.14). The results of these releases provide a basic genetic tool fordeveloping two-line rice hybrids, a further breakthrough for commercialization of hybrid rice.


Table 2.14. Some commercially used rice P (T)GMS lines developed in China. Source:Yuan 1997a.

P(T)GMS Year Development CSpx Sterility Genelines Subspecies identified province (0C) type source

Pei'ai 64s javanica 1991 Hunan 23.5 HTy Nongken 58s

7001s japonica 1989 Anhui 24.0 LDHP Nongken 58s

5088s japonica 1992 Hubei 24.0 LDHT Nongken 58s

810s indica 1995 Hunan 24.0 HT Annong s

Xiang 125s indica 1994 Hunan 23.5 HT Annong s

GD 2s indica 1995 Guangdong 23.0 HT Nongken 58s

xCSP =critical sterility point;

YHT =high temperature;

ZLDHT =long day length, high temperature.

1. Discovery ofT(P)GMS Sources. Kaul (1988) reviewed the male sterility conditioned by temperature, photoperiod, or other unknown environmental factors in his book Male Sterility in Higher Plants. Heestimated that in about 44% of research reports the major environmental factor influencing male sterility was temperature, in 12% it was photoperiod, and in the remaining 44% it was unknown environmentalfactors. Rick (1948) reported that temperature affected the male sterilityin tomato (Lycopersicon esculentum). Rundfeldt first reported the effectof photoperiod on the male sterility of cabbage (Brassica oleracea var.capitata), and also reported that the male sterility of three mutants, oneeach from cabbage, pepper, and tomato, was sensitive to both temperature and photoperiod. The cabbage mutant was male sterile in the summer and male fertile in winter, whereas the other two mutants frompepper and tomato were male fertile in summer and male sterile in winter (Rundfeldt 1960; Kaul1988). Male sterility of sorghum (Sorghum vulgare) was reported to be conditioned by photoperiod (Barabas 1962).Environment-conditioned male sterility has now been reported in pepper and tomato (Martin and Crawford 1951), cabbage (Rundfeldt 1960),sorghum (Barabas 1962; Tang et al. 1997), wheat (Jan 1974; Zhou et al.1997), barley (Ahokas and Hockett 1977), sesame (Brar 1982), pea (Kaul1988), rape (B. napus) (Xi et al. 1997), soybean (Wei et al. 1997), and rice(Shi 1981; Maruyama et al. 1991a; Oard et al. 1991).

'Nong-Ken 58s', the first rice source material for the development ofT(P)GMS lines, was discovered in the Mian-Yang County of HubeiProvince in China by Shi in 1973 (Shi 1981,1985; Shi and Deng 1986).Three male plants of this source material appeared to be physically iden-


tical to the male fertile plants, but were seven to ten days earlier duringthe initial heading stage. During 1980-1981 sequential plantings showedthat photoperiod, not daily average temperature, from seeding to 15 daysbefore initial heading was correlated with sterility performance. The correlation coefficient between daylength and male sterility was 0.91, indicating that daylength was the main factor inducing male sterility in'Nong-Ken 58s'. 'Nong-Ken 58s' and the derived T(P)GMS lines were designated as "late japonica long-daylength-sensitive genic male sterile rice"in 1983, and as "Hubei Photoperiod-sensitive Genic Male-sterile Rice"(HPGMR) in 1985. In 1987 L.P. Yuan proposed that all T(P)GMS lines beaffixed with "s" for simplification and for differentiation from the"A"line of the three-line system (2hu and Yang 1992).

In the 1980s, additional TGMS materials were discovered in China,including '5460s', 'An-Nong s-1', and 'Heng-Nong s-1'. Two so-calledreverse-TGMS materials, 'Dian-Xun -1' and 'IVA', were also reported forwhich a range of high temperatures can promote fertility (Jiang 1988;Peng et al. 1993; Jiang et al. 1997). Recently a new rice germplasm, 'YiDS', has been reported to show male sterility under short photoperiodand low temperature, which may be useful for double cropping of hybridrice seed production in southern China (Wan et al. 1997).

Outside of China, Maruyama et al. (1991a) developed a TGMS line,'Norin PL12' or 'H89-1', from 'Reimei' by irradiation with 20 Kr ofgamma rays. Another T(P)GMS line, 'X88', was also reported in Japan(Lu 1994). Oard et al. (1991) reported an environmentally influencedmale sterile material from the M7 generation of M201 treated with EMS,with its conditional male sterility controlled by two nuclear genes withepistatic effects. Rutger et al. developed an environmental-conditionedgenic male sterile material from a japonica cultivar, 'Calrose-76', usingtissue culture. This material showed male sterility under daylength of15 h, but male fertility under 12 h. These two T(P)GMS materials are stillunder study, because one produces too many selfing seeds under conditions that promote the sterility, and the other produces insufficientseed under short daylength (Mackill1995). Another mutant was recentlyidentified as a putative photosensitive genic male sterile and is currentlyunder study (Rutger 1997). The International Rice Research Institutedeveloped the TGMS line 'IR32364-20-1-3-2B' by irradiation mutationbreeding (Lu 1994). India has identified several TGMS strains, such as'SM3', 'SM5', 'F61', 'JP2', 'JP8-1-A-12', 'JP8-8-ls', 'ICI0', 'ID24', 'JP1','JP24A', 'UPRI95-140', and 'SA2(F43)', among which 'JP8-1-A-12', 'F-61',and 'SA2(F43)' have a low critical sterility-inducing temperature and'JP24A' belonged to the reverse TGMS group (Ali et al. 1995; Satyanarayana et al. 1995; Reddy et al. 1998a; Li and Pandey 1998). Three


TGMS lines, 'VN-Ol', 'VN-02', and 'TG-162', were identified in Vietnam(Minh et al. 1997), but these materials still need to be characterized indetail.

2. Responses to Temperature and Daylength

Photoperiod Sensitivity. Two photoperiodic reactions function simultaneously in PGMS rice. One affects the growth by delaying or promoting panicle differentiation and heading. The second affects developmentand determines the male sterility (Yuan et al. 1993).

Fertility Alteration Sensitive Stage (FASS). The FASS varies amongdifferent T(P)GMS lines. The FASS of 'Nong-Ken 58s' is from the secondary branch primordium differentiation to the pollen mother cell formation stage, the most sensitive being the pistil and stamen formationstage (Yuan et al. 1988). The FASS of 'Shuang 8-14s' developed from'Nong-Ken 58s' is different from that of 'Nong-Ken 58s' (Zhu and Yu1987).

Critical Daylengths for Fertility Alteration. The critical daylength is theshortest daylength inducing fertility of T(P)GMS lines. This variantdepends on the different genetic backgrounds and ecological conditions. The critical daylength of 'Nang-Ken 58s' and 'E-Yi 105s' is 13.75to 14.00 h, and their fertility alteration is not an abrupt change (Zhanget al. 1987). Under low temperature in the summer of 1989 'Nong-Ken58s' and 'N5047s' showed male fertility even with the daylength of14.17 h at Hangzhou, China (30°05' N). The fertility alteration is unaffected by daylength in some indica TGMS lines such as 'An-Nong s-l','5460s', 'Heng-Nong s-l', and W6154s (Cheng et al. 1990). The fertilityalteration stage also varies with different regions. For example, the critical daylength of 'Nong-Ken 58s' is 12.37 h at Hainan Island, but 13.67h in Fujian Province of China. 'Nong-Ken 58s' is even fertile all yearround under natural conditions in Guiyang of China (Lu 1992a,b).

Critical Light Intensity. It was reported that the lowest light intensitythat induces sterility was 50 Ix (Zhang et al. 1987). But the critical lightintensity inducing male sterility is altered by the temperature. Underhigh tempera'ture, light intensity as low as 100 Ix can induce completemale sterility, whereas under medium or low temperature the criticallight intensity inducing complete male sterility should be over 100 Ix(Liang et al. 1990).

Light vs Dark Period. For the FASS of 'Nang-Ken 58s', light interruption with 50 Ix light intensity for 1 h or 200 Ix for 5-15 min couldinduce male sterility under short daylength and 27°C (Zhang et al. 1987).The effect of light interruption under short daylength was affected by


temperature. To induce male sterility under high temperature, the lightintensity could be low and the light treatment duration could be short(Liang et al. 1990). The dark duration is more crucial than the lightperiod for inducing male sterility. If the dark duration is 10 h or less, themale sterility will be induced. But, if it is 11 h or more, the male fertility will be induced. Short duration (1-3 h) of dark under long daylengthcauses no obvious effect on the fertility (Lu and Yuan 1991).

Red Light vs. Infra-red Light. During the FASS, if red light is presentduring the dark period under short daylength, the fertility is decreased.If immediately after the red light, the far-red light is turned on the fertility will recover to the level that results from only far-red light illumination. Male sterility is determined by the last-illuminated light. Purered light incompletely converts fertility as compared with far-red light,and only red light and blue light together can induce complete fertilityconversion, which cannot again be reversed by far-red light (Yang andZhu 1990). It is concluded that in addition to phytochrome, the bluelight receptor, cryptochrome, also affects the regulation of male sterility of T(P)GMS lines.

Interaction Between Temperature and Photoperiod. Fertility of the socalled PGMS lines is also affected by temperature during the differentiation of the floret primordium, indicating that photoperiod andtemperature interact at certain levels (Wu et al. 1993; Zhang et al. 1994c).

Transmission Among Main Culm and Tillers. During the sensitive orphotoperiod induction stage, the photoperiod signal cannot be transmitted between the main culm and tillers, among tillers, or between thefirst crop and the ratooning crop (Zhu and Yang 1992).

Temperature Sensitivity. Temperature affects fertility alteration for allT(P)GMS lines (He et al. 1987; Li et al. 1989a; Sun et al. 1991; Xue andChen 1992). Expression of photoperiod-sensitive male sterility can bealtered significantly by changes in the mean daily maximum or minimum temperature during the daylength treatment (Xue and Zhao 1990).

Critical Sterility Point (CSP). Different T(P)GMS lines have differentcritical sterility-inducing temperatures, referred to as the critical sterility point (CSP). For example, the CSP is 28.1-29°C for '5460s' and24.2-26.5°C for 'An-Nong s-1' (Cheng et al. 1990; Yang 1990a; Chen etal. 1993). The CSP index evaluates the risk during commercial seedproduction for the two-line system. Molecular mapping using RFLPmarkers showed that the low-temperature sensitivity to male sterility orlow CSP was likely controlled by three independent genomic regions,among which two were on chromosome 1 and one on chromosome 12(Li et al. 1997).


Temperature Sensitive Stage. In general, the temperature sensitivestage for fertility alteration of indica T(P)GMS lines occurs at about themeiotic division of the pollen mother cells, Le. 10-15 days before heading. It requires three to seven days to induce male fertility with low temperature (Chen et al. 1993). For japonica T(P)GMS line 'Nong-Ken 58s',the temperature sensitive stage is from the second branch primodia differentiation stage to the microspore uninucleate stage, the most sensitive period being from the pistil and the stamen primodia formationstage, which is longer than for the indica T(P)GMS lines, until the meiosis of the pollen mother cells (Zhang et al. 1992).

Temperature Sensitive Part of a T(P)GMS Plant. In T(P)GMS plants,the young panicle is the critical organ that is sensitive to low temperature (Zhou et al. 1993a; Xu and Zhou 1996). The "cold water irrigation"method for effective production of T(P)GMS lines with low CSPs wasinvented by Xiaohe Luo and has been practiced extensively in China'stwo-line hybrid rice production (Hunan Hybrid Rice Research Center1992; Chu et al. 1997). Similarly, the warmer water can prevent the se1£ing of T(P)GMS lines caused by low air temperature in the F1 hybrid seedproduction (Xiao and Yuan 1997).

The Effect of Temperature or Daylength in the Vegetative Stage. Theclose correlation between the photoperiodic response of heading andmale sterility in '7001s' suggests that the genes responsible for headingand photoperiod-sensitive male sterility are not independently inherited(Tang and Shao 1997). The two photoperiodic reactions proposed byYuan et al. (1993), seemed to interact with each other, and the temperature or daylength in the vegetative stage also influenced fertility alteration in T(P)GMS lines. Higher temperatures or shorter daylengths willlower the CSP (Zhang et al. 1992, 1993b).

Model of Fertility Alteration by Interaction Between Temperature andDaylength. Based on the effect of temperature and daylength on fertility, a fertility alteration model was proposed for practical use (Zhang,Lu, and Yuan 1992). The key points are as follows: (1) When temperature is higher or lower than the physiological limit, rice grows poorlyand develops abnormal pollen. These temperature limits are 10-15°C forlower limits and 35-41°C for higher limits (Suzuki 1978; Satake andYoshida 1978). But Chinese studies indicated that the lower physicaltemperature limit was higher than 10-15°C. (2) If the temperature of aPGMS line is lower than the upper physiological limit but higher thanthe CSP, this PGMS line shows male sterility even under shortdaylength. (3) If the temperature of a PGMS line is higher than the lower


physiological limit and lower than the CSP, this PGMS line will showmale fertility or semi-sterility even under long daylength. (4) Onlywithin the daylength-sensitive temperature range, i.e. from the CSP tothe critical temperature inducing male fertility, will the PGMS line showdaylength sensitivity (Fig. 2.6). The daylength and temperature effectsare complementary, Le. if temperature increases then the criticaldaylength inducing sterility will be shortened. Confirmed from practiceis that no TGMS or PGMS materials tested under different daylength andtemperature regimes show male sterility affected only by photoperiod(Deng et al. 1997). Therefore, T(P)GMS is employed in most cases of thisreview instead of TGMS or PGMS.

3. Development of New T(P)GMS Lines. Genic male sterile lines havebeen developed primarily by chemical or irradiation mutation (Fujimaki et al. 1977; Ko and Yamagata 1980,1987; Singh and Ikehashi 1981;Fujimaki and Hiraiwa 1986) although some T(P)GMS lines have beenidentified as spontaneous mutations (Suh et al. 1989). Most breedingmethods used for inbred cultivars can be employed to develop T(P)GMSlines, including the pedigree method, cross breeding, mutation, and tissue culture.

Pedigree Method. Most Chinese source T(P)GMS materials such as'Nang-Ken 58s', '5460s', and 'An-Nang s-1' are selected from the sourcemale sterile plant(s). For example, in the case of 'An-Nong s-l' one malesterile plant was discovered in the F5 population of the cross ((Chao 40B xH285) x 6209-3] in 1987. 'An-Nang s-l' was developed from this plantusing the pedigree method.

Cross Breeding. Currently most Chinese-bred japonica T(P)GMS lines,such as 'N5047s' and '7001s', were developed using single cross breeding.Ifa japonica source T(P)GMS line is to be transferred to an indica rice cultivar, segregation among the progeny is difficult to stabilize. Therefore, themultiple cross or recurrent backcross breeding should be used (Li 1992).The indica T(P)GMS line 'W6154s' was developed from the triple cross[(Nong-Ken 58s x CS253-2-3-2) x Zhen-Shan 97]. The T(P)GMS line'8902s' was bred from the backcross F3 progeny of the cross (Shuang 8-14sx Zhen-Shan 97) using 'Zhen-Shan 97' as the recurrent parent.

A T(P)GMS line may also be developed from interspecific or intersubspecific crosses. For example, 'Heng-Nong s-1' was developed fromthe cross (long-awned wild rice x R1083) and 'Xin-Guang s' was developed from an indica-japonica cross.

Mutation Breeding. Some scientists have developed T(P)GMS linesusing irradiation or chemical mutation (Kato et al. 1990; Rutger and

CXl

o

phys

iolo

gica

lup

per

tem

pera

ture

lim

it

the

crit

ical

tem

pera

ture

indu

cing

mal

est

eril

ity

the

crit

ical

tem

pera

ture

indu

cing

mal

efe

rtil

ity

phys

iolo

gica

llow

erte

mpe

ratu

reli

mit

No

ng

-Ke

n·5

88

>36

0c

>32

0c

>24

0c

>1S

oC

An

-No

ng

8-1

>3S

0c

>260

c

>260

c

>1

S0

c

Fig

.2.

6.M

odel

off

erti

lity

alte

rati

onby

tem

per

atu

re-p

ho

top

erio

din

tera

ctio

n.S

ourc

e:Y

uan

1992

b;Z

hang

,L

u,an

dY

uan

1992

.


Schaeffer 1990; Oard et al. 1991). The International Rice Research Institute developed 'IR32364s' from 'IR32364-20-1-3-2B' using gamma rayirradiation.

Tissue Culture. Rutger et al. developed a T(P)GMS line from 'Calrose76' using tissue culture. This T(P)GMS line showed male sterility underlong daylength in California, but male fertility under short daylength inHawaii. Another TGMS line, 'T-Shan-s', was identified using tissue culture of explants from the mature embryo of 'D-Shan B' (Huang et al.1992). Other studies have shown the possibility of developing T(P)GMSlines using anther culture (Liu 1995; Niu et al. 1997; Hong et al. 1997).

Approaches to Improve Breeding Efficiency. Breeding practice in Chinahas shown that the frequency of indica T(P)GMS plants is low for crosseswith japonica T(P)GMS line as the donor (Lu et al. 1989). The averagefrequency of T(P)GMS plants in the F3 generation from crosses between'Shuang 8-14s', a japonica T(P)GMS line, and japonica cultivars was6.5%, while in crosses between 'Shuang 8-14s' and indica cultivars itwas 0.8%. Among these indica cultivars, the frequency of T(P)GMSplants for early-season indica cultivars in the Yangtze Valley of Chinawas the lowest, 0.1 %, while that ofIRRI-bred cultivars was the highest,1.5% (Zhu and Yu 1987). It is proposed that rice cultivars with low orweak photoperiod sensitivity and thermosensitivity should be selectedas recipient parents (Lu 1992a,b).

Ecological breeding is one way to increase the selection pressure fordeveloping new T(P)GMS lines. This involves the use of long daylengthand low temperature to identify and select the male sterile plants, andthe use of short daylength and high temperature to increase the efficiency of multiplication of the T(P)GMS line. In China, this method hasenhanced the efficiency of T(P)GMS line breeding (Lu et al. 1994).

Photoperiodic response of plant development is positively correlatedwith photoperiod-sensitive male sterility, but negatively correlated withtemperature-sensitive male sterility. Thermoperiodic response for plantdevelopment shows no correlation with photoperiod-sensitive and temperature-sensitive male sterility. Therefore, selection of plants with photoperiodic response would be preferable for the development ofphotoperiod-sensitive male sterile lines (Chen and Wan 1993).

4. Evaluation ofT(P)GMS Lines. China has set the following eight criteriafor the acceptance of new T(P)GMS lines (Yuan 1990; Lu et al. 1994): (1)the tested population size should be larger than 1,000 plants; (2) theseplants should express uniform agronomic traits; (3) the percentage of


male sterile plants should be 100%, and pollen sterility of the male sterile plants should be over 99.5%; (4) the fertility alteration should beobvious; (5) the seed set percentage should be more than 30% during fertility induction; (6) the duration of complete male sterility should last forat least 30 days; (7) the outcrossing seed set percentage should be higherthan that of 'V20A', 'Zhen-Shan 97A', or 'Liu-Qian-Xin A'; and (8) theCSP should be between 23-23.5°C, or even lower. In addition, the idealT(P)GMS lines should have little sensitivity to low temperature for theduration of their male sterility or F1 seed production, as well as to hightemperature for the duration of the male fertility or T(P)GMS line multiplication (Yuan 1992b).

Methods to evaluate T(P)GMS lines include field or phytotron evaluation. Field plantings involve sequential plantings at different locations.During the season that the rice plant can grow, 20-30 plants should begrown every 10-15 days. The initial heading date, pollen sterility, andselfing seed set data should be collected and analyzed to evaluate andcompare the T(P)GMS lines. However, under natural conditions, long (orshort) daylength and high (or low) temperature always coincide, so theindividual effect of the temperature and the daylength cannot beresolved. Therefore, it is impossible to distinguish between TGMS andPGMS, and to determine the temperature sensitivity of the T(P)GMSlines only using the ecological evaluation of sequential plantings.

The phytotron can be used to dissect the effects of daylength andtemperature on the fertility alteration of T(P)GMS lines. For each phytotron treatment, 10-15 plants should be evaluated using the same traitsas those measured for the ecological evaluation by sequential plantings.Variable temperature in a day is more dependable and currently used forthe evaluation ofT(P)GMS lines (Wang et al. 1994; Deng et al. 1996). Scientists at the China National Rice Research Institute (CNRRI) studied 101T(P)GMS lines under nine controlled regimes consisting of three photoperiods (15.0, 14.0, and 12.5 h) x three average temperatures (30.1,24.1, and 23.1°C) and found that 96% ofT(P)GMS lines could be dividedamong three types based on variance analysis of the seed set: (1) PGMScharacterized by significant P (photoperiod) and P x T interaction effectsbut a non-significant T (temperature) effect on fertility; (2) TGMS characterized by a significant T effect and a non-significant P effect on fertility; and (3) P-TGMS with only a significant P x T interaction effect onfertility. Among the japonica T(P)GMS lines studied, 32.3% were PGMS,9.7% were TGMS, and 51.6% were P-TGMS. In contrast, among indicaT(P)GMS lines studied, none were PGMS, 61.4% were TGMS, and35.7% were P-TGMS lines (Cheng et al. 1996).


5. Maintaining Temperature Sensitivity ofT(P)GMS. Individual plantsof a T(P)GMS line differ in their response to temperature and daylength.Seed set of plants with higher CSP is higher than for the plants withlower CSP. Consequently, there is "CSP drift" toward higher temperature in subsequent generations. Thus, the T(P)GMS line cannot be putinto commercial production, because of the increasing risk of unacceptable F1 hybrid seed production using the conventional method forinbred multiplication. For example, the CSP of 'Pei-Ai 64s' was 23.3°Cin 1991, and it increased to 24-25°C in 1993 using the conventional multiplication method. To solve the problem of CSP increase in the generation advance, the "core seed production" procedure was proposed in1994 (Yuan 1994a,b,c). The procedure consists of: (1) selection of individual plants; (2) treatment of the individuals with low temperature orlow temperature and long daylength; (3) selection of plants with lowCSP; (4) ratooning of the selected plants to obtain selfed seeds; (5)sequential development of core seeds, breeder's seeds, and foundationseeds; and (6) F1 seed production.

China's experience has demonstrated that the core seed productionprocedure maintains low CSP for commercial hybrid rice production(Deng and Fu 1998). A similar procedure for the production of nucleusand breeder's seed of TGMS lines has been proposed (Virmani et al.1997).

Multiplication of TGMS lines can also be carried out using the thinlayer cell culture technique with 50% callus induction and 100% plantregeneration (Nhan et al. 1997). Other approaches for multiplicationand maintenance of genetic stability in rice such as micropropagationor ratooning multiplication deserve further study.

C. China's Progress

The discovery by Shi (1981, 1985) of 'Nang-Ken 58s' and increasedunderstanding of the phenomenon of "wide compatibility" in rice (Ikehashi 1982) provided the genetic tools needed to achieve developmentof the two-line system for hybrid rice and the utilization of intersubspecific heterosis. With these two genetic tools, China initiated a collaborative research program on the two-line hybrid rice system in the1980s. After more than ten years of nationwide collaborative study, boththe physiological and genetic mechanisms of T(P)GMS are basicallyunderstood. Ten two-line system hybrid rice cultivars that normallyoutyield the three-line system hybrid rice cultivars by 5-10% have beenreleased and put into commercial production (Table 2.15; Yuan 1997a).


Table 2.15. Two-line system hybrid combinations certified andregistered. Source: Yuan 1997a.

Province YearCombinations certification registered

Pei'ai 64s x Teqing Hunan 1994

Pei'ai 64s x 288 Hunan 1996

Pei'ai 64s x Yuhong 1 Hunan 1997

7001s x Xiushui 04 Anhui 1994

7001s x Wanhui 9 Anhui 1994

7001s x 1514 Hubei 1995

5088s x R187 Hubei 1995

7001s x Shuangjiu Anhui 1997

Pei'ai 64s x Shanqing 11 Guangdong 1996

Shuliangyou 1 Sichuan 1996

The area under the two-line hybrid rice system has increased progressively as shown as Table 2.16 (Yuan 1997a). The area of two-line hybridrice production in China is expected to reach 1.3-1.5 million ha in theyear 2000.

D. Breeding for Two-line System Rice Hybrids UsingChemical Emasculators

When sprayed on rice plants at specific developmental stages, a gametocide or chemical hybridizing agent (CHA) can emasculate the plants,thus resulting in male sterility while maintaining normal female fertility (Lasa and Bosemark 1993). A rice cultivar having a superior specific

Table 2.16. The planting area and yield of twoline hybrid rice in China from 1993 to 1998.

Planting area YieldYear (x 1,000 hal (kg/ha)

1993 27 7170

1994 67 7005

1995 73 7215

1996 200 7190

1997 270 7150

1998 437


combining ability is used as the male parent for producing the F1 hybridseed.

Many scientists have reported that ethephon [(2-chloroethyl) phosphonic acid] induces male sterility of crops (Rowell and Miller 1971;Bennett and Hughes 1972; Perez et al. 1973; Hughes 1976; Parmar et al.1979; Chan and Cheah 1983). More than 50 chemical emasculators havebeen identified for more than 40 crops, but most also damage the pistilor cause abnormal flowering (Wang et al. 1981, 1991b,c; Yu et al. 1991).Some of these chemical emasculators are listed in detail by Kaul (1988).For rice, male sterility is induced in 'PR106A' using 0.4% EMS for 48 hat 10°C (Minocha and Gupta 1988; Minocha et al. 1991). Aswathanarayana and Mahadevappa (1991,1992) reported that 800 ppm ofGA,8000 ppm of ethephon, 0.02% maleic hydrazide (MH), and 0.8% 2,4dichlorophenoxyacetic acid (2,4-D) induced a high level of male sterility in rice. Kitaoka et al. (1991) reported that the male sterility reached95% or more using isourea at 3 kg/ha + ethephon at 5000 ppm or byalternatively using isourea at 10 kg/ha + ethephon at 2500 ppm. Otherchemicals such as RH531 and DPX [3-(p-chlorophenyl-6-methoxy-3-triazine-2,4-(lH, 3H)dione] have also been tested in rice (Perez et al. 1973;Long et al. 1973; Zhangxing and Chunnong 1980). In the 1970s and1980s, China developed chemical emasculators using arsenate, such asMale Gametocide 1 (zinc methyl arsenate, CH:lAs03Zn) and Male Gametocide 2 (sodium methyl arsenate, CH;lAsO;{Naz). While they causedexcellent emasculation of rice, they were very toxic to the environment.Later, non-arsenic chemical emasculators such as N312, HAC123,CRMS-1, and 13(a pyridazinone derivative) were developed (Luo et al.1988; Zhong et al. 1997). Currently India has identified some less toxicgametocides including ethyl 3' methoxy oxanilate and ethyl 4' fluorooxanilate (Siddiq 1994; Siddiq et al. 1994). In China the most popularhybrids produced by chemical emasculation were 'Gang-Hua 2', 'GangHua x Qing-Lan', and 'Qing-Hua x Fu-Gui', which were collectivelygrown on 60,000 ha in Guangdong and Jiangxi province during the mid1970s to early 1980s. The yield data by chemically emasculated hybridsfrom 1982 to 1985 shows that the yield increase over the check threeline hybrids ranges from 7.8-18.2% (Shao and Hu 1988). Unfortunately,the application ofthe two-line system hybrid rice using chemical emasculators was basically not successful due to the apparent contradictionbetween the seed purity and the seed yield.

1. Key Techniques for Chemical Emasculation. Theoretically, 100% ofmale sterility will result in 100% pure hybrid rice seed. Unfortunately,the highest levels of male sterility due to chemical emasculation also


cause increased female sterility, or otherwise decrease the outcrossingpotential of the female parental lines and, consequently, the F1 seedyield. To some extent, lower levels of male sterility can achieve bothhigh yield and high purity in the F1 seed. The usual criteria for chemical emasculation in China are: 95% male sterility, 30% outcrossed seedset, 85% seed purity and seed production of 1.5 t/ha.

Key techniques are careful selection of female and male parental linesand precise application of emasculators. The stamen of the femaleparental line, but not the pistil, should be very sensitive to the chemical emasculator. Furthermore, the blooming characteristics should beaffected little by the chemical emasculator. Erect leaves and uniformity of the developmental stages between tillers and plants result inmore effective emasculation. In addition to short plant height, a highstigma exsertion rate and large floret opening angle of the female parentalline contribute to high yield of hybrid seed. Possession of favorableagronomic traits, yield performance, and grain quality by the female parent leads to better performing F1 hybrids. The male parental line shouldhave high specific combining ability, large panicles, large pollen load,long blooming period, be taller than the female, and have growth duration close to that of the female parental line. Finally, uniform application of the chemicals at the correct stage is essential to F1 hybrid seedproduction.

2. Considerations of Chemical Emasculation

Advantages. Less time is needed to develop hybrids and a broadergermplasm is available for maximizing rice heterosis. Development of amale sterile line does not require several generations. The two-line system via T(P)GMS lines eliminates the maintainer line, but these linesmust be intensively evaluated prior to any F1 hybrid breeding, and stabilityof the conditional male sterility must be maintained. In comparison,chemical emasculation requires far less work, as only the identificationof an effective chemical emasculator is required prior to breeding of therice hybrids. Chemical emasculation also circumvents the genetic vulnerability of cytoplasmic-nuclear male sterile lines in the three-line system. As a result of prior progress on inbred breeding, heterosis can befurther exploited. For example, the F1 hybrid between 'Gang-Zi-Zhan'and 'IR661' showed strong heterosis. But, both parental lines were difficult to develop into A lines. Chemical emasculation made this strong heterosis commercially available in China (Guangdong Crop HeterosisUtilization Research Cooperative Group 1977, 1981). Finally, in the chemical emasculation method, segregation for male sterility in the F2 genera-


tion does not occur. For a rice hybrid between intervarietal cultivars withclose growth duration, chemical emasculation provides heterosis not onlyfor the F1 but also for the F2 generation of some combinations such as 'LiHua-Da-Zhen' (Li et al. 1986, 1989b; Tu and Hu 1989).

Disadvantages. There are several problems with chemical emasculation. Effective chemical emasculators are still needed. For example,while etheflon induces pollen sterility it also has a phytotoxic effect onthe panicle length and spikelet size (Shamsi et al. 1996). Chemical emasculators containing arsenic, which used to be employed in China'shybrid rice seed production, have a number of limitations, not the leastof which is harmful residues left following application of the chemicals.It has been shown that the arsenic content in rice stalks reaches 4.19mg/kg due to using arsinyl at 345-435 g/ha for emasculation (Liu et al.1983). A similar study also indicated that the arsenic residue in the ricegrains was 2 mg/kg and that in rice stalk was 9 mg/kg when 390 g/ha ofarsinyl was applied to the female parent of the rice hybrid combination'Hong-Yang-Ai 2' to achieve 90% purity.

Furthermore, the effective concentration for emasculation is narrow.Too small a dosage of arsenic emasculators does not emasculate completely and too large harms the pistils. To complicate the problem, therequired application time and dosage varies among different rice cultivars. The compromise between seed production yield and seed purityis also a problem. The best time for application of arsenic emasculatorsis about 10 days after the meiosis stage until the pollen filling stage, butthe development rate is not as uniform among plants and tillers asrequired, therefore, the chemical emasculation effectiveness varies.

Arsenic is harmful to the female parental line. A shortened rice culm,panicle enclosure within the flag leaf sheath, glume closure, damagedpistils, and even decreased F I seed germination can result from arsenicchemical emasculators. Not only the stamens but also the pistils are sensitive to chemical emasculators in most rice cultivars. A complete lackof F1 hybrid rice seed can be a result of severe damage to the pistils. Thisdisadvantage strongly decreases the probability of exploiting heterosisin the rice germplasm.

The interaction between emasculators and climate can be a problemas well. It usually requires 5-6 h for a chemical emasculator to takeeffect. Rain soon after the application of a chemical emasculator willdecrease the effectiveness of the emasculation. If the chemical is appliedagain, the effective dosage is difficult to determine. Hybrid rice seed cannot be produced if rainy weather lasts for the duration of effective application of the chemical emasculator. A few other climatic conditions


such as temperature, wind, and humidity will also affect the applicationeffectiveness (Zhong et al. 1992).

VI. WIDE COMPATIBILITY AND UTILIZATIONOF INTERSUBSPECIFIC HETEROSIS

A. Classification in Rice

There are 20 rice species, of which O. sativa and O. glaberrima are thetwo cultivated rice species. Ecotypes have differentiated within thesesubspecies. Classification is based on morphological and ecological features of O. sativa (Cheng 1993; Gu 1988). Kato classified Oryza sativa asindica and japonica by analyzing the morphological appearance, affinity, and serological reactions. Some scientists think there should be athird group, the tall and long-panicled Indonesian cultivars designatedas javanica by Kornicke (1885), in addition to the indica group andjaponica groups (Morinaga 1954; Chang et al. 1991). The javanica ricecorresponds to "Group Ie" of Terao and Mizushima (1944), "B planttype" of Matsuo (1952), and the "tropical insular groups IIa" and "lIb"of aka (1958). aka later designatedjavanica as "tropicaljaponica" (aka1983). Together with the aus cultivars from Bangladesh and easternIndia, which have high genetic affinity with both the indica and japonica cultivars, Indonesian cultivars were considered to be an intermediate type by Morinaga and Kuriyama (1958). Chang et al. (1991) suggestedthat the javanica group should also include the bulu and upland or hillrice. Currently, rice classification is generally based on the morphological, biochemical, and genetical features.

1. Morphological Classification. Morphological traits such as grainshape, apiculus hair length, and phenol reaction are used to classify rice.Cheng (1985) proposed a morphological index for classification in rice(Table 2.17), the accuracy of which can be as high as 95%.

For the morphological classification method, one of the most important indices is hybridization compatibility. Seed set of crosses betweenindica and japonica generally ranges from 0 to 30% except for the crossbetween the aus ecotype of indica and japonica cultivars. Seed set ofspecific crosses between indica and japonica still varies over a widerange. The seed setting percentage of the F1 between Chinese indica andChinese japonica cultivars varies from 1.3% to 80.3%, and some ricecultivars from Yunnan of China have good compatibility with bothindica and japonica cultivars, although the seed setting percentages of


Table 2.17. The grading system for rice cultivars using morphological index method.>:Source: Cheng 1985.

Grading

Variable 0 1 2 3 4

Apiculus hairLength Very short Long Medium Long Very longUniformity Very Uniform Intermediate Variable Very

uniform variableHardness Very hard.

erect Hard Medium Soft Very soft

Phenol reaction Black Light black Grey Stained on Unstainededges

Panicle internodelength (1"1 & 2'1(1) <2cm 2.1-2.5 em 2.6-3 em 3.1-3.5 cm > 3.5 cm

Glume color Green & White & Yellow & Light green Greenwhen heading white green green

Leaf pubescence Very high High Intermediate Slight None

Length-widthratio of spikelets > 3.5 3.5-3.1 3.0-2.6 2.5-2.1 < 2.0

ZNote: Indica cultivars should have a sum of grades for all items ranging from 0 to 8, andthe japonica varieties from 18 to 24. If the sum falls between 9 and 13 or between 14 and17 the eultivars are biased toward indica or japonica. respectively.

most are below 50% (Yu and Lin 1962). Based on their hybridizationcompatibility, some aus and buIu cultivars are classified as the "intermediate type" (Ikehashi and Araki 1987). Mathematical approachessuch as principal component analysis have been employed for classifying rice based on morphological traits (Zhou et al. 1988).

2. Biochemical and Genetical Classification. Glaszmann (1987) indicatedthat most japonica rice cultivars had the isozyme alleles Acp-1 2, Cat-1 2,

Est-31, and Pgc-1 2• Later Est-X for esterase isozyme was found, for which

the alleles Est-XlO, Est-X 11, Est-X 13, and Est _X14 differentiated amongindica, japonica, aus, and wild rice (Cai et al. 1992). RFLP can also beapplied for classification of rice (Tanaka et al. 1989; Zheng et al. 1990;Kawase et al. 1991). The same results should arise from different classification methods, but some RFLP analysis results differ from those of morphological classification. For example, cultivars 'Sipule' and 'KetanNangka' were classified as indica and japonica, respectively, by the morphological classification method, but as different subspecies using RFLPmarkers (Tanaka et al. 1989; Zheng et al. 1990). Consequently, integration

90 J. 11 AND L. YUAN

and comparison of the different classification methods provide a scientificbasis for rice classification.

B. Phenomenon of Wide Compatibility

The F1 seed set of intersubspecific crosses was usually below 30%,while for crosses between different ecotypes of the same subspecies itwas over 70% (Carnahan et al. 1972). There are exceptions, however.Some cultivars such as the ones from the aus (indica type) or buJu(japonica type) showed high F1 seed set when crossed with indica aswell as japonica (Terao and Mizushima 1939; Morinaga and Kuriyama1958; ,Heu 1967). Normal F1 seed set was also found in some indicajaponica crosses such as crosses between 'Ai-Zi-Zhan l' (indica) andTaiwanese japonica cultivars, between 'You-Mang-Zao-Sha-Jing' (japonica) and indica cultivars, and between a rice cultivar derived from anindica-japonica cross and indica or japonica cultivars (Min 1986).

The "intermediate type" was studied as early as in the 1930s, but Ikehashi (1982) first proposed the term "wide compatibility." Wide compatibility is the phenomenon of the F1 seed set being normal in crossesbetween some intermediate-type rice cultivars and both indica andjaponica cultivars. These cultivars are called wide compatibility varieties (WCVs). The controlling gene is called the wide compatibility gene(WCG). Discovery of wide compatibility in rice provided the opportunity to overcome the reproductive barrier exhibited in the F1 generationof crosses between the indica and japonica cultivars, and thereby to usethe strong heterosis of intersubspecific crosses. This has received muchattention from rice scientists.

C. Genetics of Wide Compatibility Traits

1. Hypotheses to Explain Semi-sterility in indica-japonica Crosses. Thefollowing hypotheses were developed to explain the semi-sterility of theF1 generation of indica-japonica crosses:

Duplicate Gametophytic Lethal Hypothesis. The existence of duplicategametophytic lethal gene(s) has been hypothesized with the duplicateloci being independent (Oka 1953, 1988). If a gamete has the doublerecessive combination it will abort during its development. These lethalgenes could affect both the male and female, or the male gametes only.

Chromosome Aberration. Yao et al. (1958) hypothesized that, in thegametic development of the F1 between indica and japonica, one of the


homologous chromosomes became aberrant resulting in pollen abortion (Chandraratna 1964; Zhou 1978).

Lack of Coordination between Cytoplasm and Nucleus. Turbin considered that coordination did not occur between the cytoplasm and nucleusof indica and japonica, so the gametes and zygotes from indica-japonicacrosses could not develop normally and the pollen aborted (Zhou 1978).

Allelic Interaction. The hypothesis is that the gamete lethality originates from the allelic interaction in the F1s between indica and japonica. Suppose the genotype of indica is Fi/Figand japonica is FiJFig, andthe F1genotype Fi/Figis gamete-abortive. The F1will be fertile if Fn/Fngis crossed with Fig/Fig or FVFig (Kitamura 1961, 1962a,b,c). This hypothesis was later confirmed and developed into the "wide compatibility"theory by Ikehashi and Araki (1986) using the triple cross method.

Multigenetic Inheritance. The semi-sterility in the F1of indica-japonicacrosses was reported to be controlled by multi-gene(s) or specific compatibility genes. At least six loci were involved in determining the semisterility of indica-japonica crosses (Zhang and Lu 1989; Shen and Xu1992).

Genetic Recombination. Seven ancestral parents of 'T984', a WCV witha wide spectrum of compatibility, were tested and no wide compatibility was identified in any of the ancestors. This suggests the hypothesisthat the wide compatibility of 'T984' arises through genetic recombination (Xiong et al. 1993).

2. Chromosomal Identification ofWCG. The first WCG, 5115 was identi

fied on chromosome 6 (lkehashi and Araki 1986; Araki et al. 1988).Wide compatibility of other Chinese-bred WCVs (e.g. 'Lun-Hui 422' and'02428') was also determined to be controlled by 5 11

5 on chromosome 6,but, the loci were ordered differently (Gu et al. 1991; Gu, You, an,d Pan1991; Gu et al. 1992; Lu and Pan 1992). It was reported that 5 11

5 wasbetween Wx and C, Le. WX-5 11

5-C in studies by Ikehashi and Araki(1986) and Liu et al. (1992). Conversely, Sll5 was linked with Wx and Ginthe order WX-C-S1l5-RG213-RG64 (Wang et al. 1994; L.S. Liu, pers. commun.). Zheng et al. (1992) confirmed that there was a locus between C andRG138 that controlled the seed set of the indica-japonica F1s, using sixRFLP markers on chromosome 6 to analyze the 'Pecos' population.

Crosses between WCVs with 5115 and some rice cultivars do not show

normal fertility and linkage with C, so additional loci are assumed to


control wide compatibility (Xu et al. 1989). A genome-wide mapping ofa three-way rice cross [(02428 x Nanjing 11) x Balilla] showed that threeloci conferred significant effects on hybrid sterility, the major locus onchromosome 6 was 5ns and the two minor loci on chromosomes 2 and12 could cause partial sterility even in the presence of 5n

s (Liu et al.1997a; Zhang et al. 1997c). Two more WCGs have been identified, 5n

7

in 'Dular' and 5y in 'Penuh Baru II'. 5n7 is located between Rc and Est-9

on chromosome 7 (Ikehashi 1991; Ikehashi et al. 1991, 1994; Yanagiharaet al. 1992), and is linked with RZ488 and RG511 in 'Aus 373' O.S. Zou,pers. commun.). Another WCG, 5n

a in 'Akihikari' is about 11.2 cM fromCat on chromosome 6 (Wan et al. 1993). The hybrid sterility of crossesbetween Chinese indica and japonica cultivars is mainly controlled bythe 5-5 locus, whereas the hybrid sterility of aus cultivars crossed toindica, japonica, or javanica cultivars is controlled by allelic interactionamong the sterility loci 5-5, 5-7, 5-9 and 5-15 (Wan and Ikehashi 1997).Six WCG loci (5-5, 5-7, 5-8, 5-9, 5-15, and 5-16) have been identified onchromosomes 6, 4,6, 7, 12, and 1, respectively (Virmani 1996).

D. Development ofWCVs

1. Screening for WCVs. Six WCVs were identified in a group of 74 ricecultivars from Indonesia, India, Bangladesh, and the Philippines, i.e.'Padi Bujang Penedak', 'Aus 373', 'Dular', 'Calotoc', 'CPSLO-17', and'Ketan Nangka' by Ikehashi and Araki (1984). Thereafter, more and moreWCVs have been identified by rice scientists at the International RiceResearch Institute and in China (Table 2.18; Zhu and Yang 1992; Min1990; Xiong et al. 1989, 1990; He and Yuan 1993; Zhang et al. 1988).Japanese rice scientists pointed out that the aus cultivars of Bengal, thebuiu cultivars of Java, cultivars from Nepal and other Himalayan tracts,and the landraces of tropical Asian countries gave fertile F1 plants whencrossed with both indica and japonica rice cultivars (Morinaga andKuriyama 1955,1958; Morinaga 1968; Oka 1988).

Three main sources of WCVs were suggested by Luo, Ying, and Wang(1991):

Primitive indica and japonica. In the region of rice origin there may existprimitive indica or japonica types with incomplete differentiation, suchas nuda from Yunnan of China or Southeastern Asian countries, javanica from Indonesia, and aus from the Indian subcontinent. M.H. Gu(pers. commun.) also indicated that the Yunnan rice landrace was animportant source for wide compatibility genes in addition to the ausand javanica-type rice cultivars.


Table 2.18. Recently screened rice cultivars with wide compatibility. Source: Zhu andYang 1992; Sun and Cheng 1994.

Country

China

Japan

Philippines

Universitiesor institutes

Beijing AgriculturalUniversity

China National HybridDevelopment Center

China National RiceResearch Institute

Guizhou Academy ofAgricultural Sciences

Huazhong AgriculturalUniversity

Fujian Academy ofAgricultural Sciences

Jiangsu Academy ofAgricultural Sciences

San-Ming PrefecturalAgricultural ResearchInstitute of FujianProvince

Sichuan AgriculturalUniversity

Wuhan University

Zhejiang AgriculturalUniversity

Tropical AgriculturalResearch Institute

IRRI

WCVs

P40, Chang-Mao-Nuo, Ai-Zi-Zhan,Bai-Lian-Dao-Gu

Pei-C312, CY85-41, CY85-43, Pei-C 116,Pei-Cl18, Pei-Ai64, Lun-Hui422,Pei-Ai64s, Lin-Lun, lin-PeL CB-l, AB78

T984, T986, Pecos, Chugoku 91, L201,Gogo Serah, Nggonemal, Tanggalasi,Senatus Madumi, Nova 76, Newbonnet,Bluebonnet, Bluebelle, Changnot

Bai-Ke-Jing-Dao, Huang-Ke-Jing-Dao

Hao-MeL Lemont, 822, 0046, B5580A1-15

Vary Lava 1312

02428, Guang-Kang-Jing 2

SMR, 68-83, CR44-38. BJ8. IR4-114-3-2-1,g4025-2, g4135-1

CA527, CA529, CA537, CA544, Lemont.Bellemont, Jian-12

MCP231-2. MCP231-4. MCP231-6,MCP231-7. 69 series. 8925s, 8926s

T8340. Er-Jiu-Feng. IR58, Xin-Guang s,Xiu-shui 117

Aus 373, Dular. CPSL017, Calotoc, KetanNangka, Tykuchern, Kuchem, NK4,DV149. KaladumanL DV52, AS35.Lepudumai. Padi Bujang Pendek.Norin PL9

BPI76, N22. Moroberekan, PBMN I,

Fossa HV, Palawan. Lambayeque 1

Intermediate Type Between indica and japonica. During the evolutionof cultivated rice, intermediate types between typical indica and typical japonica rice have arisen. These intermediate types may have widecompatibility.


Progeny from indica-japonica Crosses. 'T984' was developed from multiple crosses between '300' (a cultivar from an intersubspecific cross),'IR26' (indica) and 'C57' (a japonica R line from an intersubspecificcross). The progeny from indica-japonica crosses are similar to the intermediate type. Some rice cultivars from the United States, Korea, and theIndian 'CR' cultivar system belong to this type and may have wide compatibility. Besides the screening method, anther culture was also successfully employed to develop WCVs (Chen et al. 1997b; Yang et al.1997).

2. Evaluation of WCVs. Selection of testers is fundamental to evaluating WCVs. Ikehashi and Araki (1984) first selected 'IR26' and 'IR50' asindica testers, and 'Nihonbare' and 'Akihikari' as japonica testers. WCVsshould have over 90% pollen fertility and over 75-80% seed set whencrossed with testers. Later they suggested four tester cultivars: 'AcharBhog', 'Ketan Nangka', 'IR36', and 'Taichung 65' or 'Akihikari'. TheInternational Rice Research Institute evaluated WCVs using 'Akihikari','Toyonishiki', and 'Taichung 65' as japonica testers, and 'IR36', 'IR50',and 'IR64' as indica testers (Gu et al. 1991; Gu, You, and Pan 1991). TheChina's National Two-line System Hybrid Rice Research CooperativeGroup selected first the following as japonica testers: 'You-Mang-ZaoSha-Jing' (early-season from Shanghai, China), 'Banilla' (mid-seasonfrom Italy), and 'Akihikari' (mid-season from Japan); the first selectedindica testers were: 'Nan-Te-Hao' (early-season from Jiangxi of China),'Nan-Jing 11' (mid-season from Jiangsu of China), and 'IR36' (mid-season from IRRI). If the pollen fertility and seed set of the crosses betweenthe cultivar being tested and all six testers are over 70%, it is classifiedas a first-rate WCV. If the pollen fertility and seed set are over 70% inthe crosses with only five testers, the cultivar is considered a second-rateWCV.

In later practice the order of wide-compatibility testing ability of thesix testers was proven to be: 'Nan-Te-Hao' > 'Nan-Jing 11' > 'IR36' forthe indica testers, and 'Banilla' > 'Akihikari' > 'You-Mang-Zao-Sha-Jing'for the japonica testers (Gu et al. 1991; Gu, You, and Pan 1991). Therefore, the Cooperative Group chose 'Nan-Jing 11' and 'IR36' as indicatesters, and 'Akihikari' and 'Banilla' as japonica testers (Gu 1992). Min(1990) suggested a statistical standard for the evaluation of the seed setoftestcrossing F1s be established, rather than an absolute value becauseenvironmental conditions affect the F1 seed set. The concept "spectrumof wide compatibility" was proposed for breeding practice, using onlyWCVs having both high and a wide spectrum of compatibility to breedfor intersubspecific hybrids (Min 1990; Yuan et al. 1997).

2. HYBRID RICE: GENETICS. BREEDING, AND SEED PRODUCTION 95

E. Utilization of Intersubspecific Heterosis

Intersubspecific hybrids are expected to increase the genetic diversityof parental lines, to improve on some undesirable traits of indica, and toadd tolerance in adverse conditions. Some promising indica-japonicahybrids were developed with 10-50% yield increase over the checks,and are under different levels of field trials. Some, such as 'Pei-Ai64s xE32', 'Kanto Kou 1', and 'Ouu Kou 1', are ready for release to farmers(Ikehashi et al. 1994; Yuan 1998a).

1. Problems from Using indica-japonica Heterosis. Problems encountered during the initial use of intersubspecifc heterosis are describedbelow. Most of these have been overcome in recent years.

Low Seed Set. The compatibility of the WCVs still varies between different subspecies and even different cultivars. For 133 intersubspecificcrosses between 12 WCVs and 12 testers, the average seed set percentages were 68.1% for 33 indica-japonica crosses, 71. 7% for 45 indicajavanica crosses, and 77.3% for 60 japonica-javanica crosses. Thesequence of fertility was: indica-japonica crosses < indica-javanicacrosses < japonica-javanica crosses, which was the reverse of thesequence for the level of heterosis (Yang and Li 1989). QTL mapping ofreproductive barriers in indica-japonica hybrids indicated that nineQTLs on chromosomes 1, 3,4,5,7,8, and 12 increased sterility and onlyone QTL (stj-6) at chromosome 6 increased fertility (Liu et al. 1997c).

Superiority to Parental Lines in Plant Height and Growth Duration. Theplant height of the intersubspecific crosses is normally greater than for theparents. This is not a problem if the parental lines are selected to haveallelic semidwarf gene(s). Similarly, by selecting the proper parental lineswith short or even medium growth duration, the growth duration problems can be overcome. There was no cytoplasmic effect on the growthduration of the indica-japonica crosses (Li 1990, 1991a; Sun et al. 1993).

Poor Grain Filling. Poor grain filling was found to be caused by a number of factors, including senescence, sink-source problems, and nutrientand water barriers. Senescence of the intersubspecific cross (W6154s xAB240) arises from the WCG donor, 'CPSL017' (Zhu and Liao 1990).Root senescence may also be one of the causes for poor grain filling(Chen, Deng, and Ma 1992). The problems of coordination betweensource and sink are due to the long growth duration of intersubspecificcrosses. As a result, the panicle size or sink is large, while the source is


relatively limited (Lu et al. 1992; Li and Ren 1994). The growth durationof the hybrid 'W6154s x AB240' was 144 days and panicle size was 301spikelets per panicle, so most grains were not completely filled. The seedset was only 40.3% with three separate filling stages on a panicle. Thelack of sink-source coordination was confirmed by removing leaves andpanicles (Zhu and Liao 1990; Chen et al. 1991). Nutrient and watertransportation barriers of the rice plant were also observed to have a negative impact on grain filling. The poor flow of photosynthetic productsto the panicles of intersubspecific hybrid rice is the main cause for poorgrain-filling (Zhu et al. 1997). Experiments showed that in intersubspecific crosses only 64.5-75.4% of 14C assimilation product was transported to the panicle while 15-19% remained in the flag leaves duringspikelet formation. Furthermore, 14C was not detected in the roots of theprogeny. In contrast, 80.0% of 14C assimilation product was transportedto the panicles and 1.2% to the roots of progeny from the intercultivarcrosses (Chen, Deng, and Ma 1992).

2. Strategy for Utilization of Intersubspecific Heterosis. The intersubspecific heterosis level tends to be: indica-japonica F1 > indica-javanica F1 > japonica-javanica F1 > indica-indica F1 > japonica-japonica F1

(Yuan 1990, 1992a,c; Zeng et al. 1997; Zhang et al. 1997b). In comparison with the three-line system indica intercultivar hybrid 'Shan-You 63',the indica-japonica F1 hybrids 'Zao-Xian-Dang x 02428' and '3037 x02428' had 7.4-41.0% greater yield, 49.4-52.4% more spikelets/unitarea, 13.0-15.8% more biomass/unit area, and a 6.4-14.9% higher economic coefficient. They also showed higher photosynthetic efficiency(Gu et al. 1989; Lu et al. 1991). Heterosis of 140-170% was observed inthe crop growth rate (CGR) during the first 30 days after transplanting,110-125% heterosis in yield and higher tolerance of low temperature inthe indica-japonica hybrids (Kabaki et al. 1992). Unfortunately, incrosses between typical indica and typical japonica cultivars, though thevegetative heterosis is large, it is not coordinated with the reproductiveheterosis and thus is difficult to utilize (Yang 1990b). It was suggestedthat low seed set of indica-japonica hybrids under low temperatures possibly originated from the complementation of two pairs of genes fromboth parents (Li et al. 1996a). Furthermore, the indica-japonica hybridsgenerally have poor grain quality due to the segregation of quality traitsin hybrid grains (Khush and Aquino 1994). To overcome these problems,Yuan (1991a,b) proposed the following breeding strategy for intersubspecific heterosis utilization: (1) Instead of typical indica or typicaljaponica cultivars, javanica cultivars, biased indica or biased japonicarice cultivars should be selected as parental lines. It has been proven that


heterosis is best exploited in crosses between the U.S. rice WCVs orintermediate-type cultivars and indica or japonica rice (Yuan 1995; Liet al. 1995). IRRI's study also suggested that hybrids derived from crosses(indica x temperate japonica) had lower heterosis while hybrids fromcrosses (indica x tropical japonica) had higher yield than the ones fromindica-indica and crosses (tropical japonica x tropical japonica) (Virmani 1994a; lnt. Rice Res. lnst. 1995). (2) Plant height should be about110 em. This height facilitates larger panicles without subsequent lodging. (3) In addition to dominance, the over-dominance and additiveeffect should both be utilized in intersubspecific heterosis breeding. (4)The panicle size of the intersubspecific F1 should be 20% larger than thecurrent modern inbred cultivars and three-line system hybrids, withmore primary branches on the panicles. (5) High photosynthetic efficiency and a high ratio of grain weight/leaf area should be achieved. Ahigher ratio of grain weight to leaf area indicates a more effective transformation from vegetative heterosis to reproductive heterosis in intersubspecific crosses. (6) To overcome poor grain filling ofintersubspecifccrosses, the grains of both parental lines should be plump. A recentstudy revealed that the well-filled grains of the indica-japonica F i s of'Ce03 x Yang-Dao 4' and 'Lun-Hui 422 x 3037' resulted from the selectedparents having superior grain plumpness. In contrast, poor grain fillingoccurred in the intersubspecific cross 'PC311 x IR36', in which both parents had poor grain plumpness. These results support that selectingparents with superior filling contributes to the success of interspecificrice hybrids (Q.S. Zhu, pers. commun.). (7) To ensure good grain quality, indica parental lines can be crossed with the long-grain javanica cultivars, and japonica parental lines with short-grain javanica cultivars.

To obtain high production of hybrid rice seed, indica cultivars are generally developed as the male sterile line and japonica as the R line,because the flowering time of indica cultivars is earlier than for japonica cultivars under natural environments. However, some breeders havesuggested utilizing intersubspecific heterosis with japonica as the malesterile line and indica as the R line to solve the grain quality problems(Li and Wu 1993).

WCVs can also be used in three-line hybrid rice breeding. The progenyfrom crosses with many WCVs such as 'CPSL017', 'Calotoc', 'KetanNangka', and '02428' have unfavorable plant type or senescence. TheseWCVs are ofless value in the development of a new restorer line with widecompatibility (Zhang and Deng 1990; Zhang, Xie, and Chen 1992). Restoring gene(s) and wide compatibility gene(s) do not appear to be geneticallylinked (Cui et al.1993; Yan and Xue 1995). Hence, a parental line withboth restoring gene(s) and wide compatibility gene(s) can be developed


through genetic recombination (Liao et al. 1991; Luo et al. 1994). Forexample, it is relatively easier to develop a restorer with wide compatibility from 'Lun-Hui 422', an improved WCV (Luo and Yuan 1989).

VII. HYBRID RICE SEED PRODUCTION

A. China's Success

The success of hybrid rice in China is due to the successful developmentof parental lines and hybrids, and, even more important, the high yieldof hybrid rice seed production. During the last 20 years there have beenlarge gains in the total hybrid rice seed production and in hybrid productivity (Table 2.19). China's hybrid rice seed production can be generally divided into the following three developmental phases:

Phase 1 (1973-1980): This phase was the early developmental stageof the techniques of hybrid rice seed production and multiplication.The average seed production was 0.45 t/ha.

Table 2.19. Area, production, and productivity of hybrid rice(PI) seed production in China, 1976 to 1994. Source: Li andYuan 1996.

Area Production ProductivityYear (ha) (t) (t/ha)

1976 85,126 23,365 0.2741977 200,613 72,822 0.3631978 270,120 128,847 0.4771979 218,434 118,282 0.5421980 172,353 119,268 0.6921981 110,400 73,857 0.6991982 154,600 140,531 0.9091983 138,800 179,052 1.2901984 104,733 148,145 1.415

1985 87,667 145,045 1.654

1986 100,533 200,563 1.995

1987 154,067 309,674 2.010

1988 135,827 221,058 1.627

1989 171,866 336,170 1.9561990 191,987 431,970 2.2501991 124,733 280,898 2.2521992 139,389 333,976 2.3961993 105,959 234,593 2.2141994 117,111 261,392 2.232


Phase 2 (1981-1985): This phase was the establishment stage during which techniques for seed production and A line multiplicationwere perfected. Yield of hybrid rice seed production increased to0.91 t/ha in 1982 and to 1.65 t/ha in 1985.

Phase 3 (1986-present): This has been the exploratory stage forsuper-high-yielding techniques of hybrid seed production. The average seed production level per hectare was 2.25 tin 1991 and 2.4 t/hain 1996 (Yuan 1998b), with the highest being 7.4 t/ha (Yuan 1996;Mao et al. 1998).

B. Key Techniques

1. Choice ofFavorite Climatic Conditions. Conditions favorable for normal flowering are a daily average of 24-28°C, 70-80% relative humidity, 8-10°C difference between the day and night mean temperatures,and sunny days with a breeze. Flowering should occur when the seasonal high temperature has ended and the low temperature season hasnot yet started (Xu and Li 1988).

2. Ensuring Flowering Synchronization. Heading date of the A lineshould be one or two days earlier than that of the R line. Currently theone-date-seeding technique for the R line is practiced for high-yieldinghybrid seed production in China. The advantages are a large amount ofeffective pollen, a high effective spikelet ratio of AIR, a high pollendensity of the R line, and more pollen grains on each stigma.

Methods for Determining the Seeding Interval for the Parental Lines.Three methods are primarily used to determine the difference in seeding date that synchronizes the A and R lines (Hunan Hybrid RiceResearch Center 1993). For all three methods, the first sowing date of theR line is taken as the reference date.

In the growth duration method, prior data concerning the differencein duration from seeding to initial heading between the A and R line arechecked and used to determine the proper seeding date of both parents.This method is simple and easy to apply. In regions where temperaturevaries greatly during the vegetative growth period, however, the earlyseeded R line will have a different growth duration each year. For a seeding date of the A line adjusted only according to the growth duration,there will sometimes be a great discrepancy in the synchronization offlowering. Therefore, this method is only used in seasons or regionswhere temperature fluctuation is small.


The leaf number is used to determine the appropriate seeding datessince the leaf number is relatively stable in rice and the leaf count is agood record of the physiological age for rice plants. The leaf number onthe main culm is used to determine the difference in seeding datebetween the two parents. More than 10 seedlings are required for reliable observation, and observations must be recorded every three days.A "three-ratings" criterion (i.e. 0.2 for just emerged, 0.5 for half emerged,and 0.8 for almost opened leaf) for quantifying the leaf age has beenadopted in China's hybrid seed production practice (Yuan 1985). Counting starts when the first complete leaf emerges on the main culm.

In the effective accumulated temperature (EAT) method, the EATfrom $eeding to initial (10%) heading is relatively stable within a cultivar, but does differ with the seeding date. For rice plants, 12°C is generally used as the lower temperature limit and 27°C as the upper limit.The formula employed to calculate the EAT follows: A = I(T - H - L)where A is the EAT of the specific time duration (OC); T is the daily meantemperature (OC); H is the temperature above the upper limit (27°C), computed for only the days that the daily mean temperature is greater than27°C; L is the lower limit temperature (12°C), computed for only the daysthat the daily mean temperature is greater than 12°C, and the accumulation of the temperature is carried out from the beginning to the end ofa specific growth stage. When the EAT from seeding to initial headingis available for both the A and R lines, the seeding date for the parentalline with a shorter growth duration may be determined based on the EATdifference. The EAT of a cultivar varies by region, therefore it is best touse locally recorded temperatures.

At the beginning ofthe commercialization of hybrid rice in China, theEAT and the leaf number method were widely adopted to determine theseeding interval. Later practices demonstrated that the EAT methodsometimes was unreliable because it relied on forecasted temperaturesand there can be a difference in sensitivity to changes in temperaturesbetween parental lines. One parental line may not change its growthduration in response to the EAT change. The leaf number method ismore accurate, but the growth duration and leaf growth rate will vary forthe spring seasons of different years because the temperature varies considerably between the spring seasons of different years in some regionssuch as Hunan, China. Predictions of the three methods described aboveare closely correlated and hence can be used complementarily to determine the seeding interval. Generally, the leaf number is used as the mainmethod and the other two are used to provide supporting information,especially for China's early-season hybrid seed production. The growthduration method is effective for the single-cropping or late-season hybrid


seed production because the temperature is much less variable for different years. A recent study in India has confirmed that the leaf numberis a reliable parameter for determining the seeding intervals (Viraktamath et al. 1998; Vijayakumar et al. 1998). Predicative regression formulas are available in China based on prior EAT, leaf number, andgrowth duration data.

Prediction of Flowering Stage. The leaf growth rate of both parentallines of the seed production plots should be observed every few days topredict heading date. For hybrid rice seed production, even if the seeding interval between both parents is accurately determined, synchronization in flowering still might not be attained because of variations intemperature and/or differences in field management. The most effectiveand widely used method for prediction of heading date is by examiningthe developmental stages of young panicles. Based on their morphological features, the young panicles are classified into eight developmental stages (Table 2.20).

Table 2.20. Stages of young panicle development in rice.

Number ofDuration of days before

days heading

Stage Female Male Features Female Male

I 2 2 Differentiation of first bract primordium 25-27 30-32II 2-3 3-4 Differentiation of primary branch 22-24 27-30

primordiumIII 3-4 4-5 Differentiation of secondary branch 18-21 22-26

primordium; young panicle is about1 mm long and covered with white hairs

IV 5 6-7 Differentiation of stamen and pistil; 18-25 19-22appearance of glumes, young panicleis 0.5-1.0 cm long

V 3 3 Formation of pollen mother cells; 12-15 16-19floret about 1-3 mm long, youngpanicle 1.5-5.0 cm long

VI 2 2 From prophase I of meiosis to formation of 9-11 12-15tetrad; floret about 3-5 mm long and youngpanicle about 5-10 cm long

VII 6-7 7-9 Filling phase of pollen; floret and panicle 8-9 9-11reach full length and color turns to green

VIII 2 2 Mature pollen; panicles are to emerge shortly 2 2


Adjustment ofFlowering Stage. Current A lines in China normally havea long blooming duration. The heading date of an A line should be oneto two days earlier than the R line, in order to synchronize the peakanthesis duration of the two parental lines. Plots with predicted poorsynchronization should have their heading date adjusted as early aspossible, because earlier adjustment is more effective. Adjustment afterStage IV is only minimally effective.

The requirement for perfect synchronization of flowering of the twoparental lines means that: (1) a development of the pollen parent shouldbe one stage earlier than the seed parent during Stage I, II, and III of theyoung panicle development; (2) the seed and pollen parents should beat the same stages during the three middle stages, i.e. Stage IV, V and VI;and (3) the seed parent should be slightly earlier than the pollen parentduring the last two stages, i.e. Stage VII and VIII.

Two adjustment measures are the flowering enhancing and floweringdelay method. The flowering delay method gives a more effective adjustment and therefore it is generally used as the major method. Proper useof the two methods usually enables adjustments when difference inheading dates are five days or less. Larger differences in heading datecannot be adequately adjusted.

One major flowering delay method is application of nitrogen fertilizer(120-150 kg/ha for the A line or 30-40 kg for the R line). Granular nitrogen may be applied to the deep root system of the faster-developing parent to delay the heading date approximately four to five days. If the laborforce does not permit this, the plots to which the nitrogen will be appliedcan be drained to expose the mud surface. After one to two days forabsorption of the nitrogen, the plot can be again flooded. The effect ofthis nitrogen application method cannot last long because some of thefertilizer may be washed away. For plots with poor synchronizationand too much nitrogen, additional nitrogen should not be reappliedheavily to delay the development of young panicles. For Chinese ricehybrids, the young panicle development of most A lines can be promoted by drying, while that of most R lines will be delayed by drying.If the drying method cannot be employed, the rate of young panicledevelopment speed may be slowed by cutting some roots. To promoteyoung panicle development by about two days, solutions such as 12 gGA3 plus 60 g KHzP04 can be sprayed on the leaves of the later parent.In some cases cutting leaves and roots and removing early flowering panicles of the male can effectively adjust the flowering date (Feng 1984).Lingaraju et al. (1998) reported that, by spraying GA3 at 60 ppm at thefull boot leaf stage, the flowering of 'IR58025A' can be advanced by three


to five days, and with an application of urea or phosphorus at 20 kg/ha,flowering can be delayed by two days.

3. Population Establishment for High-yielding Hybrid Seed Production. Population establishment for high-yielding hybrid seed productioninvolves row ratio, planting density, and row orientation.

Row Ratio. The row ratio is the ratio of the male: female row numbersin hybrid seed production plots. The row ratio is adjusted according tothe growth duration, growth vigor, the pollen load, and the plant heightof the R line. The principles for determining the row ratio includemaximization of the number of A line rows based on the pollen supply of the R line and maximization of the width of each A line row inorder to reduce the shading of the A line by the R line, thus improvingthe microclimate of the field for growth and normal flowering of the Aline.

A row ratio of 1:8 to 1:10 or 2:14-16 (for early- or mid-maturing Rlines) or 2:18-20 (for late-maturing R lines) is widely used at present inindica hybrid seed production, and 1:6 or 2:8-10 in japonica hybrid seedproduction. If the R line has adequate pollen, the row ratio may beincreased even more (Li and Yuan 1996). Outside China it has beenreported that for F1 hybrid seed production the best row ratios are: 1:6,2:4, 2:8, 2:10, 2:12, and 3:10 in different seasons (dry or wet) or at different locations (Sahai and Chaudhary 1985; Sahai et al. 1987; Sharmaand Virmani 1994; Singh et al. 1997; Prabagaran and Ponnuswamy1997a; Singh et al. 1998).

Planting Density. In China, about 45,000 hills/ha are generally neededfor the R line. The plants are transplanted with two or three seedlingsper hill and a spacing of 15 cm and 200-250 cm from one row of therestorer to the next, with the A line rows in between. For seed production plots planted with double rows of the R line, the double rows arespaced at about 17-20 cm with spacing 20-35 cm between the R lineplants. The A line is transplanted, two seedlings per hill, at a spacing of12 x 13.3 cm for a density of approximately 300,000 hills/ha of the A lineplants (Huang et al. 1994).

Row Orientation. Row orientation should be nearly perpendicular to thedirection of the prevailing wind during the heading stage. This enhancesthe cross-pollination (Wan 1989).


4. Improving Outcrossing Potential

Development of Male Sterile Lines with High Outcrossing Traits. Development of good outcrossing traits in male sterile lines is essential for highyield of hybrid rice seed. A lines with higher exsertion of the stigma, especially a double-sided exsertion rate, usually have higher outcrossingpotential (Xu and Shen 1988; Tian 1991; Elsy et al. 1998). More detail onthe outcrossing mechanism in rice is described by Virmani (1994a).

GA3 Application. GA3 plays an important role in China's hybrid rice seedproduction. It can be used to adjust the physiological and biochemicalmetabolism of rice plants as described below. Its stimulation of elongation of the juvenile cells provides its major role by: (1) enlarging theangle between the flag leaf and the main culm by 15-20 degrees, (2)enhancing elongation of the three uppermost internodes, (3) increasingthe panicle exsertion from the flag leaf sheath, (4) increasing the angleof opening of glumes when flowering, (5) increasing the stigma exsertion of the female parent, and (6) increasing the 1,000-grain weight.

Because of its effect on panicle exsertion, the dosage of GA3 appliedin China was increased from 7.5-45 g/ha in the 1970s to 60-90 g/ha inthe early 1980s to 180-270 g/ha by the late 1980s and 150-180 g/ha forthe early 1990s. Some farmers use up to 300 g/ha (Duan and Ma 1992).Currently, GA3 application guidelines recommend 150-180 g/ha whenusing a knapsack sprayer (Liu 1997) and 135 g/ha when using an ultralow-volume (ULV) sprayer. A typical spraying schedule is as follows:

Knapsack sprayer: total GA3 = 150-180 g/ha1st spray: 40 ppm (30 g in 750 L/ha)2nd spray: 80-100 ppm (60-75 gin 750 L/ha)3rd spray: 80-100 ppm (60-75 gin 750 L/ha)

ULV sprayer: total GA3 = 135 g/ha1st spray: 667 ppm (15 g in 22.5 L/ha)2nd spray: 2667 ppm (60 g in 22.5 L/ha)3rd spray: 2667 ppm (60 g in 22.5 L/ha)

The time of emergence of 1-5% panicles is the best stage for GA3 application, but GA3 can be applied until 10% panicle emergence. Thespraying is best done from 7:00 to 11:00, with the next treatment administered between 15:30 to 19:00. GA3 should not be applied during blooming or at noon. The spraying interval should be as follows: (1) Threeapplications of GA3 on consecutive days, if started at 1-5% panicleemergence. The application should be from 7:00 to 11:00, from 15:30 to19:00 of the same day, and from 7:00 to 11:00 of next day. (2) Two appli-


cations of GA3 from 7:00 to 10:00 of two consecutive days if started at10% of panicle emergence, (3) One application of GA3 if sprayed at30% panicle emergence. GA3 has no effect after 50% of panicle emergence. Recent studies showed that application in the early morning gavehigher seed yield than application in the late afternoon (Tong and Ma1997) and the best time for the GA3 application was two equal splits inconsecutive days (dosage = 100g/ha) at 15-20% panicle emergence(Prabagaran and Ponnuswamy 1997b).

GA3 is dissolved in 75-90% alcohol (1 g in 30 to 50 ml) one weekahead of spraying, with 5 to 8 g neutral detergent!activator added pergram ofGA3• Plots are re-sprayed if there is rain within 6 h after a spray.Spray is applied with 3 to 5 em of standing water on the field and 30 to37.5 kg of urea/ha or 2% urea is applied with GA:l if early senescenceappears. The daily mean temperature during GA3 application should beover 25°C for the best effect. The GA] dosage should be doubled if thedaily temperature is 22°C.

GA3 is very costly outside China, so ULV sprayers should be used forGA] application. It was reported that the dosage of GA3 can be reducedto 15-45 g/ha (lnt. Rice Res. Inst. 1992; Huang et al. 1994; Ahmed et al.1997b). But the ULV sprayer should not be used if the wind velocity isabove 3 m/sec. Because of the expense of GA3 , substitutes are currentlybeing sought. Mangiferin or 1.5-2.0% urea or 1.5% boric acid is as effective as GA:l in increasing hybrid seed set. A young leaf extract of Albiziaamara may also be an alternative treatment to GA: l in hybrid rice seedproduction (Prasad et al. 1988; Singh and Sahoo 1997; Ponnuswamy andPrabagaran 1997).

Sllpplementary Pollination. Shaking the R line panicles using ropepulling or rod-driving during anthesis can greatly assist in the releaseof pollen grains from the anthers. This process is even more effective oncalm days than on breezy days.

When seed production plots are irregular in shape or uneven in topography, and where there is sufficient manual labor, the rod-drivingmethod (using a bamboo stick to stir the canopy layer of the R or B lines)is recommended (Virmani and Sharma 1993). Under other conditions,the rope-pulling method is practiced. With this rope-pulling method,panicles of the R line are shaken by pulling a long nylon rope (about 4mm in diameter) and walking against the wind at a speed of 1 to 1.5m/sec. The rope should run parallel to the parental rows.

These supplementary pollination procedures are generally conductedin the morning when the A line is flowering. If the R line is floweringbut the A line is not in the morning, these procedures should not be


used. In the afternoon, when the R line is still blooming, supplementarypollination should be continued even if the A line has closed its glumes.Generally, the supplementary pollination is repeated at intervals of 15to 30 min three to five times daily until no pollen remains on the R line.It is not needed when the wind is stronger than a moderate breeze (Virmani and Sharma 1993).

A recently established technique of supplementary pollination seemsmore efficient for increasing the outcrossing rate (P. J. Huang, pers. commun.). This method emphasizes that the best time to conduct supplementary pollination is at the peak of pollen shedding of the R line,instead of at 30-minute intervals. To predict the peak stage of pollenshedding, observations should be made every ten minutes and the number of florets blooming during this interval should be recorded. The Rline is considered to be at the beginning of the anthesis peak stage whenthe average number of blooming florets per panicle is more than fivewithin ten minutes. The best time for the supplementary pollination iswithin 30 minutes thereafter.

The rod-driving method is more effective than the rope-pullingmethod because it creates a more even pollen distribution. Under supervision by a technician, the highest hybrid seed yield can be attained byperforming the supplementary pollination simultaneously at multiplesites within a sizable seed production area. This will create a well-distributed pollen "fog" (Huang and Tang 1990).

5. Ensuring Purity

Isolation. To ensure purity of the hybrid seed, the hybrid seed production plots should be strictly isolated in space and time. An isolation distance of more than 100 m is generally necessary for (A x R) hybrid seedproduction. No other cultivars should be grown within this area duringthe same season except the pollen parent. The required isolation distanceseems to vary with location and season (Prasad and Virmani 1989). IRRIscientists reported that at least 22-31 m of isolation distance was needed(Sharma et al. 1987; Muker and Sharma 1991). Generally, time isolationrequires a period of 20 days, Le., unwanted cultivars within the 100 mdistance from the seed parent should flower at least 20 days earlier orlater than that of the pollen parent. Under some conditions, topographical features such as hills, woods, rivers, or tall crops (e.g. maize, sugarcane, and sorghum) that cover more than 30 m might provide necessaryisolation. To produce a small amount of seed for the replicated yield trials or other purposes, an isolation cloth or plastic sheet of at least 2 minheight is normally used as a barrier to prevent unwanted pollination.


Roguing. Parental lines are easily contaminated during the hybrid seedproduction, so it is necessary to thoroughly rogue the fields. Purity ofhybrid rice seed used for commercial production in China must be over98%. This requires the purity of the R line and A lines to be more than99% (Table 2.21).

Roguing should be done two or three times, including before heading,at the initiation of heading, and before harvest. Any maintainer and semisterile plants in the A line rows, and all other off-type plants in both themale and female rows should be completely removed from the field.

Characteristic features used to guide roguing include: (1) off-typeplants-This is based on color of the leaf sheath and leaf, size of leafblades, growth and developmental status, plant type, plant height, andgrowth duration; (2) maintainer florets phenotype-The basal part of themaintainer's panicles normally exerts out of the flag leaf sheath and theanthers of the maintainers should be yellow, plump, and completelydehiscent after anthesis; (3) anther phenotype-Anthers of semi-sterileplants are slightly larger than sterile anthers of the A line, yellowish incolor, and partly dehiscent after flowering; the nondehiscent anthers ofsemi-sterile plants become dark yellow several hours after anthesis.

6. Field Management

Raising Productive Seedlings. For more productive seedlings with multiple tillers, the seed parent should be seeded sparsely and evenly at aseeding rate less than 150 kg/ha. When these seedlings have two leaves,70-100 kg/ha ofurea should be applied to promote tillering. This topdressing should be repeated seven days before transplanting. Spraying40-60 g of MET (Multi-effect Triazole or Paclobutrazol) with 900 kg/hawater when seedlings have 1.1 leaves controls seedling height andpromotes tillering. The seedlings of A lines are generally transplanted

Table 2.21. The minimum standards for nucleus seeds (NS) and foundation seeds(FS). Source: Yuan 1985.

Seed Purity Cleanness Germination Moisture Sterility RestoringLine grade (%) (%) (°lc,) (%) (%) (%)

A line NS 100 >99.8 >93 <13.5 100FS >99.8 >99.5 >93 <13.5 100

B line NS 100 >99.8 >98 <13.5FS >99.8 >99.5 >98 <13.5

R line NS 100 >99.8 >98 <13.5 >85FS >99.8 >99.5 >98 <13.5 >85


at about six leaves. At present, more and more farmers are practicing thetwo-step method. This method includes raising R line seedlings at aseeding rate of 1500 kg/ha and transplanting seedlings (heel-in) temporarily with 2-3 seedling/hill at leaf number 2.5 and 10 x 13.3 em spacing. The final transplanting of early- or mid-maturing R lines should beconducted at the seventh to eighth leaf number, and late-maturing Rlines at the eighth to ninth leaves.

Water and Fertilizer Management. All field management after seedingaims to increase the total floret number and to assure the flowering synchronization. After planting, the soil moisture should cause the parentalline seeds to germinate in a short time for better subsequent synchronization. When the third leaf of the female parent appears, nitrogenshould be applied to promote the plant growth. Tillering fertilizer shouldbe applied to produce more tillers at the early tillering stage. The A lineshould have 3.1-3.5 million tillers/ha within 30-32 days after emergence. This facilitates production of 2.5 million effective panicles/ha.Shortly before panicle differentiation, healthy plants should have leaveswith slightly light green color. If the color is too light, some N, P, and Kfertilizers should be applied to promote conversion of the vegetativegrowth to reproductive development, which helps the formation of largepanicles. However, too much nitrogen will likely make the upper threeleaves droopy and fragile and have negative impact on the spread of thepollen and flowering synchronization. The best N:P:K ratio is 2:1:1.5 forhigh-yielding hybrid seed production. Application of this fertilizer at thelater stages enhances acceptivity of the pollen grains by the A line andalso increases the pollen shedding percentage of the R line. Therefore,60-90 kg/ha of urea and 45-60 kg/ha of KCl should be applied at StageV of the young panicle development, along with four applications ofKHzP04 and boron fertilizer shortly before or after heading at respectivedosages of 15 kg/ha and 1.5 kg/ha.

Before heading, water should be kept on the plot. If the plot dries out,even temporarily, at the peak tillering time, poor synchronization willresult from the different or reverse reaction of the plant development ofthe A and R lines. After the grain-filling stage begins, the seed production plots should be irrigated at intervals to fill the grains and protectthe plants from diseases.

C. Specifics for CMS Line Multiplication

The techniques of A line multiplication are basically the same as forhybrid seed production, but there are some differences. Stricter isolation


is required. At least 500 m of isolation is needed to ensure maintenanceof purity during multiplication of the CMS line. Seeding intervals varywith genotypes. An A line and its maintainer line are like twins and donot differ greatly in their growth duration. However, sporophytic indicaA lines are four to six days later in heading than their B lines, so theseA lines should be seeded earlier. The first seeding of the B line shouldbe when the A line has 1.5 leaves. The second seeding of the B lineshould be when the A line has 2.5 to 3 leaves. The gametophytic japonica A lines are similar to their corresponding B lines in days to heading,so in these cases the first seeding of the B line is at the same time as theA line. The second seeding of the B line should be when the leaf number of the A line is 1.5 to 2.0 leaves, about five to seven days after thefirst seeding. Regardless of their seeding dates, both the A and B linesare transplanted on the same day.

There is little difference in plant height between the A line and thecorresponding B line. Due to its later seeding, the B line is inferior to theA line in tillering capacity and growth vigor. Therefore, their row ratioshould be smaller than for the F1 seed production. The widely adoptedB/A row ratio for CMS line multiplication plots is 1:3 or 2:5. However,2:6-10 was reported to be the optimum row ratio for CMS line multiplication (Ahmed et al. 1997b; Singh et al. 1998).

To maximize the seed yield of the multiplication plots requirespromoting the growth of the B line. The B line generally needs to betransplanted with soil-intact seedlings to shorten duration of the transplanting shock. A quick-releasing fertilizer should also be applied to theB line.

An ideal population infrastructure for CMS line multiplication is asfollows:

A lineBasic tillers: 1.8-2.1 million/haMaximum tillers: 4.5 million/haProductive panicles: 3.0-3.6 million/haTotal florets: 300-360 million/ha

B lineBasic tillers: 0.4-0.6 million/haMaximum tillers: 2.1 million/haProductive panicles: 1.2-1.4 million/haTotal florets: 105-120 million/ha

Panicle ratio: B:A = 1:3

Floret ratio: B:A = 1:2.5-3


D. Purification of Parental Lines

1. Deterioration of Parental Lines. Deterioration of parental lines leadsto deterioration of F1 hybrids as a result of decreases in seed set and uniformity. Male sterile lines may deteriorate because of either segregationfor plant type, maturity and other traits, or due to a breakdown in sterility. Other traits may contribute to the deterioration of an A line including poor fertility restorability, decreased combining ability, increasedproportion of unopened florets, reduced proportion of stigma exsertion,less desirable flowering traits, and diffused anthesis time. Similarly, Band R lines may deteriorate and cause maintaining and restoring abilities to become weaker. Their combining ability may decrease with insufficient pollen supply and reduced pollen shedding.

2. Causes of Admixture of Parental Lines. Admixture of parental linesmay be due to pollen contamination from outcrossing, mechanical mixture during harvest and postharvest procedures such as threshing, drying, cleaning, transportation, and storage, or genetic variation present inthe parental populations.

3. Purification Method ofAIB/R Lines: Several methods are available forpurification of A, B, and R lines. The simplest and most effective methodwith regard to practical utility involves the use of testcross, identification, and multiplication nurseries, during the following four steps:

Selection of Elite Plants. Elite individual plants of the three parentallines are selected based on agronomic traits, sterility, and resistance todisease and pests.

Testcross and Backcross Nurseries. Paired crosses are made and theindividual eMS line plants selected are testcrossed and backcrossed tothe R line and to the B line. The number of paired crosses depends onavailable labor. In general, a minimum of 50 pairs of (A x B) are required,with each pair producing more than 100 hybrid seeds. Likewise, 50pairs are required for (A x R) combinations, but each should give morethan 200 hybrid seeds.

Identification Nursery. Three nurseries are used for identification.(1) The sterility identification nursery must be a well-isolated plot withthe A line and its B line planted in pairs in the plot. At the initial heading stage, the male sterility of every plant of the A line should be evaluated. If the A line has uniform traits, good flowering behavior, necks


that are not enclosed or only slightly enclosed, and the percentage ofmale sterile plants as well as the degree of sterility is 100%, these linesare retained with their corresponding B line. (2) Heterosis evaluationnursery consists of about 100 F1 plants for each pair of (A x R) crosses.The F1 plants are evaluated for heterosis including growth vigor, tillering capacity, percentage of productive tillers, seed set rate, uniformity,resistance to stress conditions, and grain yield. The best of these (A x R)crosses are then selected. (3) In the R line evaluation nursery, about100-200 plants of the R line for each of the F1s are grown in isolation.The R lines are evaluated for purity, uniformity, flowering behavior, andthe performance of the R lines and of their hybrids. The best male families (R lines) are chosen and allowed to set seed.

Bulk Multiplication. Seeds of the selected A and B lines are separatelyharvested in bulk and sown in isolation for the core seed production ofthe A and B lines. The selected restorer families are also harvested inbulk and the seeds of each are sown in another isolated plot for core seedproduction of an R line.

VIII. FUTURE PROSPECTS

World population increased from two billion in 1930 to five billion in1987 and is likely to reach eight billion in 2020 (Beachell 1989). Overthe last 30 years the population in Asian rice-growing countries, wheremore than 90% of the world rice crop is produced and consumed, hasincreased by 60%. Fortunately, rice production in these countries hasdoubled due to the spread of modern inbred and hybrid rice cultivars.Although the rice research community is proud of this remarkableprogress in increasing productivity, there is no reason for complacency.Rice production has to increase 1. 7% annually to meet the growingdemands, despite the fact that the rice-growing area continues to decline.Since 1989, the global rice production has plateaued at the level of 520million 1. Annual increase of rice production was only 1.8% during the1985-1993 period, compared to 2.8% during 1975-1985, and 3.6% forthe prior decade (Hossain 1996). It is clear that the food crisis will reemerge in some rice-growing countries without new technical breakthroughs for rice production. Hybrid rice technology developed in Chinamust be transferred to other countries, even as China continues todevelop improved methods for producing hybrid rice in the face of itsown increasing population. The following are the main objectives forfuture investigations of hybrid rice production.


A. Breeding of Diverse Parental Lines

1. Further Utilization of Rice Germplasm. Currently, only 30/0 of 1,000accessions of the germplasm of hybrid rice have been used commercially(Luo and Yuan 1990; Ying 1994). Fortunately, disruptive genetic vulnerability has not occurred in China's hybrid rice. But there is a definiterisk of genetic vulnerability due to pleiotropy, like the southern maizeleaf blight epidemic during the 1970s in the United States (Tatum 1971).This risk exists because about 93% of the A lines used in commercialhybrid production in China belong to the WA-type. As an example ofthepotential dangers facing the current commercial hybrid rice, 'Shan-You63', one of the most popular rice hybrids, has already lost its resistanceto rice blast resulting in drastic reduction in yield in mountainous regions. Diversification to broaden the rice germplasm base seems essential to solving this problem. Already a series of hybrids such as'Gang-You 22', 'Shan-You-Duo-Xi 1', and 'K-You 3' have been releasedto replace 'Shan-You 63' in China in recent years (Wang 1996). To addressthis problem, Virmani et al. (1986) considered identification and use ofadditional sources of cyto-sterility critical to preventing genetic vulnerability of the three-line system for hybrid rice to disease or insect epidemics. Some new cytoplasmic sources for male sterility have also beenidentified, such as 'V20B' (an A line with different male sterile cytoplasmfrom WA-tye A lines), 'Kalinga' (Pradhan et al. 1990), CMS-ARC (Virmaniand Dalmacio 1987; Virmani et al. 1989), 'IR66707A' from O. perennis(Acc. 104823) (Dalmacio et al. 1992, 1993, 1995), O. glumepetala and'IR62829B' (Int. Rice Res. Inst. 1995). Three new and diverse CMS sources-one is from O. rufipogon and two are from O. nivara-were recentlyidentified using substitution backcrossing and the embryo rescue technique. Among them 'RPMS1' and 'RPMS2' showed gametophytic malesterility with a restorer reaction different from WA-type CMS lines (Hoan1993; Hoan et al. 1997a,b, 1998). In addition, two novel lines were produced from BT-type CMS sources using the asymmetric protoplast fusiontechnique (Blackhall et al. 1998) and, in India, some diversified CMSsources were identified from crosses O. nivara (105343) x C045, O. barthii(100934) x IR50, and O. nivara (101508) x IR64 (Rangaswamy and Jayamani 1998). Besides the diversification of cyto-sterility sources, a recentstudy has revealed that introgression of gene(s) of agronomic importancefrom wild rice species such as O. rufipogon into the parental lines forhybrid rice might further enhance the productivity of hybrid rice (Xiaoet al. 1996b, 1998; Tanksley and McCouch 1997).

2. Further Improvements of Adaptability to Environmental Stresses.CMS lines derived from the Chinese WA sterile plant, such as 'V20A(B)'and 'Zhen-Shan 97A(B)', are not adaptable to the tropics or subtropics


(Suprihatno et aL 1994; Yogeesha and Mahadevappa 1996). Dependableand diverse parental lines need to be developed for tropical environments. Since rice hybrids generally have better drought resistance aswell as better yield performance under drought conditions, uplandhybrid rice should be studied and developed for the increasing proportion of non-irrigated rice land within and outside China.

3. "New Plant Type" and Super-high-yielding Hybrid Rice Breeding.Integration of heterosis and superior plant type should further increaseyield potentiaL If the new plant type, in association with direct seeding,can increase non-hybrid rice yield by 20-25%, and reach 13t/ha, usingthe new plant type in hybrid rice breeding might give an additional20-25% yield advantage and increase this yield to 15 t/ha of grain (Yang1987; Khush and Aquino 1994; Khush 1995; Khush and Peng 1996; Pingali et al. 1997). Breeding for "new plant type (NPT)" was initiated at theInternational Rice Research Institute in 1989. The donor germplasms aremostly from bulu cultivars, belonging to javanica or tropical japonica.Breeding objectives for this new plant type were described as follows: (1)low tillering capacity of only three to four tillers when directed seeded;(2) 200-250 grains per panicle; (3) no unproductive tillers and harvestindex of about 0.6; (4) sturdy stems; (5) dark green, thick, and erectleaves; (6) a vigorous root system; (7) 90 cm height; and (8) 100-130 daygrowth duration (Peng et al. 1994; Khush 1995). Based on the NTP design,some promising lines such as 'IR65598-112-2' have been developed.However, there are still a number of constraints, including low biomassproduction, poor grain filling, pest susceptibility, and early flag leafsenescence (Khush, Peng, and Virmani 1998). Yuan (1997b, 1998a) proposed the criterion for super-high-yielding hybrid rice of 100 kg/ha yieldper day, with the model plant type being: 100 cm plant height with a 70cm culm length, long, erect narrow, V-shape, and thick uppermost threeleaves, a moderately compact plant type and moderate tillering capacity,5 g of panicle weight and about 2.7 million panicles/ha, a 6-6.5 leaf areaindex (LAI) ofthe upper three leaves, and 0.55 for the harvest index. Thetwo-line hybrid 'Pei-Ai64s x E32' has proven this new strategy for superhigh-yielding hybrid rice breeding. This hybrid had a growth duration of130 days, yielded 13.3 t/ha on a total area of 0.24 ha at three locations inJiangsu Province of China in 1997, and reached a daily gain of 100 kg/ha,the criterion for super-high-yield breeding (Yuan 1998a).

B. Molecular Breeding

Recent advances in rice biotechnology, particularly molecular mappingand genetic transformation, have opened new avenues in hybrid ricebreeding.

114 J. 11 AND 1. YUAN

1. Marker-assisted Breeding. A PCR-mediated method for selecting Rf1 restorer lines recently used the close linkage between the Rf-l locusand a particular PCR-amplified fragment (Ichikawa et al. 1997). It hasbeen proposed that a new molecular marker (OSRRf) closely linked tothe nuclear restorer gene (Rf-l) be applied not only in the developmentof new R lines and B lines, but also for the purity management of thehybrid rice seeds (Akagi et al. 1996; Fujimura 1996). A defective atp6gene conferring the male sterility in BT type CMS lines was reported tobe located on the mitochondrial genome, so the sequence of this genewas to design PCR primers for detection of CMS lines. For the two-linesystem, a RAPD marker linked with the PGMS of'Nong-Ken 58s' and anAFLP marker (AF3) closely with tmsl in '5460s' have also been identified (Wang et al. 1995b; B. Wang, pers. commun.). The use of microsatellite markers has also been suggested as a method for screening CMSresources and removing unfavorable alleles and heterozygotic patternsfrom parental lines (Liu and Wu 1996; Liu et al. 1997b). It is predictedthat the MAS (marker-assisted selection) technique will assist in thedetermination of genetic diversity, identification and accumulation ofheterotic gene(s), and improvement of plant traits. The ability of thistechnique to detect the presence or absence of any number of alleles ofinterest in one screening is particularly attractive to rice breeders.

2. Other Potential Biotechnologies. Tissue culture and genetic transformation techniques will be more extensively employed in future hybridrice breeding. Protoplast fusion enables the direct transfer of CMS intoelite rice breeding lines as well as the development of alloplasmic lineshaving cytoplasms from various wild species and related genera. PlantGenetic Systems in Belgium has pioneered the research and development of a genetically engineered "male sterility gene" for producinghybrid rice. The genes for nuclear male sterility (barnase) and fertilityrestoration gene (bastar) were cloned and transferred to crops includingtobacco and rape (Lasa and Bosemark 1993). Two more organizations,Paladin Hybrids in Canada, and ICI in the UK, have also patented genetically engineered dominant nuclear male sterility systems (Cutler 1991).These systems provide an opportunity to develop robust systems fortwo-line hybrid rice (Mariani et al. 1990 and 1992; Brar et al. 1994; Jefferson and Nugroho 1998).

C. Apomixis Breeding

Since Navashin and Karpechenko in the 1930s demonstrated the valueof apomixis in fixing F1 heterosis, apomixis has been an attractive


research subject, because through the use of apomixis it is possible todevelop true-breeding hybrids, produce high-quality pure seed withoutisolation requirements, and enhance the selection of a variety of morehighly adapted genotypes (Solntzeva 1978). Apomixis, which has beencriticized as a threat to the seed industry and ignored by the seed business in a number of developing countries (Koltunow et al. 1995), couldbe protected potentially by intellectual property rights and a patentingsystem or potentially through the engineering of technical tricks inapomixis breeding (Savidan 2000). If successful, the development ofapomictic rice would also enable resource-poor farmers in developingcountries to adopt high-yielding hybrid rice technology (Khush, Brar,and Bennett 1998).

1. Definition and Classification. "Apomixis," practically defined (Nogler1984a) as asexual reproduction through seeds, can be classified into thefollowing three major categories: (1) apospory; (2) diplospory; and (3)adventitious embryony (Savidan 2000). Apomixis can be either obligateor facultative. Apomixis is the predominant mode of reproduction in theobligate type, while apomixis is combined with sexual reproduction tosome extent in the facultative type. Breeding facultative apomicts generally is more difficult than breeding obligate apomicts (Bashaw 1980a).Apomixis exists in more than 300 plant species from 35 families(Bashaw 1980a; Hanna and Bashaw 1987), including a number of fruitcrops, and in the wild relatives of some important agronomic crops.Apomixis is also common in the grasses and in several polyploid plantspecies. Among the major cereals, maize, wheat, and pearl millet haveapomictic relatives.

2. Inheritance. Genes governing apomixis may be governed by a singledominant gene as in Panicum maximum (Savidan 1983), Ranunculus(Nogler 1975, 1984b), Poa pratensis (Matzk 1991), Brachiaria (Lutts etal. 1994; do Valle et al. 1994; do Valle and Savidan 1996; Miles andEscandon 1997), Amelanchier (Campbell and Wright 1996), F1 maizeTripsacum hybrids (Leblanc et al. 1995), Tripsacum (Grimanelli et al.1997), and Citrus (Parlevliet and Cameron 1959; Iwamasa et al. 1967).Apomixis has also been reported to be controlled by two or three genesin Parthenium argentatum (Powers 1945), Bothriochloa spp. (Harlan etal. 1964), Pennisetum ciliare (Taliaferro and Bashaw 1966; Gustine et al.1989), and Poa pratensis (Funk and Han 1967) or a group of genes (Gerstel et a1. 1953; Savidan 1982). Apomixis is generally thought to be controlled by dominant genets), but some studies have indicated that arecessive gene or genes control apomixis in Paspalum notatum (Burton


and Forbes 1960; Asker 1970), Panicum maxicum (Hanna et al. 1973),and Paa pratensis (Akerberg and Nygren 1959; Grazi et al. 1961). In 1977Carman suggested that apomixis did not originate from specific apomictic genes or alleles but from asynchronously-expressed duplicate genescontrolling female development. Apomixis probably is so complex thatit cannot result from a single-locus mutation. The whole process mightbe divided into individual components and the mutations affecting eachof those may be identified separately (Savidan 2000). Environmental factors such as temperature, photoperiod, and a number of chemical agentssuch as MH and gibberellic acid (den Nijs and van Dijk 1993), also affectthe expression or stability of apomixis, especially facultative apomixis.

3. Breeding Approaches. Various apomixis breeding procedures wereproposed by Taliaferro and Bashaw (1966) and Bashaw and Funk (1987)for bufflegrass (Cenchrus ciliaris) based on a two-gene model, Pernes etal. (1975) and Hanna (1995) for guineagrass based on a one-dominantallele model, and Hanna (1995) based on a recessive-gene model. Several modified procedures for Brachiaria, Tripsacum, Paspalum, andCitrus are detailed in Savidan's review (2000). Peacock also proposed asynthetic lethal system in screening apomictic mutants. The firstapomictic cultivar was the Japanese forage cultivar 'Natsukaze' (Sato etal. 1990).

4. Breeding Apomictic Rice. The basic requirements for fixing rice heterosis using apomixis should involve: (1) embryo development fromnucellar cells or 2n embryo sac cell without meiosis; (2) obligate type ofapomixis; (3) dominant inheritance involving one or a few gene(s); (4)normal endosperm development; and (5) stable expression of apomixisover environments (Sun and Cheng 1994). Major strategies for developing apomictic rice are (1) screening germplasm of tetraploid wild speciesas a source of apomixis and transferring the apomictic trait to rice cultivars; (2) inducing apomictic mutants in rice through mutagenesis(seeds and fertilized egg cells can be induced with gamma rays, X rays,EMS, and NED; and (3) use of molecular approaches.

In the screening of 108 accessions of tetraploid Oryza species forapospory (multiple embryo sac development) and 86 accessions fordiplosory (based on callose detection), including five related genera, noevidence of apomixis was found (Brar et al. 1995). Rutger (1992) alsoreported a similar negative result after screening 547 accessions ofrelated wild rice species having the AA genome. Since screening andconfirming rice apomixis from both rice cultivar and wild relativesfailed, the most promising approach would be transfer of apomixis in


other grasses via protoplast fusion, direct DNA transfer, and genetictransformation. Molecular markers have been identified in progenyof maize (Tripsacum) and in crosses of sexual and apomictic wildspecies of Pennisetum. The ORSTOM-CIMMYT project is attempting totransfer apomixis from Trisacum dactyloides to maize through widehybridization (Savidan et al. 1994). Two markers, one RFLP marker(UGT197) and one RAPD marker (OPC-04), were reported to be linkedto apospory in Pennisetum (Ozias-Akins et al. 1993). In maizeTripsacum F1 hybrid, five markers, Le. umc28, csu68, umc62, umc71,and CD0202, were also found linked to apomixis (Leblanc et al. 1995;Grimanelli et al. 1997). Cloning of gene(s) for apomixis from apomicticplant species including Tripsacum, Pennisetum, Brachiaria, andCenchrus is presently under way. Once such genes become available,they could be introduced into elite breeding lines of rice using transformation technology. But genetic engineering of apomixis requires adetailed understanding of the genetic basis and the molecular mechanisms that control megasporogenesis, megagametogenesis, fertilization,and seed development. For the selection of progeny of rice apomicts, Virmani (1994a) proposed the following indicators: (1) identical maternalprogeny from plants of cross-pollinated species, or progeny of F1 crosses;(2) limited or no genetic variation in the F2 population of a cross betweentwo distinct parents; (3) recessive genotypes from a cross of parentswith recessive genes pollinated with a parent possessing a dominantmarker gene; (4) unusually high seed fertility in aneuploids, triploids,and wide crosses normally expected to be sterile; (5) aneuploid chromosome number or structural heterozygosity remaining constant fromparent to progeny; and (6) multiple seedlings per seed, multiple stigma,multiple ovules per floret, and double or fused ovaries. There are reportsof mutants possessing twin seedlings per seed (Yuan et al. 1990; Sharmaand Virmani 1990) and multiple pistillate ovaries (Suh 1988). A recentstudy by Shi et al. (1996a,b) showed that no apomictic phenomenaexisted in the rice line '84-15', which was once presumed to be anapomictic rice, as many papers from other crops referred to occasional,spontaneous, or induced haplo- or diplo- parthenogenesis as apomixis(Chen et al. 1988, 1992; Chen 1989; Asker and Jerling 1992). Besides '8415', China identified some rice materials with abnormal sexual reproduction process, including SAR-l (Zhou et al. 1991b, 1993b), HDAR (Caiet al. 1991; Yao et al. 1997), Cl00l (Guo et al. 1991; Wu et al. 1991), APIAPIV (Li, Deng, and Yuan 1990; Li and Yuan 1990); PDER (Ye et al.1995), 322B (Huang 1988), PYl and PJ5 (Liu, Chen, and Zheng 1990),W3338 (Luo, Zhou, and Wang 1991), CDAR (Yan et al. 1991), and ABF(Zhao et al. 1992). However, cytoembryological studies indicated that


these materials were of little use in fixing rice heterosis (Sun and Liu1996). All egg cells are produced via meiosis in SAR-1, C1001, andAPIV. HDAR only has low-frequency abnormality of megasporogenesisand occurrence of double embryo sacs. A recently identified material'TAR' (3n 36) is under study. In this material the meiosis of the megaspore was hindered and an unreduced embryo sac was produced viamitosis, in which embryos were formed via egg cell parthenogenesis, andendosperms were formed via pseudogamy of polar nuclei (Sun and Liu1996).

More and more new genetic tools and techniques may contribute torice breeders' ability to handle apomixis, including tissue culture,embryo rescue, somatic embryogenesis, the Herr-clearing technique,and biochemical or molecular marker-aided selection, thus enhancingthe breeding for apomixis. Either the discovery of practically usablesource material(s) from the huge rice gene pool or introduction ofapomixis gene(s) from other plant species to rice through biotechnologies will make this "utopian scheme" (Hermsen 1980) for using apomixisto fix rice heterosis come true.

D. Hybrid Seed Production

1. Mechanical Seed Production. Simplification of the labor-intensiveand complicated procedures of China's hybrid rice seed productionpractices should be explored. F1 seed production in China utilizes theplanting of the male sterile line and a pollinator line in alternative rowsrequiring much labor. Separation of selfing R line seeds and the F1 seedson the basis of difference in color, size, or other traits after planting thepollinator and male sterile plants as mixed seed has been proposed. Atechnique for mechanical separation of hybrid and inbred seed using aphotoelectric seed-sorting apparatus and hull color has been patented,and in the future may provide the necessary hybrid seed purity (Barabas1974; Kato et al. 1994; Suzuki et al. 1990). In addition, use of a femalesterile pollinator and incorporation of a herbicide-sensitive gene into apollinator were suggested to facilitate mechanical harvest of the hybridseed (Maruyama et al. 1991b). Herbicide sensitivity to chemicals suchas bentazon (3-[1-methyethyl]-[IH]-2,1,3-benzothiodiazin-4-[3H]-one2,2-dioxide) has been introduced into parental lines to destroy the pollenparent before it sets seed, thereby eliminating R line seed contamination of the F1 seed (MoTi 1984). There was also a report in China thatherbicide-resistance genes were successfully introduced into parentallines of hybrid rice via particle bombardment transformation to protectthe F 1 seed producing plants from herbicide injury (Huang et al. 1998).


Exploration of possible mechanization of other processes involved inhybrid rice seed production is under way, such as GA3 application byan airplane and modified techniques of pollen collection, storage, andspraying (Li et al. 1996c).

2. Monitoring Purity of Hybrid Rice Seed. In the 1980s it was proposedthat some isozyme markers such as esterase isozymes might be used tomonitor seed purity (Yan 1987; Glaszmann et al. 1987; Shi et al. 1988a,b;Li 1991b). Since the 1980s a practically usable technique incorporatingisozyme markers and seed scanning has proven to be helpful in monitoring purity of hybrid rice seed at the Hunan Hybrid Rice Research Center. A PCR technique was recently used to determine seed purity of'Shan-You 63' by using the P18 primer to amplify a specific band of 0.8kb from the restorer line 'Ming-Hui 63' and thereby separate the truehybrids (Qian et al. 1996). Based on the low cost and effectiveness, B.Wang (pers. commun.) proposed that STS and AFLP markers also canbe used to monitor the hybrid rice seed purity. It is predicted that seedpurity can be monitored more accurately and economically by usingmolecular markers.

E. Socioeconomic Impact

Apart from the technological aspects, the success of hybrid rice technology in China is due primarily to its profitability and government support. Other countries with a high labor-land ratio and a high proportionof irrigated area, such as India, Indonesia, the Philippines, Sri Lanka, andVietnam, are likely to have the highest potential demand for hybrid ricetechnology. The availability of market opportunities for hybrid rice willnot be a limiting factor for the private sector if government policies aremade less restrictive and unfair competition from the public sector iseliminated. Hybrid rice could have the same catalytic effect on thehybrid rice seed industry that hybrid maize had on the seed industry inNorth America (Sehgal 1994). To popularize the use of hybrid rice, boththe NARS and private sectors should identify target areas for hybrid ricecultivation or seed production. An effective hybrid seed productionand distribution system must be established to stabilize the price of certified hybrid seed at a reasonable level and maintain hybrid seed purityin the long term. For example, the "seed-producing village" has beenproved to be an effective practice for hybrid rice seed production bothin China and India. However, policies for hybrid seed production anddistribution of quality hybrid seeds are still on the drawing board inmost countries other than China. With the successful establishment of


a commercialization system for hybrid rice in countries, includingChina, it is probable that 18-20 million ha will be planted each year inthe next three to five years. That would mean an increase of more than18-20 million t of rice production, which would be worth US$1,800-2,000 million per year for the whole rice world.

LITERATURE CITED

Ahmed, M. 1. 1997. Development and use of hybrid rice technology-Lessons learned fromthe Indian experience. Int. Rice Comm. News!. 46:75-83.

Ahmed, M. 1., K. Krishnaiah, and E. A. Siddiq. 1997a. Enhancement of yield heterosis forsustenance of hybrid rice technology in India. p. 78-79. In: J. G. Coors, S. Pandey, M.V. Grinkel, A. R Hallauer, D. C. Hess, K. R Lamkey, A. E. Melchinger, G. Srinivasan,and C. W. Stuber (eds.), The genetics and exploitation of heterosis in crops. Proc. Int.CIMMYT Symp., 17-22 August 1997. ASA, CSSA, SSSA, Madison, WI, CIMMYT. Mexico. (Abstr.)

Ahmed, M. 1., S. Singh, C. H. M. Vijayakumar, and M.S. Ramesha. 1997b. Standardizationof row ratios and GA3 application in hybrid rice seed production in India. p. 122-123.In: J. G. Coors, S. Pandey, M. V. Grinkel, A. R Hallauer, D. C. Hess, K. R. Lamkey, A.E. Melchinger, G. Srinivasan, and C. W. Stuber (eds.), The genetics and exploitation ofheterosis in crops. Proc. Int. CIMMYT Symp., 17-22 August 1997. ASA, CSSA, SSSA,Madison, WI, CIMMYT. Mexico. (Abstr.)

Ahokas, H., and Hockett, E. A. 1977. Male sterile mutants of barley. IV. Different fertilitylevels of msg9ci (cv Vantage) an ecoclinal response. Barley Genet. News!. 7:10-11.

Akagi, H., M. Sakamoto, C. Shinjyo, H. Shimada, and T. Fujimura. 1994. A uniquesequence located downstream from the rice mitochondrial atp6 may cause male sterility. Current Genet.25:52-58.

Akagi, H., T. Taguchi, and T. Fujimura. 1995. Stable inheritance and expression of the CMStraits introduced by asymmetric protoplast fusion. Theor. App!. Genet. 91:563-567.

Akagi, H., Y. Yokozeki, A. Inagaki, A. Nakamura, and T. Fujimura. 1996. A codominantDNA marker closely linked to the rice nuclear restorer gene, Rf-1, identified with interSSR fingerprinting. Genome 39:1205-1209.

Akbar, M., and T. T. Yabuno. 1975. Breeding for saline-resistant varieties to rice. III.Response of F1 hybrids to salinity in reciprocal crosses between Jhona 349 and Magnolia. Japan. J. Breed. 25:215-220.

Akerberg, E., and A. Nygren. 1959. Poa pratensis, trivalis, palustris, compressa und verwandte Arten. p. 392-418. In: H. Kappert and W. Rudorf (eds.), Ziichtung der Futterpflanzen.

Ali, A. J., M. Rangaswamy, R Rajagopalan, S. E. N. Mohamed, and T. S. Manickam, 1998.TNRH16: A salt-tolerant rice hybrid. Int. Rice Res. Notes 1998(2):22.

Ali, J., E. A. Siddiq, F. U. Zaman, M. J. Abraham, and 1. Ahmed. 1995. Identification andcharacterization of temperature sensitive genic male sterile sources in rice (Oryza sativaL.). Indian J. Genet. Plant Breed. 55:243-259.

Ali, S. S., and M. G. Khan. 1998. Hybrid rice research in Pakistan. Int. Rice Res. Notes1998(2):17.

Anandakumar, C. R, and S. R Sreerangasamy. 1986. Heterosis and inbreeding depressionin rice. Oryza. 23:96-101.


Araki, H., K. Toya, and H. Ikehashi. 1988. Role of wide-compatibility genes in hybrid ricebreeding. p. 79-83. In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10,1986,Changsha, China). Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Armenta-Soto, J. L. 1988. Agronomic performance of hybrid rice in northern Mexico. p.291. In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10, 1986, Changsha,China). Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Asker, S. 1970. Apomixis and sexuality in the PotentjJla argentea complex. II. Crosseswithin the complex. Hereditas 66:189-204.

Asker, S. K, and 1. Jerling. 1992. Apomixis in plants. CRC Press, Boca Raton, Florida.Aswathanarayana, S. c., and M. Mahadevappa. 1991. Method of application of gameto

cides in hybrid seed production of rice. J. Maharashtra Agr. Univ. 1991(2):198-200.Aswathanarayana, S. C., and M. Mahadevappa. 1992. Effects of gametocides in hybrid seed

production ofrice. J. Maharashtra Agr. Univ. 1993(1):14-16.Athwal, D. S., and S. S. Virmani. 1972. Cytoplasmic male sterility and hybrid breeding in

rice, p. 615-620. In: Rice breeding. Int. Rice Res. Inst., P.O. Box 933, 1099 Manila,Philippines.

Bai, D. 1., and X. H. Luo. 1996. Liangyou Peite, a new two-line hybrid rice released inChina. Int. Rice Research Notes 1996(1):42-43.

Bai, S. N., and K. H. Tan. 1990. The discovery of the specific protein in HPGMR rice. p.245. The 5th Symp. of Chinese Society of Plant Physiology. (Abstr.).

Bai, S. N., and Y. H. Xiao. 1988. A study of the root growth and respiration of hybrid rice.Acta Agron. Sinica 1988(1):53-59.

Barabas, Z. 1962. Observation of sex-differentiation in sorghum by use of induced malesterile mutants. Nature 195:257-259

Barabas, Z. 1974. U.S. Patent Number: 3842538.Barwale, B. R. 1994. Hybrid rice: Food security in India. Macmillan India Ltd.Bashaw, E. C. 1980a. Apomixis and its implication in crop improvement. p. 45-68. In: W. R.

Fehr and H. H. Hadley (eds.), Hybridization of crop plants. Am. Soc. Agron., Madison, WI.Bashaw, K C. 1980b. Registration of Nueces and Llano Buffelgrass. Crop Sci. 20:112.Bashaw, K c., and C. R. Funk. 1987. Breeding apomictic grasses. In: W. R. Fehr (ed.), Prin

ciples of cultivar development: Vol. 2. Crop species. Macmillan, New York.Beachell, H. M. 1989. The need for a global rice research system. p. 315-324. In: Progress

in irrigated rice research. Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.Bennett, M. D., and W. G. Hughes. 1972. Additional mitosis in wheat pollen induced by

Ethrel. Nature 240:566-568.Bharaj, T. S., S. S. Bains, G. S. Sidhu, and M. R. Gagneja. 1991. Genetics of fertility restora

tion of wild abortive cytoplasmic male sterility in rice Oryza sativa 1. Euphytica56:199-204.

Bharaj, T. S., S. S. Virmani, and G. S. Khush. 1995. Chromosomal location offertility restoring genes for 'wild abortive' cytoplasmic male sterility using primary trisomics in rice.Euphytica 83:169-173.

Blackhall, N. W., H. Akagi, T. FujimakL J. P. Jotham, M. R. Davey, J. B. Power, and E. C.Cocking. 1998. Diversification of cytoplasmic male sterility systems through somaticcell hybridization. In: S.S. Virmani, KA. Siddiq, and K. Muralidharan (eds.), Advancesin hybrid rice technology (Proc. 3rd Int. Symp. Hybrid Rice, Hyderabad, India, October 1996). Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Bollich, C. N. 1957. Inheritance of several economic quantitative characters in rice. Diss.Abstr. 17:1638.

Bollich, C. N., J. N. Rutger, and B. D. Webb. 1992. Developments in rice research in theUnited States. Int. Rice Comm. News!. 41:32-35.


Bollich, C. N., and B. D. Webb. 1973. Inheritance of amylose in two hybrid populationsof rice. Cereal Chern. 50:631-636.

Bong, B. B., and V. P. Singh. 1993. Cooking quality characteristics of hybrid rice. IndianJ. Genet. Plant Breed. 53:8-13.

Borkakati, R, and S. S. Virmani. 1993. Inheritance of a thermosensitive genic male-sterilemutant of indica rice. Rice Genet. Newsl. 10:92-94.

Borkakati, R P., and S. S. Virmani. 1996. Genetics of thermosensitive genic male sterilityin rice. Euphytica 88:1-7.

Brar, D. S., R M. Elloran, M. C. Lanuang, V. S. Tolentino, and G. S. Khush. 1995. Screening wild species of rice for apomixis. Philippine J. Crop Sci. 20:6.

Brar, D. S., T. Fujimura, S. R McCouch, and F. J. Zapata. 1994. Application of biotechnology in hybrid rice. p. 51-62. In: S. S. Virmani (ed.), Hybrid rice: New developmentsand future prospects, Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Brar, G. S. 1982. Male sterility in sesame. Indian J. Genet. Plant Breed. 42:23-27.Brewbaker, J. L. 1964. Agricultural genetics. Prentice-Hall, Englewood Cliffs, NJ.Brown, F. B. 1953. Hybrid vigor in rice. Malaysian Agr. J. 36:226-236.Bruce, A. B. 1910. The mendelian theory of heredity and the augmentation of vigour. Sci

ence 32:627-628.Burton, G. W., and I. Forbes. 1960. The genetics and manipulation of obligate apomixis

in common bahiagrass (Paspalum notatum Flugge). p. 66-71. In: Proc. the VIIIth Int.Grassl. C~ngr., Univ. of Reading, U.K.

CaL D. T., P. F. Ma, H. X. Guan, J. L. Yao, Z. A. Wang, and H. Zhu. 1991. Study of highfrequency apomixis in rice. J. Huazhong Agr. Univ. 10:223-227.

CaL H. W., X. Q. Wang, K. S. Cheng, and Y. Z. Zhang. 1992. Classification of Asian ricesby esterase isozymes. Southwest China J. Agr. Sci. 1992(5):19-24.

Campbell, C. S., and W. A. Wright. 1996. Apomixis, hybridization, and taxonomic complexity in eastern north American Amelanchier (Rosaceae). Folia Geobotania & Phytotaxonomica 31 :345-354.

Capinpin, J. N., and P. Singh. 1938. A study of varietal crosses and hybrid vigor in rice.Philippine Agr. 27:255-277

Carman, J. G. 1997. Asynchronous expression of duplicate genes in angiosperms may causeapomixis, bispory, tetraspory, and polyembryony. BioI. J. Linn. Soc. 61:51-94.

Carnahan, H. L., J. R Erickson, S. T. Tseng, and J. N. Rutger. 1972. Outlook for hybrid ricein the USA. p. 603-607. In: Rice breeding, Int. Rice Res. Inst., P.O. Box 933, Manila 1099,Philippines.

Castle, W. E. 1946. Genes which divide species or produce hybrid vigor. Proc. Nat. Acad.Sci. (USA) 32:145-149.

Chalam, G. V., and J. Venkateswarlu. 1965. Introduction to agricultural botany in India.Vol. I. Asia Publish. House, New Delhi.

Chan, Y. W., and C. H. Cheah. 1983. Evaluation of etherel as a selective gametocide in thebreeding ofrice (Oryza sativa L.). SABRAO J. 15:149-155.

Chandraratna, M.F. 1964. Genetics and breeding of rice. Longman, London.Chang, T. M. 1974. Studies on the inheritance of grain shape of rice. J. Taiwan Agr. Res.

Inst. 23:9-15.Chang, T. T., C. C. Li, and O. Tagumpay. 1973. Genotypic correlation, heterosis, inbreed

ing depression and transgressive segregation of agronomic traits in a diallel cross of rice(Olyza sativa L.) cultivars. Bot. Bul. Acad. Sinica (Taipei) 1973(2):83-93.

Chang, T. T., Y. B. Pan, Q. R Chu, R Peiris, and G. C. Loresto. 1991. Cytogenetic, electrophoretic, and root studies of javanica rices. p. 21-32. In: Rice genetics II. Proc. 2ndInt. Rice Genetics Symp., Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.


Cheema, A. A., and M. A. Awan. 1985. Heterosis, gene action and combining ability studyof yield and some of the yield components of four-parent diallel cross in rice. Pak. J.Sci. Ind. Res. 28:175-178.

Cheema, A. A., M. A. Awan, G. R. Tahir, and M. Aslam. 1988. Heterosis and combiningability studies in rice. Pakistan J. Agr. Res. 1988(1):41-45.

Chen, C. L., Z. C. Deng, P. F. Ma. 1992. The characteristics of photosynthesis and transportation and distribution of 14C photosynthates in hsien keng rice hybrid combination.J. Huazhong Agr. Univ. 11:312-316.

Chen, J. G., G. Q. Song, S. H. Qu, and y. Zhou. 1998. Direct and maternal genetic effectson grain quality characters in early hybrid rice. Chinese J. Rice Sci. 1998(2):79-84.

Chen, J. S. 1989. Identification of apomictic inheritance using dominant marker genes.Genet. Manipulation Plants 1989(1):19-23.

Chen, J. S., Q. S. Jia, L. Yin, and W. J. Cao. 1992. Study on fixing heterosis in hybrid rice.p. 69-82. In: J. S. Chen (ed.), Collection of papers on breeding of apomictic rice. ChinaAgr. Sci. Tech. Press.

Chen, J. S., Y. Lin, and C. W. Jun. 1988. A study of apomixis in rice. Genet. ManipulationsCrops News!. 1988(2):3-10.

Chen, L. B., X. Z. Li, and G. Q. Zhou. 1993. Effects of temperature on the sterility of ricein photo-sensitive and thermosensitive male sterile lines. Acta Agron. Sinica1993(1):47-54.

Chen, L. B., G. Q. Zhou, and X. X. Huang. 1994. Effects of temperature and photoperiodon fertility and physiological activities of rice Annong S-1 and Hengnong S-1. Acta Bot.Sinica 36(suppl.):119-123.

Chen, L. Y., G. T. Li, G. H. Liu, and C. L. Xiao. 1997a. Breeding of the newly-developedthree-line hybrid rice Xingxiangyou 80. J. Hunan Agr. Univ. 23:316-319.

Chen, P., and Y. H. Xiao. 1987. A preliminary study on the peroxidase activity of the anthersduring the process of pollen abortion in HPGMR. J. Wuhan Univ. (HPGMR): 39-42.

Chen, S. G., S. H. Cheng, Z. X. Sun, H. M. Si, S. K. Min, and C. S. Ying. 1997b. Selectionof 064A, a WA type male sterile line of rice with wide compatibility. Sci. Agr. Sinica1997(2):21-26.

Chen, X. B., X. J. XU. and R. Q. Liao. 1991. Study on nutritional physiology of two-linesystem hybrid rice: 1. Characteristics of sink and source and flow of photosyntheticproducts in two-line system hybrid rice. Hunan Agr. Sci. 1991(1):7-9.

Chen, X. F., and C. Y. Liang. 1991. Accumulation of hydrogen peroxide and lipid peroxidation in male-sterile anthers of Oryza sativa. Acta Phytophysiol. Sinica 17:44-48.

Chen. X. F., and C. Y. Liang. 1992. Energy and activated oxygen metabolisms in anthersof Hubei photoperiod-sensitive genic male-sterile rice. Acta Bot. Sinica 34:416-425.

Chen. X. H., and B. H. Wan. 1993. Studies on the relationship between photoperiodicresponse. thermoperiodic response and photoperiod-temperature sensitive male sterility in rice. Acta Agron. Sinica 1993(3):282-287.

Chen, Y. W. 1985. Hybrid rice in China. J. Hunan Sci. Tech. Univ. 1985(1):1-9.Cheng, K. S. 1985. A statistical evaluation of the classification ofrice cultivars into hsien

and keng subspecies. Rice Genet. Newsl. 2:46-48.Cheng, K. S. 1993. The evaluation of indica and japonica subspecies in Oryza sativa L.

Yunnan Sci. Tech. Press, Kunming, China.Cheng, S. H., H. M. Si, L. S. Zhuo, and Z. X. Sun. 1996. Classification of environmentally

induced genetic male sterile lines of rice based on their fertility responses to photoperiod and temperature. J. Agr. Sci. 127:161-167.

Cheng, S. H., Z. X. Sun, S. K. Min, Z. M. Xiong. and C. S. Ying. 1990. Studies on responseto photoperiod and temperature of the photoperiod-sensitive genic male sterile rice


(PGMSR): 1. Observation of fertility in PGMSR under natural conditions at Hangzhou(30 0 05'N). Chinese J. Rice Sci. 1990(4):157-162.

Chiang, C. L., J. Y. Zao, and Z. W. Hu. 1981. Cytological studies on pollen abortion 'V20A'and Honglian A in rice. Dept. Biology, Hunan Teachers' College, Changsha, China.

Chinese Academy of Agricultural Sciences and Hunan Hybrid Rice Research Center, 1991.The development of hybrid rice in China. Agr. Press, Beijing, China.

Chitrakon, S. 1987. Hybrid rice: Status and future in Thailand. Research Monitor (Thailand) 1987(1):4-10.

Chitrakon, S., P. Khambanonda, and P. Senawong. 1986. Hybrid rice: Status and future inThailand. Thai Agr. Res. J. 1986(2):149-157.

Choi, J. H. 1991. Status and prospects of hybrid rice research in the Democratic People'sRepublic of Korea. Int. Rice Comm. Newsl. 40:56-60.

Chou, S. T. 1978. Cytological observations on tissue structure and opening inner factorsof anther of male fertile and male sterile lines in rice. Acta Genet. Sinica 5:125-132.

Chu, C. S., X. F. Lu, K. M. Yao, X. M. Zong, C. S. Zhou, J. M. Li, and J. B. Liu. 1997. Studyon applying "cold fertility" method to low-temperature sensitive variety of sterile strainPei'ai64s. J. Nanjing Inst. Meteorology 20:425-433.

CNHRRDC, 1997. Summary report on the International Training Courses on Hybrid Rice,China National Hybrid Rice Research & Development Center (CNHRRDC), Changsha,Hunan, China.

Cress, C. E. 1977. Heterosis of the hybrid related to gene frequency differences betweentwo populations. Genetics 53:86-94.

Crow, J. F. 1997. Dominance and overdominance. In: J. G. Coors, S. Pandey, M. V. Grinkel,A. R Hallauer, D. C. Hess, K. R Lamkey, A. E. Me1chinger, G. Srinivasan, and C. W.Stuber (eds.), The genetics and exploitation of heterosis in crops: Proc. Int. CIMMYTSymp., ASA, CSSA, SSSA, Madison, WI (in press), 17-22 August 1997, CIMMYT,Mexico. p. 10 (Abstr.)

Crow, J. F. 2000. The rise and fall of overdominance. Plant Breed. Rev. 17: Chapter 5.Cui, J., Z. Y. Yang, and Y. Gao. 1993. The relationship between wide compatibility and

restorability in rice. Chinese J. Rice Sci. 1993(1):11-16.Cutler, K. 1991. Canadian seed firm offers" Male sterility technology for hire." Seed Ind.

42:4-21.Dai, Y. R, Z. R Sun, Y. R Xu, M. Z. Zhao, J. H. Zong, and Z. Z. Ling. 1978. Comparative

investigations on some respiratory enzymes and free histones in anthers from male sterile and maintainer lines of rice. Acta. Genet. Sinica 5:227-234.

Dalmacio, R, D. S. Brar, T. Ishii, L. A. Sitch, S. S. Virmani, and G. S. Khush. 1993. Identification and transfer of a new source of cytoplasmic male sterility from wild speciesOryza perennis into rice. Philipp. J. Crop Sci. 1993(suppl.1):40 (Abstr.).

Dalmacio, R, D. S. Brar, T. Ishii, L. A. Sitch, S. S. Virmani, and G. S. Khush. 1995. Identification and transfer of a new cytoplasmic male sterility source from Oryza perennisinto indica rice (0. sativa). Euphytica 82:221-225.

Dalmacio, R, D. S. Brar, L. A. Sitch, S. S. Virmani, and G. S. Khush. 1992. Oryza perennis: a new source of cytoplasmic male sterility in rice. Rice Genet. Newsl. 9:108-110.

Darwin, C. 1877. The effects of cross and self fertilization in the vegetable kingdom. D.Appleton, New York.

Davenport, C. B. 1908. Degeneration, albinos and inbreeding. Science 28:454-455.Davis, M. D. and J. N. Rutger. 1976. Yield ofF1 , Fz and F3 hybrids ofrice (Oryza sativa L.).

Euphytica 25:587-595.Deng, J. X., W. F. Uu, and Y. H. Xiao. 1990. The synthesis of nuclear acids and proteins

and ATP content in anthers of HPGMR at the developmental stage in rice. J. WuhanUniv. (Ser. Natural Science) 1990(3):85-88.


Deng, Q. Y., and X. Q. Fu. 1998. Studies of fertility stability of P(T)GMS rice. III. Drift ofcritical temperature inducing male sterility and its controlling technology. J. Hunan Agr.Univ. 1998(1):8-13.

Deng, Q. Y., X. Q. Fu, and L. P. Yuan. 1997. On fertility stability of the P(T)GMS lines andtheir identification technology. p. 76-85. Proc. Int. Symp. on Two-line System HeterosisBreed. in Crops, Changsha, Hunan, China, September 6-8,1997.

Deng, Q. Y., A. H. Ou, X. Q. Fu, and Q. R Zhu. 1996. A preliminary study on the methodfor identifying the practical photo- and thermo-sensitive genic male sterile rice. HybridRice 1996(2):23-27.

Devarathinam, A. A. 1984. Study of heterosis in relation to combining ability and per seperformance in rainfed rice. Madras Agr. J. 71:568-572.

Dhulappanavar, C. V., and S. W. Mensikai. 1967. Study of heterosis in rice. Mysore J. Agr.Sci. 1:117-122.

Dorosti, H. K H. 1997. Evaluation of heterosis among Iran's rice cultivars. In: J. G. Coors,S. Pandey, M. V. Grinkel, A. R Hallauer, D. C. Hess, K R Lamkey, A. E. Melchinger,G. Srinivasan, and C. W. Stuber (eds.), The genetics and exploitation of heterosis incrops. Proc. Int. CIMMYT Symp., ASA, CSSA, SSSA, Madison, WI (in press), 17-22August 1997, CIMMYT, Mexico. p. 342-343 (Abstr.).

Duan, X. M., and H. S. Ma. 1992. Effects of gibberellic acid application on seed yield andquality of hyhrid rice. Seed Sci. Tech. 20:209-214.

Edwardson, J. R 1956. Cytoplasmic male sterility. Bot. Rev. 22:696-738.Elsy, C. R, M. Rangaswamy, and K N. Ganesan. 1998. Significance of stigma exsertion in

enhancing outcrossing in male sterile lines. lnt. Rice Res. Notes 1998(1):25.Erickson, J. R 1969. Cytoplasmic male sterility in rice (Oryza sativa L.). Agron. Abstr. p. 6.Feng, Y. Y. 1984. Regulation of flowering date in seed production of hybrid rice. GlIang

dong Agr. Sci. 1984(6):5-9.Fu, A. J., and H. Wang. 1988. Genetical study on rice male sterility. J. Hunan Agr. Univ.

1988(1):1-6.Fujii, K, T. Kato, A. Shumiya, T. Ito, and S. Kudo. 1991. A rapid and laborsaving method

of introducing fertility restoring gene Rf-1 in hybrid rice breeding and its applicationto general back cross hreeding. Res. Bul. Aichi Ken Agr. Res. Cent. p. 27-39.

Fujimaki, H., and S. Hiraiwa. 1986. Genetic analysis of male sterile lines induced hy artificial mutation. Japan. J. Breed. 36: 401-408.

Fujimaki, H., S. Hiraiwa, K Kushihuchi, and S. Tanaka. 1977. Artificially induced malesterile mutants and their usages in rice breeding. Japan. J. Breed. 27:70-77.

FlIjimaki, H., and H. Yoshida. 1984. Development of male sterile-restoring system forhybrid rice production. 2. Variation of days to heading ofhalf-sih F 1 hyhrids with a common female parent "Akihikari". Japan. J. Breed. 36(suppl.1):196-197.

Fujimura, T. 1996. Application of molecular markers for rice breeding in Japan. PlantGenome IV, San Diego, CA.

FlIkuta, Y., M. Yano, and K Fukui. 1992. Linkage analysis of the restorer gene (Rf-1) inrice using restriction fragment length polymorphism markers. Japan. J. Breed. 42(suppI.1):164-165.

Funk, C. R., and S. J. Han. 1967. Recurrent interspecific hybridization; aproposed methodof breeding Kentucky bluegrass Poa pratensis. New Jersey Agr. Expt. Sta. Bul. 818:3-14.

Ganesan, K N., and M. Rangaswamy. 1997. Genetics offertility restoration for WA sourceof CMS lines in rice. Madras Agr. J. 84:42-43.

Gao, M. W. 1981. A preliminary analysis of genotype of hybrid hsien rice with wi Id ricecytoplasm. Acta Genet. Sinica 8:66-74.

Gerstel, D. U., B. L. Hammond. and C. Kidd. 1953. An additional note on the inheritanceof apomixis in guayule. Bot. Gaz. 115:89-93.


Ghosh, A. K., and S. Govindaswamy. 1972. Inheritance of starch iodine blue value andalkali digestion value in rice and their genetic association. Riso. 21:123-132.

Gillham, N. W. 1978. Organelle heredity. Raven, New York.Glaszmann, J. C. 1987. Isozymes and classification of Asian rice. Theor. Appl. Genet.

74:21-30.Glaszmann, J. c., O. Edralin, and S. S. Virmani. 1987. Isozyme markers to monitor seed

purity in indica hybrid rice. Int. Rice Res. Newsl. 1987(5):17.Govinda, R. K., and E. A. Siddiq. 1984. Genetics of fertility restoration and biochemical

basis of male sterility-fertility restoration system in rice. Rice Genetics Newsl.1:103-104.

Govinda, R. K., and S. S. Virmani. 1988. Genetics of fertility restoration of WA type cytoplasmic male sterility in rice. Crop Sci. 28:787-792.

Grazi, F., M. Umaerus, and E. Akerberg. 1961. Observations on the mode ofreproductionand the embryology of paa pratensis. Hereditas 47:489-541.

Grimanelli, D., O. Leblanc, E. Espinosa, E. Perotti, D. Gonzalez de Leon, and Y. Savidan.1997. Mapping diplosporous apomixis in tetraploid Tripsacum: one gene or severalgenes? Heredity 80:33-39.

Gu, F. L., C. G. Lu, M. L. Lu, H. Y. Zhu, and J. S. Zou. 1989. Biological basis of highyielding capacity in indica-japanica hybrid rice. Jiangsu Agr. Sci. 1989(suppl.):31-38.

Gu, M. H. 1988. The inheritance of wide compatibility gene (s) and its utilization in ricebreeding. J. Jiangsu Agr. College 1988(2):19-26.

Gu, M. H. 1992. Current situation and perspective for wide compatibility study in Oryzasativa, p. 23-29. In: L. P. Yuan (ed.), Current status of two-line hybrid rice research. Agr.Press, Beijing, China.

Gu, M. H, X. B. Pan, Z. X. Chen, and W. C. Yang. 1991. A study on compatibility of standard wide compatibility testers of rice in China. Sci. Agr. Sinica 1991(6):27-32.

Gu M. H., A. Q. You, and X. B. Pan. 1991. Genetic analysis of wide compatibility varietiesofrice (Oryza sativa L.). p. 768-769, In: G. S. Khush (ed.), Rice genetics II. Proc. 2ndInt. Rice Genetics Symp. Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Gu, M. H., A. Q. You, and X. B. Pan. 1992. Genetic analysis on allelic relationship of widecompatibility genes among several WCV (Oryza sativa L.), p. 259-267. In: L. P. Yuan(ed.), Current status of two-line hybrid rice research. Agr. Press, Beijing, China.

Guangdong Crop Heterosis Utilization Research Cooperative Group. 1977. Heterosis utilization by way of chemical emasculation in rice. Sci. Agr. Sinica 1977(2):47-50.

Guangdong Crop Heterosis Utilization Research Cooperative Group. 1981. Chemical emasculation and heterosis utilization in rice. Agr. Press, Beijing, China.

Guangxi Teachers' College. 1975. Cytological studies in Guan-Xuan 3 A, "WP" type of rice.p. 147-150. In: Proc. of China's National Symp. for Cooperative Research on HybridRice, November 1975, Changsha, China.

Guo, X. X., X. Y. Zeng, G. Liu, T. He, F. L. Kong, Z. X. Li, K. Luo, and D. Q. Wu. 1991. Preliminary study on apomixis of a rice line CI00l. Southwest. China J. Agr. Sci.1991(2):10-16.

Guok, H. P. 1994. Hybrid rice research in Malaysia, p. 207-211. In: S. S. Virmani (ed.),Hybrid rice: New developments and future prospects. Int. Rice Res. Inst., P.O. Box 933,Manila 1099, Philippines.

Gustafsson, A. 1946. The effect of heterozygosity on variability and vigour. Hereditas32:263-286.

Gustine, D. L., R. T. Sherwood, and J. Gounaris. 1989. Regulation of apomixis in buffelgrass. p. 411-412. In: Proc. XVIth Int. Grassland Congr., Nice, Association FraIl<;:aisepour la Production Fourragere.


Hanna, W. W. 1995. Use of apomixis in cultivar development. Adv. Agron. 54:333-350.Hanna, W. W., and W. C. Bashaw. 1987. Apomixis: Its identification and use in plant breed

ing. Crop Sci. 27:1136-1139.Hanna, W. W., J. B. Powell, J. C. Millot, and G. W. Burton. 1973. Cytology of obligate sex

ual plants in Panicum maximum Jacq. and their use in controlled hybrids. Crop Sci.13:695-697.

Hara, S. 1946. Linkage between factors for sterility and anthocyanin pigmentation in riceplant. Japan. J. Genet. 21 :32.

Haraiwa, S., and S. Tanaka. 1980. Induction of male-sterile mutation in rice. GammaField Symp. 19:103-115.

Harlan, J. R., M. H. Brooks, D. S. Borgaonkar, and J. M. J. de Wet. 1964. The nature andinheritance of apomixis in Bothriochloa and Dichanthium. Bot. Gaz. 125:41-46.

Hayes, H. K. 1952. Chapter 3. Development of the heterosis concept. p. 49-65. In: J. W.Gowen (ed.J, Heterosis. Iowa State College Press, Ames.

He, H. H., and S. e. Yuan. 1993. Research and applications of two-line system hybrid rice.Jiangxi Sci. Tech. Press, Nanchang, China.

He, H. H., Z. G. Zhang, and S. C. Yuan. 1987. Response on development and fertilitychanges induced by light under different temperature conditions in HPGMR. J. WuhanUniv. (HPGMR): 87-93.

He, Z. e. 1990. The role of the activity of superoxide dismutase in the prediction of riceheterosis. Hubei Agr. Sci. 1990(11):10-12.

He, Z.e., Y. H. Xiao, and S. Y. Feng. 1992. Distribution ofP:l2 in the plants in HPGMR (58s).J. Wuhan Univ. (Ser. Natural Science) 1992(1):127-128.

Hermsen, J. 1980. Breeding for apomixis in potato, pursuing a utopian scheme. Euphytica 29:595-607.

Heu, M. H. 1967. Studies on the growth duration and hybrid sterility in remote cross breeding of cultivated rice. J. Korean. Soc. Crop. Sci. 4:31-71.

Hoan, N. T. 1993. Diversification of male sterile cytoplasm and exploitation of hybrid vigorusing in vitro techniques in cultivated rice (Oryza sativa L.). Ph.D. Diss. Anghra PradeshAgr. Univ.

Hoan, N. T., N. N. Kinh, B.B. Bong, N. T. Tram, T.D. Qui, and N.V. Bo. 1998. Hybrid riceresearch and development in Vietnam. In: S. S. Virmani (ed.), Advances in hybrid ricetechnology (Proc. 3rd Int. Symp. Hybrid Rice, Hyderahad, India, October 1996). Int. RiceRes. Inst., P.O. Box 933, 1099 Manila, Philippines.

Hoan, N. T., N. P. Sarma, and E. A. Siddiq. 1997a. Diversification of male sterile cytoplasmwith the aid of embryo rescue technique in inter-specific hybridization for rice. p.393-394. In: J. G. Coors, S. Pandey, M. V. Grinkel, A. R. Hallauer, D. C. Hess, K. R.Lamkey, A. E. Melchinger, G. Srinivasan, and C. W. Stuber (eds.), The genetics andexploitation of heterosis in crops. Proc. Int. CIMMYT Symp., 17-22 August 1997. ASA,CSSA, SSSA, Madison, WI, CIMMYT. Mexico. (Abstr.)

Hoan, N. T., N. P. Sarma, and E. A. Siddiq. 1997b. Identification and characterization ofnew sources of cytoplasmic male sterility in rice. Plant Breed. 116:547-551.

Hoan, N. T., N. P. Sarma, and E. A. Siddiq. 1998. Wide hybridization of CMS in rice. Int.Rice Res. Notes 1998(1):5-6.

Hoff, B. J., and D. Chandrapanya. 1973. Inheritance of two male sterile characters in rice.Am. Soc. Agron. p. 7 (Abstr.)

Honda, I., S. Iwasaki, K. Sudo, H. Kato, K. Maruyama, M. Hasegawa, I. Yamaguchi, N.Murofushi, T. Yanagisawa, and N. Takahashi. 1997. Semi quantification of gibberellinsin the anthers of thermosensitive genetic male sterile rir:e (Oryza sativa L. cv. PL12).BiascL, Biatechnol. Biachem. 61:1748-1750.


Hong, L. F., J. P. Liu, B. Q. Liu, D. F. Hu, and Z. W. Mao. 1997. 1647s-a photosensitivegenic male sterile japonica rice (PGMS) developed via anther culture. Acta Agr. BorealiSinica 1997(1):41-45.

Hossain, M. 1996. Recent developments in the Asian rice economy: challenge for riceresearch, p. 17-33. In: R E. Evenson, RW Herdt, and M. Hossain (ed.), Rice researchin Asia. CAB Int. Oxford & IRRI, Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Hu, J. G., and Z. B. Li. 1985. A preliminary study on the inheritance of male sterility ofrice male sterile lines with four different kinds of cytoplasms. J. Huazhong Agr. Col.1985(2):15-22.

Hu, J., and J. N. Rutger. 1992. Pollen characteristics and genetics of induced and spontaneous genetic male-sterile mutants in rice. Plant Breeding 109:97-107.

Huang, C. S., T. H. Tseng, and C. G Chen. 1987. Genetic analysis of fertility restoration incytoplasmic male sterile indica rice. J. Agr. Res. China. 1987(2):137-150.

Huang, D. N., S. Q. Zhang, R Xue, Z. H. Hua, X. B. Xie, and X. L. Wang. 1998. A newmethod to identify and improve the purity of hybrid rice with herbicide resistant gene.Chinese Rice Res. Newsl. 1998(1):1.

Huang, G. S., Y. Y. Hu, and P. Li. 1992. Preliminary study ofthermosensitive genic malesterile lines by somatic culture and breeding in indica rice. J. Sichuan Agr. Univ.1992(1):1-6.

Huang, H. Z., S. L. Lou, H. C. Wang, and R M. Chen. 1984. Auxin destruction and malesterility in rice. J. Xiamen Univ. (Ser. Natural Science) 1984(1):82-87.

Huang, P. J., K. Maruyama, H. L. Sharma, and S. S. Virmani. 1994. Advances in hybrid riceseed production technology. p. 63-70. In: S. S. Virmani (ed.), Hybrid rice: New developments and future prospects. Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Huang, P. J., and S. D. Tang. 1990. A supplementary pollination technique for hybrid riceseed production. Int. Rice Res. Newsl. 1990(5):5-6.

Huang, Q. L., X. H. Tang, and J. L. Mao. 1994. Photoperiod and temperature response ofa photoperiod-sensitive male-sterile japonica rice line 7001s and their effect on fertility and anther protein. Acta Agron. Sinica 1994(2):156-160.

Huang, Q. L., X. H. Tang, and J. L. Mao. 1996. Effects of temperature on pollen fertility andanther protein in thermosensitive genetically male-sterile rice. Acta Physiol. Sinica1996(1):69-73.

Huang, R H. 1988. Preliminary report on the multiple embryos in rice. 1988(2):10-14.Hughes, W. G. 1976. Ethrel induced male sterility in wheat (Triticum aestivum L.). p.

111-114. In: A. Janossy and F.G.H. Lupton (eds.), Heterosis in plant breeding (Proc. 7thCongr. EUCARPIA, Budapest, June 24-29, 1974). Elsevier Sci. Pub. Co., Amsterdam.

Hull, F. H. 1945. Recurrent selection for specific combining ability in corn. J. Am. Soc.Agron.37:134-145.

Hunan Hybrid Rice Research Center. 1992. Annual report. Changsha, Hunan, China.Hunan Hybrid Rice Research Center. 1993. Manual for International Hybrid Rice Train

ing Course. Changsha, Hunan, China.Hunan Teachers' College. 1972. Morphological and physiological analysis on pollen devel

opment in male-sterile rice. Proc. of China's National Symp. for Cooperative Researchon Hybrid Rice, October 1972, p. 81-88, Changsha, China.

Ichikawa, N., N. Kishimoto, A. Inagaki, A. Nakamura, Y. Koshino, Y. Yokozeki, M. Oka,S. Samoto, H. Akagi, K. Higo, C. Shinjyo, T. Fujimura, and H. Shimada. 1997. A rapidPCR-aided selection of a rice line containing the Rf-1 gene which is involved in restoration of the cytoplasmic male sterility. Molec. Breed. 3:195-202.


Idasumi, Y. 1936. Investigations in heterosis of rice plant (first report). Proc. Crop Sci. Soc.Japan 8:504-515.

Ikehashi, H. 1982. Prospects for overcoming barriers in the utilization of indica-japonicacrosses in rice breeding. Oryza 19:69-77.

Ikehashi, H. 1991. Genetics of hybrid sterility in wide hybridization in rice (Oryza sativaL.). p. 113-127. In: Biotechnology in agriculture and forestry. Springer-Verlag, Berlin.

Ikehashi, H., and H. Araki. 1984. Varietal screening for compatibility type revealed in F j

fertility of distant crosses in rice. Japan. J. Breed. 34:304-312.Ikehashi, H., and H. Araki. 1986. Genetics of F j sterility in remote crosses of rice. p.

119-130. In: Rice genetics. Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.Ikehashi, H., and H. Araki. 1987. Screening and genetic analysis of wide-compatibility in

F j hybrids of distant crosses in rice, Oryrza sativa. Tech. Bul. Trop. Agr. Res. Center(Japan) 1987(23):1-79.

Ikehashi, H., H. Araki, and S. Yanagihara. 1991. Screening and analysis of wide compatibility loci in wide crosses of rice. p. 33-43. In: Rice Genetics II. Int. Rice Res. Inst., P.O.Box 933, Manila 1099, Philippines.

Ikehashi, H., J. S. Zou, H. P. Moon, and K. Maruyama. 1994. Wide compatibility gene(s)and indica-japonica heterosis in rice for temperate countries. p. 21-31. In: S. S. Virmani(ed.), Hybrid rice: New developments and future prospects. Int. Rice Res. Inst., P.O. Box933, Manila 1099, Philippines.

Int. Rice Res. Inst. 1980. Hybrid rice work revived at IRRI. IRRI Reporter. P.O. Box 933,Manila 1099, Philippines.

Int. Rice Res. Inst. 1992. Program Report for 1991. P.O. Box 933, Manila 1099, Philippines.Int. Rice Res. Inst. 1995. Program Report for 1994. P.O. Box 933, Manila 1099, Philippines.Ishikawa, J. 1927. Studies on the inheritance of sterility in rice. J. College Agr. Hokkaido

Univ.20:79-201.Iwamasa, M., I. Ueno, and M. Nishiura. 1967. Inheritance of nucellar embryony in Citrus.

Bul. Hort. Res. Sta. Japan. Ser. B. 1967(7):1-8.Jan, c. C. 1974. Genetic male sterility in wheat (Triticum aestivum L.): expression, stabil

ity, inheritance and practical use. Ph.D. Thesis. Univ. California, Davis.Janick, J. (ed.). 1989. Classic papers in horticultural science. Prentice-Hall, Englewood

Cliffs, NJ. p. 544-553.Jefferson, R. A., and S. Nugroho. 1998. Molecular strategies for hyhrid rice: male sterility

and apomixis. In: S. S. Virmani, E. A. Siddiq, and K. Muralidharan (eds.), Advances inhybrid rice technology (Proc. 3rd Int. Symp. Hybrid Rice, Hyderabad, India, Octoher1996). Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Jiang, Y. M. 1988. Studies on the effect of high temperature on fertility of the male sterileline in Dian-type hybrid rice. J. Yunnan Agr. Univ. 1988(2):99-107.

Jiang, Y. M., Y. Rong, and G. X. Tao. 1997. A new type of thermo-sensitive genic male sterility bred by hybridization. p. 188-192. In: Proc. Int. Symp. on Two-line System HeterosisBreed. in Crops, Changsha, Hunan, China, September 6-8,1997.

Jiang, Z. S., Q. G. Xu, Y. W. Dong, and C. L. Kang. 1993. Fertility change and heredity inindica photoperiod sensitive genic male sterile rice "An-Nong s-1." J. Hunan Agr. College 19:217-222.

Jin, D. M., Z. B. Li, and J. M. Wan. 1987. The use and value of the gene for photosensitivemale sterility in rice breeding. Sci. Agr. Sinica 1987(1):6-12.

Jones, D. F. 1917. Dominance of linked factors as a means of accounting for heterosis.Genetics 2:466-479.

Jones, H. A., and A. E. Clarke. 1943. Inheritance of male sterility in the onion and the production of hybrid seed. Proc. Am. Soc. Hort. Sci. 43:189-194.


Jones, H. A., and S. L. Emsweller. 1936. Development of the flower and macrogametophyteof Allium cepa. Hilgardia 10:415-428.

Jones, J. W. 1926. Hybrid vigor in rice. J. Am. Soc. Agron. 18:423-428.Jones, J. W. 1952. Inheritance of natural and induced mutation in caloro rice and obser

vations on sterile caloro types. J. Hered. 43:81-85.Julfiquar, A. W. 1998. Hybrid rice: status and future in Bangladesh. Int. Rice Res. Notes

1998(2):17-18.Julfiquar, A. W., S. S. Virmani, and A. 1. Carpena. 1985. Genetic divergence among some

maintainer and restorer lines in relation to hybrid breeding in rice (Oryza sativa L.).Theor. Appl. Genet. 70:671-678.

Kabaki, N., H. Hasegawa, H. Yamaguchi, and T. Kon. 1992. Growth and yield ofjaponicaindica hybrid rice. Bul. Hokuriku National Agr. Expt. Sta. 1992(34):111-139.

Kadam, B. S., G. G. Patil, and V. K. Patankar. 1937. Heterosis in rice. Indian J. Agr. Sci.7:118-125.

Kadowaki, K., T. Ishige, and S. Suzuki. 1986. Differences in the characteristics of mitochondria DNA between normal and male sterile cytoplasms of japonica rice. Japan. J.Breed. 36:333-339.

Kadowaki, K. 1., T. Suzuki, and S. Kazama. 1990. A chimeric gene containing the 5' portion of Atp6 is associated with cytoplasmic male-sterility of rice. Mol. Gen. Genet.224:10-16.

Kadowaki, K., K. Yazaki, T. Osumi, K. Harada, and M. Nakaghara. 1988. Mitochondrial DNApolymorphism in male-sterile cytoplasm of rice. Theor. Appl. Genet. 75:234-236.

Kato, H., et al. 1990. Temperature response and inheritance of a thermosensitive genic malesterility in rice. Japan. J. Breed. 40 (suppI. 1):352-369.

Kato, H., K. Maruyama, and H. Uchiyamada. 1994. Hybrid rice research in Japan, p.149-156. In: S. S. Virmani (ed.), Hybrid rice: New developments and future prospects.Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Katsuo, K., and U. Mizushima. 1958. Studies on the cytoplasmic difference among ricevarieties, Oryza sativa L. 1. On the fertility of hybrids obtained reciprocally between cultivated and wild varieties. Japan. J. Breed. 8:1-5.

Kaul, M. L. H. 1986. Male-sterility in doubled haploid rice. Rice Genet. Newsl. 3:109.Kaul, M. L. H. 1988. Male sterility in higher plants. Springer-Verlag, Berlin and Heidel

berg.Kawase, M., N. Nishimoto, T. Tanaka, A. Yoshimura, S. Yoshimura, K. Saito, A. Saito, M.

Yano, N. Takeda, T. Nagamine, and M. Nakagahra. 1991. Intraspecific variation andgenetic differentiation based on restriction fragment length polymorphism in Asian cultivated rice, Oryza sativa L. p. 467-473. In: Rice genetics II. Int. Rice Res. Inst., P.O. Box933, Manila 1099, Philippines.

Khalique, M. A., O. 1. Joarder, and A. M. Euus. 1977. Heterosis and combining ability ina diallel cross of rice (Oryza sativa L.). Bangladesh J. Agr. Sci. 1977(2):137-145.

Khush, G. S. 1995. Breaking the yield frontier ofrice. GeoJournal 35:329-332.Khush, G. S., and R. C. Aquino. 1994. Breeding tropical japonicas for hybrid rice pro

duction. p. 3:j-36. In: S. S. Virmani (ed.), Hybrid rice technology: New developmentsand future prospects, Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Khush, G. S., D. S. Brar, and J. Bennett. 1998. Current status of research on apomixis inrice and prospects for its use in heterosis breeding. In: S. S. Virmani (ed.), Advances inhybrid rice technology (Proc. 3rd Int. Symp. Hybrid Rice, Hyderabad, India, October1996). Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Khush, G. S., 1. Kumar, and S. S. Virmani. 1988. Grain quality of hybrid rice. p. 201-215.In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10,1986, Changsha, China).Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.


Khush, G. S., and S. Pengo 1996. Breaking the yield frontier ofrice. In: Proc. Workshop onRaising Yield Potential in Wheat: Breaking the Barriers. CIMMYT, Mexico.

Khush, G. S., S. Peng, and S. S. Virmani. 1998. Improving yield potential by modifyingplant type and exploiting heterosis. p. 150-170. In: J. C. Waterlow, D. G. Armstrong, L.Fowden, and R. Riley (eds.), Feeding a world population of more than eight billion people: A challenge to science. Oxford, U.K.

Kim, C. H., and J. N. Rutger. 1988. Heterosis in rice. p. 39-54. In: Hybrid rice (Proc. Int.Symp. on Hybrid Rice, October 6-10,1986, Changsha, China). Int. Rice Res. Inst., P.O.Box 933, Manila 1099, Philippines.

Kim, K. H., and M. H. Heu. 1979. A study on heterosis in cross between semi-dwarf ricecultivars. Korean J. Breed. 1979(2):127-132.

Kinoshita, T. 1992. Report of the committee on gene symbolization nomenclature and linkage groups. Rice Genet. Newsl. 9:2-4

Kinoshita, T. 1997. Gene symbols and information on male sterility. Rice Genet. Newsl.14:13-22.

Kirk, J. T. 0., and R. A. E. Tilney-Bassett. 1978. The plastids: Their chemistry, structure,growth and inheritance. Elsevier/North-Holland Biomedical Press, Amsterdam.

Kitamura, E. 1961. Genetic studies on hybrid sterility in the cross between japonica andindica type of rice (in Japanese). Rec. Adv. Breed. Res. 2:53-63.

Kitamura, E. 1962a. Genetic studies on sterilities observed in hybrids between distantlyrelated varieties of rice, Oryza sativa L. (in Japanese with English summary). Bul.Chugoku Agr. Expt. Sta. Ser. A8:141-205.

Kitamura, E. 1962b. Studies on cytoplasmic sterility of hybrids in distantly related varieties of rice, Oryza sativa L. I. Fertility of the F1 hybrids between strains derived froma certain Philippine x Japanese variety crosses and Japanese varieties. Japan. J. Breed.12:81-84.

Kitamura, E. 1962c. Studies on cytoplasmic sterility of hybrids in distantly related varieties of rice, Oryza sativa L. II. Analysis of nuclear genes in Japanese varieties controlling cytoplasmic sterility. Japan. J. Breed. 12:166-168.

Kitaoka, M., K. Ota, and M. Ogawa. 1991. Synergistic effects of Ethephon and Isourea onmale sterility of rice. Japan. J. Crop Sci. 60:349-354.

Ko, T., and H. Yamagata. 1980. Studies on the utility of artificial mutations in plant breeding: XII. Induction and gene analysis of male sterile mutants in rice. Japan. J. Breed.30:367-374.

Ko, T., and H. Yamagata. 1985. Studies on the induction of male sterile strains in rice:VIII. Linkage relationships of male sterility genes to marker genes. Japan. J. Breed.35:86-87.

Ko, T., and H. Yamagata. 1987. Studies on the utility of artificial mutations in plant breeding: XVI. Gene analysis of male sterility induced in rice. Japan. J. Breed. 37:192-198.

Ko, T., and H. Yamagata. 1989. Studies on the induction of male sterile strains in rice. 9.Linkage groups of few male-sterility genes. Japan. J. Breed. 39 (suppi. 1):234-235.

Koh, J. C. 1987. Studies on combining ability and heterosis in F1 hybrids using cytoplasmic genetic male sterile lines of rice (Oryza sativa L.). Res. Rep. Rural Dev. Adm.Korea. 1987(2):1-21.

Koltunow, A. M., R. A. B. Bicknell, and A. M. Chaudhury. 1995. Apomixis: molecularstrategies for the generation of genetically identical seed without fertilization. PlantPhysioi. 108:1345-1352.

Kornicke, F. U. W. 1885. Handbuch des Getreidebaues, V. 2. Die Sorten und der Anbaudes Getreides. Berlin, Germany.

Kumar, I., and G. S. Khush. 1986. Genetics of amylose content in rice (Oryza sativa L.). J.Genet. 65:1-11.


Kumar, I., and G. S. Khush. 1987. Genetic analysis of different amylose levels in rice. CropSci. 27:1167-1172.

Kumar, I., and G. S. Khush. 1988. Inheritance of amylose content in rice. Euphytica.38:261-269.

Kumar, I., G. S. Khush, and B. O. Juliano. 1987. Genetic analysis of waxy locus in rice.Theor. Appl. Genet. 73:481-488.

Kumar, I., K. Maruyama, and H. P. Moon. 1994. Grain quality consideration in hybrid rice.p. 123-130. In: S. S. Virmani (ed.), Hybrid rice technology: New developments andfuture prospects, lnt. Rice Res. lnst., P.O. Box 933, 1099 Manila, Philippines.

Kumar, I., and S. S. Saini. 1981. Diallel analysis in rice combining ability for various quantitative characters. Genet. Agr. 35:243-252.

Kumari, S. 1., M. Mahadevappa, and R. S. Kulkarni. 1998. Fertility restoration studies infour WA CMS lines ofrice. lnt. Rice Res. Notes 1998(1):9.

Lara, R. J., I. M. Dela Cruz, M. S. Ablaza, H. C. Dela Cruz, and S. R. Obien. 1994. Hybridrice research in Philippines. p. 173-186. In: S. S. Virmani (ed.), Hybrid rice: Newdevelopments and future prospects. lnt. Rice Res. lnst., P.O. Box 933, Manila 1099,Philippines.

Lasa, J. M., and N. O. Bosemark. 1993. Chapter 14. Male sterility. p. 213-228. In: M. D. Hayward, N. O. Bosemark, and I. Romagosa (eds.), Plant breeding: Principles and prospects.Chapman & Hall, London.

Laser, K. D., and N. R. Lerstern. 1972. Anatomy and cytology of microsporogenesis in cytoplasmic male sterile angiosperms. Bot. Rev. 38:425-454.

Leblanc, 0., D. Grimanelli, D. Gonzalez de Leon, and Y. Savidan. 1995. Detection of theapomictic mode ofreproduction in maize-Tripsacum hybrids using maize RFLP markers. Theor. Appl. Genet. 90:1198-1203.

Lei, J. C. 1983. The recessive restoration gene in WA-type male sterile lines in rice and itsapplication in breeding. Fujian Agr. Sci. Tech. 1983(5):16-19.

Lei, J. X., and Z. B. Li. 1989. A study on hereditary regularity of Hubei Photo-sensitiveGenetic Sterile Rice. Hybrid Rice 1989(2):39-43.

de Leon, J. c., E. D. Redofia, I. A. dela Cruz, M. F. Ablaza, F. M. Malabanan, R. J. Lara, andS. R. Obien. 1998. Hybrid rice in the Philippines: progress and prospects. In: S. S. Virmani, E. A. Siddiq, and K. Muralidharan (eds.), Advances in hybrid rice technology(Proc. 3rd lnt. Symp. Hybrid Rice, Hyderabad, ~ndia, October 1996). lnt. Rice Res. lnst.,P.O. Box 933,1099 Manila, Philippines.

Li, C. Q., and S. F. Ang. 1988. Genetic distance and heterosis in japonica rice. p. 257. In:Hybrid rice (Proc. lnt. Symp. on Hybrid Rice, October 6-10, 1986, Changsha, China).lnt. Rice Res. lnst., P.O. Box 933, Manila 1099, Philippines.

Li, D. H., B. S. Luo, and Y. L. Qu. 1996. Ethylene biosynthesis of photoperiod-sensitivegenic male-sterile rice and fertility alteration. Acta Phytophysiol. Sinica. 22:320-326.

Li, D. M., S. R. Liang, Q.L Xuan, and X. Y. Qin. 1989a. A study on utilization of Hubei photosensitive genic sterile rice in south China. Hybrid Rice 1989(1):27-31.

Li, H. B., J. Wang, A. M. Liu, K. D. Liu, Q. F. Zhang, and J. S. Zou. 1997. Genetic basis oflow-temperature-sensitive sterility in indica-japonica hybrids ofrice as determined byRFLP analysis. Theor. Appl. Genet. 95:1092-1097.

Li, H. B., Q. Zhang, A. M. Liu, J. S. Zou, and Z. M. Chen. 1996a. A genetic analysis oflowtemperature-sensitive sterility in indica-japonica rice hybrids. Plant Breed.115:305-309.

Li, J. M. 1990. A preliminary study on the inheritance of growth duration in the progeniesof indica-japonica crosses in rice (Oryza sativa 1.) (I). Hybrid Rice 1990(5)32-35.


Li, J. M. 1991a. A preliminary study on the inheritance of growth duration in the progenies of indica-japonica crosses in rice (Oryza sativa L.) (II). Hunan Agr. Sci.1991(4)12-13.

Li, J. M. 1995. Hybrid rice in Vietnam. Hybrid Rice 1995(1):43-44.Li, J. M., and L. P. Yuan. 1996. Hybrid rice seed production in China. October 29-Novem

ber 1, 1996. Grupo Tecnico de Trabajo de Hibridos de Arroz Para America Latina y elCaribe (GRUTHA), Ibaqu8, Colombia.

Li, P., Y. Wang, and H. Liu. 1990. Physiological bases of high yielding heterosis in indicatype F1 hybrid rice. Sci. Agr. Sinica 1990(5):39-44.

Li, P., K. D. Zhou, Y. Chen, P. He, R. D. Li, and L. H. Zhu. 1996b. RFLP mapping of genesconferring fertility restoration of wild cytoplasmic-genetic male sterility in rice (Oryzasativa subsp. indica). Acta Genet. Sinica. 23:357-362.

Li, P. H. 1977. How we studied hybrid rice. Acta Bot. Sinica 19:7-10.Li, R. B., and M. P. Pandey. 1998. Genetics of the thermosensitive genic male sterility in

rice. Int. Rice Res. Notes 1998(2):9-10.Li, R. Q., J. B. Wang, and X. M. Wang. 1993. Ultrastructural changes in microsporogene

sis and pollen development of photoperiod sensitive genic male-sterile rice under different daylength. Chinese Rice J. 1993(2):65-70.

Li, X. F., D. W. Hu, Y. H. Jiang, and F. Y. Huang. 1989b. Utilization of rice F2 hybrids bredby chemical castration technique. Guangdong Agr. Sci. 1989(4):5-7.

Li, X. F., F. Y. Huang, and Y. H. Jiang. 1986. Discussion on the utilization of F2 generationin the hybrid combination Gang-Hua-Qin-Lan. Hybrid Rice 1986(3):12-13.

Li, X. Q. 1992. The use of successive backcrosses in transfer of dual-purpose sterile lines.Hybrid Rice 1992(1):29-31.

Li, X. Z., and C F. Ren. 1994. A study on grain plumpness of inter-subspecific hybrid rice(Oryza sativa L.). J. Southwest Agr. Univ. 16:456-460.

Li, X. Z., G. Q. Zhou, M. L. Xu, L. B. Chen, C. D. Huang, and H. Q. Huang. 1996c. Effectsof mechanical collection and pollination of rice pollen on the vigor and seed set inhybridization. Acta Agron. Sinica 1996(3):353-357.

Li, Y. C, H. D. Deng, and L. P. Yuan. 1990. The research progress on rice apomixis. HybridRice 1990(1):29-32.

Li, Y. C, and L. P. Yuan. 1985. Genetic analysis offertility restoration in male sterile linesof rice. p. 617-632. In: Rice genetics. Proc. Int. Rice Genetics Symp. Int. Rice Res. Inst.,P.O. Box 933, Manila 1099, Philippines.

Li, Y. c., and L. P. Yuan. 1990. Studies on genetics of twin seedlings in rice (Oryza sativaL.). Acta Agron. Sinica 1990(2):176-182.

Li, Z. B. 1980a. A preliminary discussion about the classification of male sterile lines ofrice in China. Acta Agron. Sinica 1980(1):17-26.

Li, Z. 8., Y. H. Xiao, Y. G. Zhu, R. Q. Li, C. L. Liu, and J. M. Wan. 1982. Study and practice of hybrid rice. Shanghai Sci. Tech. Press, Shanghai, China.

Li, Z. 8., and Y. G. Zhu. 1988. Rice male sterile cytoplasm and fertility restorations. p.85-102. In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10,1986, Changsha, China). Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Li, Z. C., X. K. Wang, J. P. Wang, and Y. H. Huang. 1995. Studies on the position of American rice varieties in heterotic utilization in F1 hybrid of rice. Acta Agr. Boreali Sinica1995(1):22-28.

Li, Z. J. 1991b. Evaluation on seed purity ofT(P)GMS system hybrid rice variety, KS-9/ZhiLong. Seed. 1991(3):1-3.

Li, Z. Y. 1980b. Dian-type hybrid rice. Yunnan People's Press.


Li, Z. Y., and J. L. Wu. 1993. A new combining pattern in utilization of heterosis betweenrice subspecies. Hybrid Rice 1993(1):6-8.

Liang, C. Y. 1991. Research status of physiological and biochemical basis for rice heterosis and the prediction of heterosis. p. 211-214. In: Papers on basic research of rice breeding techniques. China Sci. Tech. Press, Beijing, China.

Liang, C. Y., and X. F. Chen. 1993. Polyamine contents and antioxidative metabolism inanthers of cytoplasmic male-sterile rice lines and their corresponding maintainer lines.Acta Phytophysiol. Sinica 19:82-88.

Liang, C. Y., M. W. Huang, and X. J. Pan. 1991. A preliminary research on the relationshipbetween the esterase and yield heterosis in hybrid rice. p. 279-284. In: Papers on basicresearch of rice breeding techniques. China Sci. Tech. Press, Beijing, China.

Liang, J. H., B. X. Wang, S. C. Yuan, and H. L. Zeng. 1990. The critical light intensitiesinducing two photoperiodic relations in HPGMR. J. Huazhong Agr. Univ.1990(4):363-368.

Liao, C. L., Z. H. Huang, K. Y. Ni, Y. S. Liu, and G. N. Shang. 1991. Selection for generalrestorers in hybrid rice using special accumulative model. Southwest China J. Agr. Sci.1991(5):1-5.

Lin, J. Y. 1994. Impact of hybrid rice on input demand and productivity. Agr. Econ.10:153-164.

Lin, M. J. 1978. Genetic study of the relationship between the lowland rice varieties andthe traditional upland rice varieties. M.S. Thesis (Univ. Philippines).

Lin, S. c., and L. P. Yuan. 1980. Hybrid rice breeding in China. p.33-51. In: Innovativeapproaches to rice breeding. Int. Rice Res. Inst., P.o. Box 933, Manila 1099, Philippines.

Lin, X. H., G. X. Yu, D. P. Zhang, Y. F. Xie and F. L. Qin. 1996. Location of one PGMS genein Nongken 58s on chromosome 5 ofrice. J. Huazhong Agr. Univ. 1996(1):1-5.

Ling, D. H., C. Y. Liang, R. Ma, and M. F. Chen. 1989. Somaclonal male-sterile mutants inindica rice. p. 737-740. In: S. Iyama and G. Takeda (eds.), Proc. 6th Int. Congr. SABRAO.

Lingaraju, S., B. V. Chandra, V. Bhaskar, and S. Gangadhariah. 1998. Adjusting floweringof a CMS line in hybrid rice seed production. Int. Rice Res. Notes 1998(2):28.

Liu, A. M., Q. F. Zhang, and H. B. Li. 1992. Location of a gene for wide compatibility inthe RFLP linkage map. Rice Genet. Newsl. 9:134-136.

Liu, J. P. 1995. The development of japonica T(P)GMS lines by way of anther culture. Beijing Agr. Sci. 1995(4):22-24.

Liu, K. D., J. Wang, H. B. Li, C. G. Xu, A. M. Liu, X. H. Li, and Q. F. Zhang. 1997a. Agenome-wide analysis of wide compatibility in rice and the precise location of the S5locus in the molecular map. Theor. Appl. Genet. 95:809-814.

Liu, L. J., and G. X. Xue. 1995. Studies on specific proteins of the photoperiod sensitivegenic male-sterile genes of rice. Acta Agron. Sinica 21:251-253.

Liu, P. 1997. Optimum use of gibberellins for seed production in japonica hybrid rice.China Rice 1997(3):22.

Liu, Q. K., C. S. Wang, P. L. Chen, and B. Chen. 1983. Studies on the arsentic residue of"male-gametocide No.2" used to induce male-sterility in the production of hybrid rice.Acta Agr. Univ. Zhejiangensis 1983(4):331-337.

Liu, X. c., M. A. S. Maroof, J. L. Wu, and L. Pan. 1997b. SSR characterization of heterozygotic patterns of parents for predicting heterosis in rice breeding. p. 72. In: J. G.Coors, S. Pandey, M. V. Grinkel, A. R. Hallauer, D. C. Hess, K. R. Lamkey, A. E.Melchinger, G. Srinivasan, and C. W. Stuber (eds.), The genetics and exploitation ofheterosis in crops. Proc. Int. CIMMYT Symp., 17-22 August 1997. ASA, CSSA, SSSA,Madison, WI, CIMMYT. Mexico. (Abstr.)


Liu. X. c., and J. L. Wu. 1996. A novel approach for discovery and exploitation of new malesterile cytoplasm resources for hybrid rice. Chinese Rice Res. News!. 1996(3):12.

Liu, X. c., and J. L. Wu. 1998. SSR heterogenic patterns of parents for marking and predicting heterosis in rice breeding. Molec. Breed. 4:263-268.

Liu, X. D., Q. F. Chen, and S. K. Zheng. 1990. Preliminary study on multiple seedlings inrice. J. Fujian Agr. College 1990(2):131-137.

Liu, Y. B., Y. W. Sun, and Q. P. Cai. 1990. Analysis on heterosis in protein content of hybridrice. Acta Agr. Univ. Jiangxiensis (Special issue for quality rice breeding). p. 9-19.

Liu, Y. S., S. M. Wang, Y. Z. Wang, and D. Y. Zuo. 1988. Structural variance analysis ofmitochondia COl and COIl genes in normal and cytoplasmic male-sterile cultivars ofrice. Acta. Genet. Sinica 15:348-354.

Liu, Y. S., L. H. Zhu, J. S. Sun, and P. He. 1997c. Mapping quantitative trait loci for reproductive barriers occming in hybrid between indica and japonica rice. Acta Bot. Sinica.39:1099-1104.

Liu, Z. C. 1986. Chloroplast genome translated products and cytoplasmic male sterility.Acta Genet. Sinica 13:201-206.

Long, J. D., M. B. Weed, D. J. Fitzgerald, and G. E. Barrier. 1973. Chemical control of pollination. Proc. Am. Soc. Agron. Madison. p. 69. (Abstr.)

Lu, c., and X. G. Pan. 1992. Inheritance of wide compatibility in rice cultivars 02428 and8504. Chinese J. Rice Sci. 1992(3):113-118.

Lu, C. G., F. L. Gu, M. L. Lu, and J. S. Zou. 1991. Studies on physiological basis of highyield of indica-japonica hybrid rice Yayou 2. Jiangsu J. Agr. Sci. 1991(1):15-19.

Lu, K. Y., and S. C. Yuan. 1991. Effects of dark-duration in photoperiod for inducing fertility transformation in photoperiod-sensitive genic male sterile rice. J. Huazhong Agr.Univ.1991(3):238-241.

Lu, X. G. (ed.). 1992a. Current status on the research of photoperiod or temperaturesensitive male sterility in rice. Agr. Press, Beijing, China.

Lu, X. G. 1992b. Considerations on the development of T(P)GMS lines in rice. Hi-techNewsl. 1992(5):1-6.

Lu, X. G. 1994. Retrospects on selection and breeding of photo-thermo-sensitive geneicmale sterile rice lines in China. Hybrid Rice 1994(3-4):27-30.

Lu, X. G., T. M. Mu, G. C. Fang, M. H. Zhou, and J. X. Zhou. 1989. Studies on the selection and utilization of indica type photoperiod-sensitive genic male sterile lines. I. Effectiveness of controlling the PGMS genes into the indica type rice varieties. Hybrid Rice1989(5):29-32.

Lu, X. G., and T. L. Wang. 1986. Research and utilization of HPGMR. II. Observations onthe genetical behavior of fertility performance. Hybrid Rice 1986(4):6-9.

Lu, X. G., Z. G. Zhang, K. Maruyama, and S. S. Virmani. 1994. Current status of two-linemethod of hybrid rice breeding. p. 37-49. In: S. S. Virmani (ed.), Hybrid rice: Newdevelopments and future prospects. In1. Rice Res. Inst., P.O. Box 933, Manila 1099,Philippines.

Lu, X. Y., F. C. Kung, X. K. Li, F. E. Xiao, and Z. M. Luo. 1992. Physiological factors forhigh proportion of empty-unfilled grains in two-line intersubspecific hybrid rice. J.Hunan Agr. College 18:509-515.

Lu, Y. G., and G. Q. Zhang. 1986. Breeding for an isogenic restorer line ot'CMS Zhen-Shan97A. Rice Genet. Newsl. 3:90-91.

Lu, Z. T., S. Q. Tang, Z. M. Xun, S. K. Min, and Z. D. Sheng. 1988. Analyses of yield components in conventional and hybrid rice. Zhejiang Agr. Sci. 1988(4):156-158.

Lua, B. S., W. B. Li, Y. L. Qu, D. H. Liu, and H. S. Li. 1990. A preliminary investigation of


fertility alteration in Hubei photoperiod sensitive genic male-sterile rice. J. HuazhongAgr. Univ. 1990(1):7-12.

Luo, C. S., and L. P. Yuan. 1990. Status of research in and utilization of hybrid ricegermplasm. Crop Gen. Res. 1990(1):5-9.

Luo, L. J., c. S. Ying, S. K. Min, X. Q. Yu, H. W. Mei, Y. P. Wang, L. B. Guo, and D. B. Zhong.1994. Wide compatible and restoring ability of Zhong 413 and its utilization in hybridrice improvement. Chinese J. Rice Sci. 1994(4):193-199.

Luo, L. J., c. S. Ying, and X. K. Wang. 1991. Sources and selection for universally compatible cultivars of rice. Crop Gen. Res. 1991(1):14-15.

Luo, M., K. D. Zhou, and X. D. Wang. 1991. Selection effect and cytoembryological studies of rice twin seedlings. p. 65-73. In: K. D. Zhou (ed.), Study on rice apomixis.Chengdu, Sichuan Sci. Tech. Press, China.

Luo, X. H., and L. P. Yuan. 1989. The development of WCVs in rice. Hybrid Rice1989(2):35-38.

Luo, Y. c., C. Q. Li, S. H. Wang, and D. Z. Wang. 1996. A study on the allelism ofT(P)GMSgenes. Proc. Annual Hainan Meeting of China's National "863" Program. p. 163-166.

Luo, Z. M., A. P. Zhong, and Q. X. Zhou. 1988. Effects of a new nontoxic chemical gametocide on rice. p. 147-156. In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October6-10,1986, Changsha, China). Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Lutts, S., J. Ndikumana, and B. P. Louant. 1994. Male and female sporogenesis and gametogenesis in apomictic Brachiaria brizantha, Brachiaria decumbens and F1 hybrids withsexual colchicine-induced tetraploid Brachiaria ruziziensis. Euphytica 78:19-25.

Mackill, D. J. 1995. Applying genetics to rice improvement in California's temperate environment. Int. Rice Commission News!. 44:62-65.

Mackill, D. J., and J. N. Rutger. 1994. Public sector research on hybrid rice in the UnitedStates. p. 235-239. In: S. S. Virmani (ed.), Hybrid rice: New developments and futureprospects. Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Maekawa, M. 1982. Studies on genetical differences between distantly related rice varieties. Mem. Fac. Agr. Hokkaido Univ. 1982(2):146-177.

Mahadevappa, L. M. M., and W. R Coffman. 1980. Karnatka male sterile rice KMS 1. Int.Rice Res. Newsl. 1980(3):3.

Mahalanobis, P. C. 1936. On the generalized distance in statistics. Proc. Nat. Int. Sci. India.B2:49-55.

Mallick, E. H., H. N. Ghosh, and P. Bairagi. 1978. Heterosis in indica rice. Indian J. Agr.Sci. 48:384-387.

Manuel, W. W., and S. Palanisamy. 1989. Heterosis and correlation in rice. Oryza26:238-242.

Mao, C. X., S. S. Virmani, and 1. Kumar. 1998. Technological innovations to lower the costsof hybrid rice seed production. In: S. S. Virmani, E. A. Siddiq, and K. Muralidharan(eds.), Advances in hybrid rice technology (Proc. 3rd Int. Symp. Hybrid Rice, Hyderabad, India, October 1996). Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Mariani, c., V. Gossele, M. de Beuckeleer, M. de Block, RB. Goldberg, W. de Greef, andJ. Leemans. 1992. A chimeric ribonuclease-inhibitor gene restores fertility to male sterile plants. Nature (Lond.) 357:384-387.

Mariani, C., M. D. Beuckeber, J. Truettver, J. Leemans, and R. B. Goldberg. 1990. Induction of male sterility in plants by a chimeric ribonuclease gene. Nature 347:737-741.

Martin, J. A., and J. H. Crawford. 1951. Several types of sterility in Capsicum frutesceus.Proc. Am. Soc. Hort. Sci. 57:335-338.

Maruyama, K., H. Araki, and H. Kato. 1991a. Thermosensitive genetic male sterility


induced by irradiation. p. 227-232. In: Rice genetics II. Int. Rice Res. Inst., P.o. Box 933,Manila 1099, Philippines.

Maruyama, K., H. Kato, and H. Araki. 1991b. Mechanized production of F1 seeds in riceby mixed planting. Japan. Agr. Res. Quarterly 24:243-252.

Matsuo, T. 1952. Genecological studies on the cultivated rice. BuI. NatI. Inst. Agr. Sci.Japan D3:1-111.

Matzk, F. 1991. Studies towards novel methods in breeding of the apomictic species Poapratensis L. p. 191-192. In: A. P. M. den Nijs and A. Elgersma (eds.), Fodder crops breeding: achievements, novel strategies and biotechnology. Pudoc, Wageningen, Netherlands.

Maurya, D. M., and D. P. Singh. 1977. Genetic divergence in rice. Indian J. Genet. 37:395-402.

Maurya, D. M., and D. P. Singh. 1978. Heterosis in rice. Indian J. Genet. Plant Breed. 38:71-76.

Maximos, M. A., and 1. R Aidy. 1994. Hybrid rice research in Egypt. p. 227-233. In: S. S.Virmani (ed.), Hybrid rice: New developments and future prospects. Int. Rice Res. Inst.,P.O. Box 933, Manila 1099, Philippines.

McDaniel, R G., and 1. V. Sarkissian. 1966. Heterosis complementation by mitochondria.Science 152:1640-1642.

McKenzie, K. S., and J. N. Rutger. 1983. Genetic analysis of amylose content, alkali spreading and grain dimension in rice. Crop Sci. 23:306-313.

McWilliam, J. R, H. Ikehashi, and S. K. Sinha. 1995. Progress in hybrid rice in India. Int.Rice Commission News!. 44:80-87.

Mei, G. Z., X. M. Wang, and M. Q. Wang. 1990. Genetical analysis of the photoperiod sensitive male sterility of Nongken 58s and its derivatives. J. Huazhong Agr. Univ.9:400-406.

Mese, M. D., L. E. Azzini, and J. N. Rutger. 1984. Isolation of genetic male-sterile mutantsin a semidwarf japonica rice cultivar. Crop Sci. 24:523-525.

Mignouna, H., S. S. Virmani, and M. Briquet. 1987. Mitochondrial DNA modificationsassociated with cytoplasmic male sterility in rice. Theor. App!' Genet. 74:666-669.

Miles, J. W., and M. 1. Escandon. 1997. Further evidence on the inheritance of reproductive mode in Brachiaria. Can. J. Plant Sci. 77:105-107.

Min, S. K. 1986. Cross breeding between indica and japonica. China's Rice Science. Agr.Press, Beijing, China.

Min, S. K. 1990. A study on the standard of evaluation for WCVs in rice. J. Zhejiang Agr.1990(1):1-6.

Minh, H. T., T. D. Quy, N. T. Nhan, and 1. Q. Du. 1997. Development ofTGMS lines withdiversified source of sterility. p. 152-153. In: J. G. Coors, S. Pandey, M. V. Grinkel, A.R Hallauer, D. C. Hess, K. R Lamkey, A. E. Melchinger, G. Srinivasan, and C. W. Stuber (eds.), The genetics and exploitation of heterosis in crops. Proc. Int. CIMMYTSymp., 17-22 August 1997. ASA, CSSA, SSSA, Madison, WI, CIMMYT. Mexico.(Abstr.)

Minocha, J. 1., and R K. Gupta. 1988. Induction of male sterility in rice using chemicalmutagens. Mutation Breed. NewsI. 1988(32):5-6.

Minocha, J. L., J. S. Sidhu, and R K. Gupta. 1991. Mutagen-induced male sterility in rice.p. 752-753. In: Rice Genetics II, 2nd Int. Rice Genetics Symp. Int. Rice Res. Inst., P.O.Box 933, Manila 1099, Philippines.

Mitra, G. N. 1962. Inheritance of grain size in rice. Curro Sci. 31:105-106.Miyazawa, B. 1932. On the two cases of semisterility in Oryza sativa. BuI. Miyazaki Col

lege Agr. Forestry 4:193-197.


Mohamad, 0., H. P. Guok, and O. Othman. 1987. Hybrid rice breeding in Malaysia. Malays.Appl. BioI. 1987(1):129-138.

Mohanty, H. K., and K. C. Mohapatra. 1973. Diallel analysis of yield and its componentsin rice. Indian J. Genet. Plant Breed. 33:264-270.

Moon, H. P. 1988. Hybrid rice research in South Korea. p. 290. In: Hybrid rice (Proc. Int.Symp. on Hybrid Rice, October 6-10,1986, Changsha, China). Int. Rice Res. Inst., P.O.Box 933, 1099 Manila, Philippines.

Moon, H. P., M. H. Heu, and C. H. Kim. 1994. Hybrid rice research in the Republic of Korea.p. 217-226. In: S. S. Virmani (ed.), Hybrid rice: New developments and future prospects.Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Mori, K. 1984. Inheritance of susceptible mutant in rice plant to herbicide, bentazon. Japan.J. Breed. 34(suppl.):421-422.

Morinaga, T. 1954. Classification ofrice varieties on the basis of affinity. Japan. J. Breed.4:1-14.

Morinaga, T. 1968. Origin and geographical distribution of Japanese rice. Japan. Agr. Res.Quart. 18:1-5.

Morinaga, T., and H. Kuriyama. 1955. ]aponica type in the subcontinent ofIndia and Java.Japan. J. Breed. 5:149-153.

Morinaga, T., and H. Kuriyama. 1958. Intermediate type of rice in the subcontinent ofIndiaand Java. Japan. J. Breed. 8:253-259.

Muker, H. S., and H. L. Sharma. 1991. Isolation distance for producing hybrid rice seed.Int. Rice Res. Newsl. 1991(1):6.

Munoz, D. B. 1992. Hybrid rice research in Colombia. Arroz 1992(379):20-28.Munoz, D. B. 1994. Hybrid rice research in Colombia. p. 242-248. In: S. S. Virmani (ed.),

Hybrid rice: New developments and future prospects. Int. Rice Res. Inst., P.O. Box 933,Manila 1099, Philippines.

Murayama, S. 1973. The basic studies on utilization of hybrid vigor in rice. 1. The degreeof heterosis and its phenomenon. Japan. J. Breed. 23:22-26.

Murayama, S., T. Omura, and K. Kimazato. 1974. Basic studies on utilization of hybridvigor in rice. IV. Heterosis under different cultural conditions. Japan. J. Breed.24:287-290.

Nagai, 1. 1926a. Studies on the mutations in Oryza sativa L. II. On awned sterile, compactpanicled and dwarf mutants. Japan. J. Bot. 3:55-56.

Nagai, 1. 1926b. Studies on the mutations in Oryza sativa 1. III. On paleaceous sterilemutant. Japan. J. Bot. 3:67-84.

Nagai, 1. 1958. ]aponica rice: its breeding and culture. Yokendo, Tokyo.Nagamine, T., K. Kadowaki, A. Goto, and T. Ajioka. 1995. A new source of cytoplasmic

male-sterility found in Bangladesh Bora rice. Rice Genet. Newsl. 12:232-235.Nakajima, M., 1. Yamaguchi, S. Kizawa, N. Murofushi, and N. Takahashi. 1991. Semi

quantification of Ga-1 and Ga-4 in male-sterile anthers of rice by radioimmunoassay.Plant Cell Physiol. 32:511-513.

Nakatat, S., and B. R. Jackson. 1973. Inheritance of some physical grain quality characteristics in a cross between a Thailand and Taiwanese rice. Thai J. Agr. Sci. 6:223-235.

Namboodri, K. M. N. 1963. Hybrid vigor in rice. Rice News Teller 11:92-96.Neves, P. C. F., E. M. Castro, P. H. N. Rangel, and 1. P. Yokoyama. 1994. Hybrid rice

research in Brazil. p. 249-252. In: S. S. Virmani (ed.), Hybrid rice: New developmentsand future prospects. Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Nguyen, C. T. 1994. Progress in the hybrid rice programme in Viet Nam. Int. Rice Comm.Newsl. 43:23-28.


Nguyen, V. L., B. B. Bong, and J. C. Mohan. 1985. Evaluation ofF] hybrids on the Cuu LongDelta, Vietnam. Int. Rice Res. News!. 1985(3):19.

Nguyen, V. L., H. T. Minh, and N. V. Suan. 1994. Hybrid rice research in Vietnam. p.187-193. In: S. S. Virmani (ed.), Hybrid rice: New developments and future prospects.Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Nguyen, V. 1., V. S. Nguyen, and S. S. Virmani. 1995. Current status and future outlookon hybrid rice in Vietnam. p. 73-80. In: G. 1. Denning and V. T. Xuan (eds.), Vietnamand IRRI: A partnership in rice research. Int. Rice Res. Inst., P.O. Box 933, Manila 1099,Philippines.

Nhan, N. 1'., H. T. Minh, N. T. Hoai, and V. T. Thuy. 1997. Using the thin layer cell culture technique for multiplication of TGMS lines in rice. p. 160-161. In: J. G. Coors, S.Pandey, M. V. Grinkel, A. R. Hallauer, D. C. Hess, K. R. Lamkey, A. E. Melchinger, G.Srinivasan, and C. W. Stuber (eds.), The genetics and exploitation of heterosis in crops.Proc. Int. CIMMYT Symp., 17-22 August 1997. ASA, CSSA, SSSA, Madison, WI, CIMMYT. Mexico. (Abstr.)

den Nijs, A. P. M., and G. E. van Dijk. 1993. Chapter 15. Apomixis. p. 229-245. In: M. D.Hayward, N. O. Bosemark, and I. Romagosa (eds.), Plant breeding: Principles andprospects. Chapman & Hall, London.

Niu, J., Y. P. Li, and Z. X. Niu. 1997. Studies on the breeding of japoniea photoperiodsensitive male sterile line via anther culture. p. 164-169. In: Proc. Int. Symp. on Twoline System Heterosis Breed. in Crops, Changsha, Hunan, China, September 6-8,1997.

Nagler, G. A. 1975. Genetics of apospory in Rant/neulus aurieomus. IV. Embryology of VIand F4 backcross offspring. Phytomorphology 25:485-490.

Nagler, G. A. 1984a. Gametophytic apomixis. p. 475-518. In: B.M. Johri (ed.), Embryology of angiosperms. Springer-Verlag, Berlin.

Nagler, G.A. 1984b. Genetics of apospory in apomictic Ranunculus aurieomus. V. Conclusions. Bot. Helvet. 94:411-422.

Oard, J. H., and J. Hu. 1995. Inheritance and characterization of pollen fertility in photoperiodically sensitive rice mutants. Euphytica 82:17-23.

Oard, J. H., J. Hu, and J. N. Rutger. 1991. Genetic analysis of male sterility in rice mutantswith environmentally influenced levels of fertility. Euphytica 55:179-186.

Oka, H. 1953. The mechanism of sterility in the intervarietal hybrid. Japan. J. Breed.3:217-224.

Oka, H. 1957. Phylogenetic differentiation of the cultivated rice. XVIII. Sterility and vigorof lines derived from hybrids between distant varieties of rice in crosses with parentalvarieties. Japan. J. Breed. 7:7-11.

Oka, H. I. 1958. Intervarietal variation and classification of cultivated rice. Indian J. Genet.Plant Breed. 18:79-89.

Oka, H. I. (ed.). 1983. The indica-japoniea differentiation of rice cultivars: A review.SABRAO, Malaysia.

Oka, H. I. 1988. Origin of cultivated rice. Japan Scientific Societies Press, Tokyo.Osman, M., H. P. Guok, and O. B. Omar. 1988. Hybrid rice breeding in Malaysia. p. 289.

In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10,1986, Changsha, China).Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Ozias-Atkins, P. E. L. Lubbers, W. W. Hanna, and J. W. McNay. 1993. Transmission of theapomictic mode of reproduction in Pennisetum: Co-inheritance of the trait and molecular markers. Theor. Appl. Genet. 85:632-638.

Pan, K. Q. 1979. Cytological and histological investigations on pollen abortion in male sterile line of wild rice. Acta Genet. Sinica 1979(2):211-215.


Pan, K. Q., and 1. Q. He. 1981a. The relationship between pollen abortion and tapetum inmale-sterile line of rice 424, 131. J. South China Agr. College 1981(3):39-45.

Pan, K. Q., and L. Q. He. 1981b. Cytological and anatomical observation of stamens of arice hybrid with "wild pollen-abortive" cytoplasm and its three parental lines. Guangdong Agr. Sci. 1981(1):18-20.

Pan, K. Q., Z. X. Wang, and Q. R. He. 1982. Cytological studies on pollen abortion ofTianrei 409 A in rke. Yunnan Agr. Sci. 1982(2):28-30.

Panawar, D. V. S., R S. Paroda, and A. Singh. 1983. Heterosis in rice. Indian J. Genet.43:363-369.

Parlevliet, J. K, and J. W. Cameron. 1959. Evidence on the inheritance ofnucellar embryony in Citrus. Proc. Am. Soc. Hort. Sci. 74:252-260.

Parmar, K. S., E. A. Siddiq, and M. S. Swaminathan. 1979. Chemical induction of malesterility in rice. Indian J. Genet. 39:529-541.

Parmasivian, K. S. 1986. Estimation of heterosis in hybrids and segregating pattern in F2

generation for apiculus pigmentation in spikelets of rice (Oryza sativa L.). Madras Agr.J. 73:573-578.

Pei, X. S. 1980. A new view on inheritance of male sterility in plant. Hunan Agr. Sci. Tech.1980(3):1-12.

Peng, G. F., Z. Y. Li, W. H. Li, and C. X. Zhang. 1993. Studies on the fertility of the temperature sensitive male sterile line 913 in Dian-type hybrid rice and its application. J.Yunnan Agr. Univ. 1993(4):343-346.

Peng, J. Y., J. C. Glaszmann, and S. S. Virmani. 1988. Heterosis and isozyme divergencein indica rice. Crop Sci 28:561-563.

Peng, J. Y., and S. S. Virmani. 1990. Combining ability for yield and four related traits inrelation to hybrid breeding in rice. Oryza 27:1-10.

Peng, J. Y., S. S. Virmani, and A. W. Julfiquar. 1991. Relationship between heterosis andgenetic divergence in rice. Oryza 28:129-133.

Peng, S., G. S. Khush, and K. G. Cassman. 1994. Evolution of the new plant ideotype forincreased yield potential. p. 5-20. In: K. G. Cassman. (ed.), Breaking the yield barrier.Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Peng, Y. F., and Y. R Wang. 1991. Studies on starch and protein contents of HPGMR during pollen abortion. Acta Scientiarum Naturalium Universitatis Sunyatseni1991(4):92-99.

Perez, A. T., T. T. Chang, H. M. Beachell, B. S. Vergara, and A. P. Marciano. 1973. Induction of male sterility in rice with Etherel and RH-531. SABRAO Newsl. 1973(2):133-139

Pernes, J., R Rene-Chaume, J. Rene, and Y. Savidan. 1975. Schema d'amelioration genetique des complexes agamiques du type Panicum. Cah ORSTOM, ser. Bio. 10:67-75.

Pham, T. M., M. H. Vu, C. B. Bui, V. L. Nguyen. 1991. Hybrid rice yield trials in the MekongDelta. Int. Rice Res. Newsl. 1991(5):8.

Pillai, M. S. 1961. Hybrid vigor in rice. Rice News Teller 1961(1):15-17.Pingali, P. 1., M. Hossain, and R V. Gerpacio. 1997. Asian rice bowls: The returning cri

sis? p. 208-233. CAB Int. and Int. Rice Res. Inst., Oxon, U.K.Ponnuswamy, A. S., and S. R Prabagaran. 1997. A sustainable substitute to gibberellic acid

for hybrid rice seed production. Madras Agr. J. 84:384-385.Powers, L. 1945. Fertilization without reduction in guayule (Parthenium argentatum

Gray) and a hypothesis as to the evolution of apomixis and polyploidy. Genetics30:323-346.

Prabagaran, S. R, and A. S. Ponnuswamy. 1997a. Determination of ideal row ratio forhybrid rice seed production. Madras Agr. J. 84:230-231.

Prabagaran, S. R, and A. S. Ponnuswamy. 1997b. Determination of optimum growth stage


for gibberellic acid application in hybrid rice seed production. Madras Agr. J.84:231-232.

Pradhan, S. B., S. N. Ratho, and P. J. Jachuck. 1990. Development of new cytoplasmicgenetic male sterile lines through indica-japonica hybridization in rice. Euphytica51:127-130.

Prakash, B. G., and M. Mahadevappa. 1987. Evaluation of some experimental rice hybridsfor field performance and standard heterosis. Olyza 24:75-78.

Prasad, M. N., and S. S. Virmani. 1989. Optimum distance of isolation for hybrid rice seedproduction. Int. Rice Res. News!' 1989(3):4-5.

Prasad, M. N., S. S. Virmani, and A. D. Gamutan. 1988. Substituting urea and boric acidfor gibberellic acid in hybrid rice seed production. Int. Rice Res. News!' 1988(6):9-10.

Purohit, D. C. 1972. Heterosis in rice. Madras Agr. J. 59:335-339.Qian, Q., H. Chen, Z. X. Sun, and L. H. Zhu. 1996. The study on determining true and false

hybrid rice II You 63 using RAPD molecular markers. Chinese J. Rice Sci.1996(4):241-242.

Qian, Q., X. D. Zhu, D. L. Zen, Z. M. Xiong, and S. K. Min. 1995. Linkage analysis ofHubeiphotoperiod-sensitive genic male sterile locus in rice. Acta Agr. Zhejiangensis7:429-433.

Raj, K. G., and E. A. Siddiq. 1986. Biochemical characterization of normal and male sterile anthers in rice Oryza sativa L. Indian J. Genet. Plant Breed. 46:541-549.

Raj, K. G., and S. S. Virmani. 1988. Genetics of fertility restoration of WA type cytoplasmic male sterility in rice. Crop Sci. 28:787-792.

Ram, J., and D. V. S. Panwar. 1970. Interspecific divergence in rice. Indian J. Genet.1970(1):1-10.

Ramalingam, J., N. Nadarajan, C. Vanniarajan, and P. Rangaswamy. 1995. Genetics of fertility restoration and its allelic relationship in rice (Olyza sativa L.). J. Gen. Breed.49:265-268.

Ramanujam, S. 1935. Male sterility in rice. Madras Agr. J. 23:190.Ramiah, K. 1935. Rice genetics. Proc. Assoc. Econ. Bio. Coimbatore 3:51-61.Ramiah, K., S. Jobiraz, and S. D. Mudaliar. 1931. Inheritance of characters in rice. Part IV.

Mem. Dept. Agr. India. Bot. Ser. 18:229-250.Ramiah, K., and N. Parthasarathy. 1933. Inheritance of grain length in rice (Olyza sativa

L.). Indian J. Agr. Sci. 3:808-819.Ramiah, K., and K. Rangaswamy. 1941. Hybrid vigor in rice (Oryza sativa L.). Indian J.

Genet. Plant Breed. 1:4-12.Ramiah, K., and M. B. V. N. Rao. 1953. Rice breeding and genetics. Sci. Monogr., Indian

Council Agr. Res., New Delhi. Vol. 19.Rangaswamy, M., and P. Jayamani. 1998. Diversifying cytoplasmic sources in rice. Int. Rice

Res. Notes 1998(2):5.Rangaswamy, M., M. N. Prasad, S. R. S. Rangasamy, S. S. Virmani. E. A. Siddiq, T. B. Ran

ganathan, W. W. Manual, K. Thiyagarajan, P. Jayamani, V. Palanisamy, K. Angamuthu,A. S. Ponnusamy, G. J. Martin, P. Thangamani, and R. Velusamy. 1994. CORH1: Thefirst rice hybrid for Tamil Nadu, India. Int. Rice Res. Notes 1994(3):19.

Rao, A. V., A. S. R. Prasad, S. T. Krishina, D. V. Seshu, and T. E. Srinivasan. 1981. Geneticdivergence among some brown planthopper resistant rice varieties. Indian J. Genet.1981(2):179-185.

Rao, G. M. 1965. Studies on hybrid vigor in intervarietal hybrid of rice (Olyza sativa L.).Andhra Agr. J. 12:1-12.

Raa, Y. S. 1988. Cytohistology of cytoplasmic male sterile lines in hybrid rice. p. 115-128.

142 J. LI AND 1,. YUAN

In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10,1986, Changsha, China).lnt. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Rao, Y. S., and S. C. Xie. 1983. Cytological studies on pollen abortion in 'V20A' offspringsof anther culture of rice. Department of Biology, Hunan Teachers' College, Changsha,China.

Razzaque, C. A. 1975. Genetics of male sterility in rice. Indian J. Genet. Plant Breed.34:303-308.

Reddy, O. U. K, E. A. Siddiq, J. Ali, A. J. Hussain, and M. 1. Ahmed. 1998a. Fertilityaltering conditions of promising thermo-sensitive genic male sterile lines in rice. Int.Rice Res. Notes 1998(2):6.

Reddy, O. U. K, E. A. Siddiq, J. Ali, A. J. Hussain, and M. I. Ahmed. 1998b. Genetics ofthermosensitive genic male sterile lines in rice. Int. Rice Res. Notes 1998(1):10.

Reddy, O. U. K, E. A. Siddiq, J. Ali, A. J. Hussain, P. Arti, M. I. Ahmed, and N. P. Sharma.1998b. Identifying molecular markers for the gene(s) governing thermosensitive genicmale' sterility in rice. Int. Rice Res. Notes 1998(2):10-11.

Reddy, R. P., and K Sathyanarayanaiah. 1981. Inheritance of aroma in rice. Indian J.Genet. 40:327-329.

Rick, C. M. 1948. Genetics and development of nine male sterile tomato mutants. Hilgardia 18:599-633.

Rothschild, G. H. 1,. 1998. IRRI's role and vision for hybrid rice. In: S. S. Virmani, E. A.Siddiq, and K. Muralidharan (eds.), Advances in hybrid rice technology (Proc. 3rd Int.Symp. Hybrid Rice, Hyderabad, India, October 1996). Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Rowell, P. 1,., and D. G. Miller. 1971. Induction of male sterility in wheat with 2chloroethylphosphonic acid (Ethrel). Crop Sci. 11:629-631.

Rundfeldt, H. 1960. Untersuchungen zur Ziichtung des kopfkohls (B. oleracea L. var. capitata). Z. Pflanzenziicht. 44:30-62.

Rutger, J. N. 1992. Searching for apomixis in rice. p. 36-39. In: Proc. Apomixis Workshop.U.S. Dept. Agr., Beltsville, MD.

Rutger, J. N. 1997. Rice genetics and germplasm enhancement. Research Ser.-ArkansasAgr. Expt. Sta. 1997(456):22-27.

Rutger, J. N., and G. W. Schaeffer. 1990. An environmental sensitive genetic male sterilemutant in rice. p. 25. In: Proceedings of Twenty-third Rice Technical Working Groupin USA, February 26-28, 1990. Biloxi, Mississippi.

Rutger, J. N., and C. Shinjyo. 1980. Male sterility in rice and its potential use in breeding.In: Innovative approaches to rice breeding: Selected papers from 1979 International RiceResearch Conference. Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Sahai, V. N., and R. C. Chaudhary. 1985. Standardization of parameters for seed production techniques in hybrid rice. Seeds & Farms 11:9-14.

Sahai, V. N., S. Saran, and R. C. Chaudhary. 1987. Hybrid rice research in Bihar, India.Int. Rice Res. Newsl. 1987(2):23.

Saini, S. S., and I. Kumar. 1973. Hybrid vigor for yield and yield components in rice. IndianJ. Genet. Plant Breed. 1973(2):197-200.

Saini, S. S., I. Kumar, and M. R. Gagneia. 1974. A study on heterosis in rice (Oryza sativa1,.). Euphytica 23:219-224.

Sakamoto, W., K Kadowaki, and T. Yabuno. 1990. Analysis of mitochondria DNAs fromOryza glaberrima and its cytoplasmic substituted line for Oryza sativa associated withcytoplasmic male sterility. Japan. J. Genet. 1990(1):1-6.

Sampath, S., and H. K Mohanty. 1954. Cytology of semi-sterile rice hybrids. Curro Sci.23:182-183.


Sano, Y. 1985. Interspecific cytoplasm substitutions of an indica strain of Oryza sativa andOryza glaberrima. Euphytica 34:587-592.

Sano, Y, and M. Eiguchi. 1991. A nuclear gene inducing fertility restoration in cytoplasmic male-sterile rice. Rice Genet. Newsl. 8:113-115.

Sano, Y., M. Eiguchi, H. Y. Hirano, and M. Yamada. 1992. A nuclear gene modifying instability of fertility restoration in cytoplasmic male-sterile rice. Genet. Res. Camb.60:195-200.

Satake, T., and S. Yoshida. 1978. High temperature-induced sterility in indica rice at flowering. Japan. J. Crop Sci. 47:6-7.

Sato, H., N. Shimizu, H. Nakagawa, and K. Nakajima. 1990. A new registered cultivar "Natsukaze" of guineagrass (Panicum maximum). Japan. Agr. Res. Quarterly 23:196-201.

Sato, S., L. Sakamoto, and S. Nakasone. 1985. Location ofEf-1 for earliness on Nishimura'sseventh chromosome. Rice Genet. Newsl. 2:59-60.

Sato, S., and C. Shinjyo. 1991. Genetical studies on genic male sterile mutant obtainedthrough breeding of isogenic translocation line of Taichung 65 in rice Oryza sativa L.Japan. J. Breed. 41:451-460.

Sattari, M. 1997. Identification heterotic hybrid for exploitation of hybrid vigor in Iranianrice cultivars. p. 344-345. In: J. G. Coors, S. Pandey, M. V. Grinkel, A. R. Hallauer, D.C. Hess, K. R. Lamkey, A. E. Melchinger, G. Srinivasan, and C. W. Stuber (eds.), Thegenetics and exploitation of heterosis in crops. Proc. Int. CIMMYT Symp., 17-22 August1997. ASA, CSSA, SSSA, Madison, WI, CIMMYT. Mexico. (Abstr.)

Satyanarayana, P. V., I. Kumar, and M. S. S. Reddy. 1995. A new source of thermosensitive genic male sterility for 2-line hybrid rice breeding. Int. Rice Res. Notes 1995(1):10.

Savidan, Y. 1982. Nature et heredite de l'apomixie chez Panicum maximum Jacq. Travauxet documents ORSTOM, Paris. 153:1-159.

Savidan, Y. 1983. Genetics and utilization of apomixis for the improvement of guineagrass(Panicum maximum Jacq.). p. 182-184. In: J. A. Smith and V. W. Hayes (eds.J, Proc. Int.Grassl. Congr., 14th, Lexington, KY, June 15-24, 1981. Westview Press, Boulder. CO.

Savidan, Y. 2000. Apomixis: Genetics and breeding. Plant Breed. Rev. 18: chapter 2.Savidan, Y., O. Leblanc. and J. Berthaud. 1994. Transferring apomixis to maize. In: G. S.

Khush (ed.), Apomixis: exploiting hybrid vigor in rice. Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Schwartz, D., and W. J. Laughner. 1969. A molecular basis for heterosis. Science.166:626-627.

Seetharaman, R. 1959. Inheritance of iodine value in rice and its association with othercharacters. Diss. Abstr. 20:856.

Sehgal, S. M. 1994. Capabilities and limitations of the global seed industry in hybrid ricedevelopment. p. 257-266. In: S. S. Virmani (ed.), Hybrid rice technology: New developments and future prospects. Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Sen, P. K., and G. N. Mitra. 1958. Inheritance of photoperiodic reaction in rice. Nature(Lond.) 182:119.

Shamsi, M. A., B. K. Murthy, D. S. Manyam, and M. V. B. Rao. 1996. Effect of chemicalgametocides on pollen sterility and seed set in rice (Oryza sativa L.). Indian J. Genet.Plant Breed. 56:100-103.

Shanghai Plant Physiology Research Institute. 1977. Index of physiology and biochemistryin hybrid rice. Hunan Agr. Sci. Tech. 1977(1):40-51

Shao, K. X., and D. W. Hu. 1988. Chemical emasculators for hybrid rice. In: Hybrid rice(Proc. lnt. Symp. on Hybrid Rice, October 6-10,1986, Changsha. China). Int. Rice Res.Inst., P.O. Box 933, 1099 Manila, Philippines.


Shao, Y., and X. H. Tang. 1992. Analysis ofrestorer genes for photoperiod sensitive malesterile rice. Rice Genet. Newsl. 9:52-54.

Shao, Y., and X. H. Tang. 1993. Studies on the relationship of allelic sterile genes betweenphotoperiod-sensitive sterile rice and BT-type sterile rice. J. Shanghai Agr. College1993(4):273-277.

Shao, Y., and X. H. Tang. 1995. Allelic relation between photoperiod-sensitive genicmale-sterility and cytoplasmic-genic male-sterility ofBT type in rice. Rice Genet. Newsl.12:231-232.

Shao, Y., X. H. Tang, Q. L. Huang, and G. Z. Pan. 1993. Studies on the sterile genes andits restoring genes in photoperiod-sensitive male sterile rice. Acta Agr. Shanghai1993(3):22-29.

Sharma, H. L., H. Singh, and D. P. Joshi. 1987. Minimum isolation distance for hybrid riceproduction. Int. Rice Res. Newsl. 1987(2):24.

Sharma, H. L., and S. S. Virmani. 1990. Some varietal sources of twin seedlings trait inrice. Rice Genet. Newsl. 7:134-135.

Sharma, H. L., and S. S. Virmani. 1994. Optimum pollen load and male-female row ratiofor hybrid rice seed production. p. 294. In: S. S. Virmani (ed.), Hybrid rice technology:New developments and future prospects. Int. Rice Res. Inst., P.O. Box 933, 1099 Manila,Philippines.

Shen, J. H. 1980. Rice breeding in China. In: Rice improvement in China and other Asiancountries. Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Shen, Y., Q. CaL M. Gao, and X. Wang. 1996a. Isolation and genetic characterization of afertility-restoring revertant induced from cytoplasmic male sterile rice. Euphytica90:17-23.

Shen, Y., Z. Guan, H. Qiang, J. Lu, K. Zheng, and X. Wang. 1996. Identification of RAPDmarkers linked to fertility restoring gene of rice cytoplasmic male-sterile line. RiceGenet. Newsl. 13:115-117.

Shen, Y. W., Q. H. Cai, and M. W. Gao. 1993a. Preliminary characterization of fertilityrestoring mutants induced from cytoplasmic male-sterile rice. Rice Genet. Newsl.10:90-92.

Shen, Y. W., Q. H. Cai, and M. W. Gao. 1993b. A new thermosensitive radiation-inducedmale-sterile rice line. Rice Genet. Newsl. 10:97-98.

Shen, Z. T., and Y. B. Xu. 1992. Genetic analysis for hybrid sterility of intersubspecificcrosses in rice. Chinese J. Rice Sci. 1992(3):105-112.

Sheng, X. B. 1987. Genetic effects of cytoplasm on agronomic characteristics in hybrid rice.Chinese J. Rice Sci. 1987(3):155-170.

Sheng, X. B. 1992. Genetics of photoperiod sensitive genic male sterility of Nong-Ken58s (Oryza sativa). Chinese J. Rice Sci. 1992(1):5-14.

Sheng, X. B. 1994. The review and prospect of hybrid rice male sterility genetics researchin China. Hybrid Rice 1994(3-4):58-62.

Shi, C. H., G. L. Chen, J. Zhu, R. C. Zang, and J. G. Wu. 1998. Analysis of genetic effectsand heritabilities for amylose content of indica rice in different environments. ChineseRice Res. Newsl. 1998(2):10-12.

Shi, G. C., P. C. Ni, and J. X. Song. 1996a. The identification of apomixis in rice variety84-15. Sci. Agr. Sinica. 1996(5):56-61.

Shi, G. c., P. C. Ni, J. X. Song, X. J. Mu, and T. B. Liang. 1996b. A cytoembryological studyon rice 84-15, a rice line presumed to be apomictic. Acta Agron. Sinica. 22:657-660.

Shi, M. S. 1981. The preliminary report on the breeding and application of a late-seasonjaponica natural dual-purpose male sterile line in rice (in Chinese). Hubei Agr. Sci.1981(7):1-3.


Shi, M. S. 1985. Discovery of and preliminary studies on a photosensitive recessively malesterile rice. Sci. Agr. Sinica 1985(2):44-48.

Shi, M. S., and J. Y. Deng. 1986. The discovery, determination and utilization of the HubeiChina photosensitive genic male-sterile rice Oryza-sativa-ssp-japonica. Acta Genet.Sinica 13:107-112.

Shi, S. Y., A. M. Liu, Q. H. Yi, and Z. K. Zhong. 1988a. Application of isozyme analysisto selection for restorer lines in hybrid rice. Jiangsu J. Agr. Sci. 1988(3):9-14.

Shi, S. Y., Q. H. Yi, J. R. Jiang, and A. M. Liu. 1988b. Using isozymes to determine purityof hybrid rice seed. p. 274. In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October6-10,1986, Changsha, China). Int. Rice Res. lnst., P.O. Box 933, 1099 Manila, Philippines.

Shibuya, N. 1973. Studies on causal genes of partial male-sterility in rice and their utilization in testing cool temperature tolerance. Bul. Yamagata Univ. Agr. Sci. 6:571-625.

Shinjyo, C. 1969. Cytoplasmic genetic male sterility in cultivated rice (Oryza sativa L.).II. The inheritance of male sterility. Japan. J. Genet. 44:149-156.

Shinjyo, C. 1975. Genetic studies of cytoplasmic male sterility and fertility restoration inrice (Ol}'za sativa L.). Sci. Bul. College Agr. Univ. Ryukyus 22:1-57.

Shinjyo, C. (ed.). 1984. Cytoplasmic male sterility and fertility restoration in rice havinggenome A. p. 321-338. In: Biology of rice. Japan Sci. Soc. Press, Tokyo.

Shinjyo, C. 1990. Identification of male sterile cytoplasms and fertility restoring genesderived from wild species. p. 106-111. In: N. Iwata (ed.), Exploitation and identification of rice gene resources (Report of Grant-in-Aid for Sci. Res.).

Shinjyo, c., and T. Omura. 1966. Cytoplasmic male sterility in cultivated rice (Ol}'za sativaL.). I. Fertility ofF1 , F2 and offsprings obtained from their mutual reciprocal backcrosses;and segregation of completely male sterile plants (in Japanese). Japan. J. Breed. 16(suppl.):179-180.

Shinjyo, c., and S. Sato. 1994. Chromosomal location of fertility-restoring gene Rf-2. RiceGenet. Newsl. 11 :93-95.

Shinjyo, c., and Y. Watanabe. 1977. Allelism test of two genes showing fertility-restoringeffect for male sterile cytoplasm of Lead rice and their inheritance. Japan. J. Breed.27:70-71.

Shu, Q. F., S. S. Wang, and Q. M. Yi. 1989. Two-dimensional gel electrophoresis ofendosperm proteins in male sterile rice. Hereditas (Beijing) 1989(3):5-8.

Shull, G. H. 1908. The composition of a field of maize. Am. Breed. Assoc. Rep. 4:296-301.Shull, G. H. 1952. Chapter 2. Beginnings of the heterosis concept. p. 14-48. In: J. W. Gowen

(ed.), Heterosis. Iowa State College Press, Ames.Siddiq, E. A. 1994. Hybrid rice in India. Int. Rice Comm. Newsl. 43:29-31.Siddiq, E. A., P. J. Jachuck, M. Mahadevappa, F. U. Zaman, R. V. Kumar, B. Vidyachan

dra, G. S. Sidhu, I. Kumar, M. N. Prasad, M. Rangaswamy, M. P. Pandey, D. V. S. Panwar, and I. Ahmed. 1994. Hybrid rice research in India. p. 157-171. In: S. S. Virmani(ed.), Hybrid rice: New developments and future prospects. Int. Rice Res. Inst., P.O. Box933, Manila 1099, Philippines.

Singh, G. B., A. S. Randhawa, G. S. Magnat, and G. S. Sidhu. 1997. Studies on relative seedset and seed yield in different CMS lines of rice. J. Res. (Punjab Agr. Univ.)1997(2):125-130.

Singh, N. B., H. G. Singh, and P. Singh. 1977. Heterosis and combining ability for qualitycomponent in rice. Indian J. Genet. 37:347-352.

Singh, R. 1997. Studies on heterosis for certain physiological and biochemical parametersin rice. p. 164-165. In: J. G. Coors, S. Pandey, M. V. Grinkel, A. R. Hallauer, D. C. Hess,K. R. Lamkey, A. E. Melchinger, G. Srinivasan, and C. W. Stuber (eds.), The genetics


and exploitation of heterosis in crops. Proc. Int. CIMMYT Symp., 17-22 August 1997.ASA, CSSA, SSSA, Madison, WI, CIMMYT. Mexico. (Abstr.)

Singh, R., and S. K. Sahoo, 1997. Artificial and natural parameters influencing outcrossing of cyto-sterile lines in rice. p. 162-163. In: J. G. Coors, S. Pandey, M. V. Grinkel, A.R. Hallauer, D. C. Hess, K. R. Lamkey, A. E. Melchinger, G. Srinivasan, and C. W. Stuber (eds.), The genetics and exploitation of heterosis in crops. Proc. Int. CIMMYTSymp., 17-22 August 1997. ASA, CSSA, SSSA, Madison, WI, CIMMYT. Mexico.(Abstr.)

Singh, R. J., and H. Ikehashi. 1981. Monogenic male sterility in rice: induction, identification and inheritance. Crop Sci. 21:286-289.

Singh, R. P., and R. R. Singh. 1979. Heterosis in rice. 0IYza 17:119-122.Singh, S., B. C. Viraktamath, C. H. M. Vijayakumar, M. S. Ramesha, and M. I. Ahmed. 1998.

Standardizing hybrid rice seed production practices. Int. Rice Res. Notes 1998(1):22.Singh, S. P., and H. G. Singh. 1977. Heterosis in rice. 0IYza. 15:173-175.Singh, S. P., H. G. Singh, and A. K. Singh. 1984. Hybrid vigor for yield and its components

in rice. Genet. Iber. 36:53-59.Singh, S. P., R. P. Singh, and R. V. Singh. 1980. Heterosis in rice. OIyza 18:109-113.Singh, U. S. 1983. Hybrids excel standard cultivar. Deep Water Rice 1:1.Sivasubranian, S., and M. Menon. 1973. Heterosis and inbreeding depression in rice.

Madras Agr. J. 60:1139-1144.Sohu, V. S., and P. S. Phul. 1995. Inheritance of fertility restoration of three sources of cyto

plasmic male sterility in rice. J. Gen. Breed. 49:93-96.Solntzeva, M. P. 1978. Apomixis and hemigamy as one ofits forms. Proc. Indian Nat. Sci.

Acad. Part B. 44,78.Somoto, S., and K. Hamamura. 1973. Inheritance of grain characters in rice plant. Hokuriku

Crop Sci. 8:11-13.Somrith, B., 1'. T. Chang, and B. R. Jackson. 1979. Genetic analysis of traits related to grain

characteristics and quality in two crosses of rice. IRRI Res. Pap. Ser. 35.Song, X. F., Z. M. Xiong, and S. K. Min. 1990. Biological characteristics of competitive het

erosis in indica/japonica intersubspecific rice hybrids. p. 129-134. In: Selected paperson the development of P(T)GMS rice lines and utilization of intersubspecific heterosis.China's Ministry Agr., Beijing.

Sood, B. C., and E. A. Siddiq. 1978. A rapid technique for scent determination in rice.Indian J. Genet. 38:268-271.

Sprague, G. F. 1983. Chapter 2. Heterosis in maize: Theory and practice. p. 47-70. In: R.Frankel (ed.), Heterosis: Reappraisal of theory and practice. Springer-Verlag, Berlin.

Srivastava, H. K. 1972. Mitochondrial complementation and hybrid vigor. Indian J. Genet.Plant Breed. 32:215-228.

Srivastava, H. K. 1983. Chapter 10. Heterosis and intergenomic complementation: mitochondria, chloroplast, and nucleus. p. 260-286. In: R. Frankel (ed.), Heterosis: Reappraisal of theory and practice. Springer-Verlag, Berlin.

Srivastava, M. N., and D. V. Seshu. 1983. Combining ability for yield and associated characters in rice. Crop Sci. 23:741-744.

Stansel, J. W. 1966. Influence of heredity and environment on endosperm characteristicsofrice (Oryza sativa 1.). Diss. Abstr. 27:48b.

Stuber, C. W., S. E. Lincoln, D. W. Wolff, 1'. Helentjaris, and E. S. Lander. 1992. Identification of genetic factors contributing to heterosis in a hybrid from two elite maizeinbred lines using molecular markers. Genetics 132:823-839.

Subandi, B., B. Suprihatno, D. A. Sihombing, and S. Suemardhi. 1987. Research anddevelopment of hybrid variety of maize and rice in Indonesia. Indonesian Agr. Res. Dev.J. 1987(1-2):13-18.


Subramanian, S., and M. Rathinam. 1984. Heterosis in rice. Madras Agr. J. 71:402-405.Subudhi, P. K., R. P. Borkakati, S. S. Virmani, and N. Huang. 1995. Identification ofRAPD

markers linked to rice thermosensitive genetic male-sterility gene by bulk segregantanalysis. Rice Genet. News!. 12:228-231.

Subudhi, P. K., R. P. Borkakati, S. S. Virmani, and N. Huang. 1996. Inheritance and molecular mapping of the thermosensitive genetic male-sterility gene in rice. p. 601-606.In: Rice Genetics II. Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Subudhi, P .K., R .P. Borkakati, S. S. Virmani, and N. Huang. 1997. Molecular mapping ofa thermosensitive genetic male sterility gene in rice using bulked segregant analysis.Genome 40:188-194.

Suh, H. S. 1988. Multiple pistillate male sterile rices for hybrid seed production. p.181-187. In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10,1986, Changsha, China). Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Suh, H. S., J. D. Kim, M. H. Heu, and G. S. Chung. 1991. Linkage relationships betweengenetic male sterile and marker genes in rice. p. 121-125. In: Rice Genetics II. Proc. 2ndInt. Rice Genetics Symp., Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Suh, H. S., J. C. Koh, and S. K. Lee. 1989. Identification of four genes for genetic male sterility in rice. p. 317-320, In: S. Iyama and G. Takeda (ed.), Breeding research: The key tothe survival of the earth. Proc. 6th Int. Congr. of SABRAO.

Suherman, 0.1989. Performance of hybrid rice in Indonesia. Int. Rice Res. News!. 1989(4):9.Sun, J. S., and Y. S. Liu. 1996. Current status and potential of utilization of apomixis in

rice breeding. China Rice 1996(6):1-2.Sun Vat Sen Univ. 1976. The study of biological characters of "three lines" in crops: III. The

comparative research to the formation and embryological characters of pollen cells,metabolic block and development of the vascular bundle in anther septum of severalMS types in rice. Acta Genet. Sinica 3:119-127.

Sun, Y. W., J. Q. Li, and Y. B. Liu. 1993. Heterosis and combining ability for the days toheading of the 'two-line system' intersubspecific hybrid rice. Chinese J. Rice Sci.1993(2):105-108.

Sun, Z. X., and S. H. Cheng. 1994. Hybrid rice breeding: from three-line system, two-linesystem to one-line system (in Chinese). China Agr. Sci. Tech. Press, Beijing, China.

Sun, Z. X., S. H. Cheng, S. K. Min, Z. M. Xiong, C. S. Ying, and H. M. Si. 1991. Studies onresponse of photoperiod and temperature to the photoperiod sensitive genic male sterile rice (PGMSR): II. Identification of japonica PGMSR in growth chamber. Chinese J.Rice Sci. 1991(2):56-60.

Sun, Z. X., S. K. Min, and Z. M. Xiong. 1989. A temperature-sensitive male-sterile linefound in rice. Rice Genet. News\. 6:116-117.

Suprihatno, B. 1986. Hybrid rice: its prospects and problems in Indonesia. Indonesian Agr.Res. Dev. J. 1986(3-4):51-58.

Suprihatno, B., B. Sutaryo, and T. S. Silitonga. 1994. Hybrid rice research in Indonesia.p. 196-206. In: S. S. Virmani (ed.), Hybrid rice: New developments and future prospects.Int. Rice Res. Inst., P.O. Box ~l33, Manila 1099, Philippines.

Suzuki, 1., Y. Vagi. H. Hashimoto, A. Shimizu, and T. Kato. 1990. The development of asorting machine for rice seeds. Japan. J. Breed. 40(suppl 2):374-375.

Suzuki. S. 1978. Anther and pollen abnormalities induced by cold treatment and their varietal differences in rice plants. Japan. J. Breed. 28:21-32.

Swaminathan, M. S., E. A. Siddiq, and S. D. Sharma. 1972. Outlook for hybrid rice in India.In: Rice breeding. p. 609-G13. Int. Rice Res. Inst., P.O. Box 933, Manila 1099, Philippines.

Taillebois, J. 1991. F 1 hybrid rice: prospects for Mediterranean climates. Int. Rice Comm.News!. 40:33-35.


Taillebois, J. 1994. Hybrid rice research at CIRAD/IRAT. p. 253-256, In: S. S. Virmani (ed.),Hybrid rice: New developments and future prospects. Int. Rice Res. Inst., P.O. Box 933,Manila 1099, Philippines.

Takeda, K. 1987. Open hull male sterile mutant in rice. Rice Genet. Newsl. 4:78-79.Takezaki, y. 1932. Inheritance of a few sterile plants. I. Male sterile rice. Agr. and Hortic.

Japan 7:387-397.Taliaferro, C. M., and E. C. Bashaw. 1966. Inheritance and control of obligate apomixis in

breeding bufflegrass, Pennisetum ciliare. Crop Sci. 6:473-476.Tamaru, N. 1994. Breeding studies on genetic male-sterility and hybrid sterility in rice

(01)1za sativa 1.). Mem. Fac. Agr. Hokkaido Univ. 1994(2):203-256.Tanaka, T., M. Kawase, A. Saito, M. Yano, N. Kishimoto, K. Saito, A. Yoshimura, H.

Takeda, T. Nagamine, and M. Nakagahra. 1989. Intraspecific variation on restrictionfragment length polymorphism of nuclear DNA in rice, Oryza sativa L. p. 599-602. In:S. Iyama and G. Takeda (ed.), Breeding research: The key to the survival of the earth.Proc. 6th Int. Congr. SABRAO, Tsukuba-machi, Japan.

Tang, C. Q., Z. Tong, D. C. Peng, and Q. D. Zhang. 1994. Photosynthesis characteristics ofchloroplasts in photoperiod-sensitive genic male sterile rice (Nongken 58s Rice). ActaPhytophysiol. Sinica 20:257-262.

Tang, S. X. 1987. A study on the cooking and eating qualities of Chinese hybrid rice (01)1zasativa 1.). Sci. Agri. Sinica 1987(5):17-22.

Tang, W. G., G. 1. Yang, G. 1. Li, T. 1. Ye, and Z. B. Yin. 1997. Breeding of the dual-purpose genic male sterile line in sorghum and its utilization. p. 206-214. In: Proc. Int.Symp. on Two-line System Heterosis Breed. in Crops, Changsha, Hunan, China, September 6-8,1997.

Tang, X. H., and Y. Shao. 1997. Relationship between the rice heading sensitivity and themale sterile sensitivity to photoperiod in view of developmental genetics. Acta Bot.Sinica. 39:618-622.

Tanksley, S. D., and S. R. McCouch. 1997. Seed banks and molecular maps: unlockinggenetic potential from the wild. Science 277:1063-1066.

Tatum, 1. A. 1971. The southern corn leaf blight epidemic. Science 171:1113-1116.Teng, 1. S., and Z. T. Shen. 1994a. Genetic studies on fertility restoration in cytoplasmic

male-sterile rice. Hybrid Rice 1994(3-4):63-66.Teng, 1. S., and Z. T. Shen. 1994b. Inheritance of fertility restoration for cytoplasmic male

sterility rice. Rice Genet. Newsl.11:95-97.Teng, 1. T. 1982. Cytological studies on development of sperm and pollen abortion in Nong

Jin 2, Fu You 1 A, and B of rice. p. 130-136. In: Selections of graduate theses, Dept. Biology, Hunan Teachers' College, Changsha, China.

Terao, H., and U. Mizushima. 1939. Some considerations on the classification of 01)1zasativa 1. into subspecies, so-called japonica and indica. Japan. J. Bot. 1939(3):213-258.

Terao, H., and U. Mizushima. 1944. On the affinity of rice varieties cultivated in East Asiaand America. Bur. Agr. Expt. Sta., Ministry Agr. Commerce, Japan.

Tian, c., X. Cheng, and Z. Liang. 1980. Several views on popularization of Xian (indica)hybrid rice. Yunnan Agr. Sci. Tech. 1980(2):12-18.

Tian, D. C. 1991. Cultivation for outcrossing rice. Sichuan Sci. Tech. Press, Chengdu, China.Tomar, J. B., and J. S. Nanda. 1983. Inheritance of aroma in rice (OJYza sativa L.). Madras

Agr. J. 70:491-494.Tong, H. J., and H. S. Ma. 1997. Cultivation techniques for increasing the outbred seed set

ting percentage in seed production of hybrid rice. China Rice 1997(5):14-17.Trees, S. c., and J. N. Rutger. 1978. Inheritance of four genetic male steriles. J. Hered.

69:270-272.Trinh, T. T. 1992. The FAO hybrid rice programme. Int. Rice Comm. Newsl. 41:51-59.


Trinh, T. T. 1993. New developments in hybrid rice. Int. Rice Comm. Newsl. 42:28-34.Trinh, T. T. 1994. FAO's contribution to hybrid rice development. p. 267-274. In: S. S.

Virmani (ed.), Hybrid rice: New developments and future prospects. Int. Rice Res. Inst.,P.O. Box 933, Manila 1099, Philippines.

Tu, Z. P., and D. W.Hu. 1989. Using chemical male gametocides in hybrid rice breedingin China. p. 237-243. In: Progress in irrigated rice research: Selected papers andabstracts from the Int. Rice Res. Conference, September 21-25,1987, Hangzhou, China.

United States Department of Agriculture. 1963. Rice gene symbolization and linkagegroups. U.S. Dept. Agr. Res. Ser. 34-38:1-56.

Vaidyanath, K., and G. M. Reddy. 1985. Studies on genetic divergence in the genus Olyza.J. Cytol. Genet. 20:59-68.

Vairavan, S., E. A. Siddiq, V. Arunachalam, and M. S. Swaminathan. 1973. A study on thenature of genetic divergence in rice from Assam and northeast Himalayas. Theor. Appl.Genet. 43:213-221.

do Valle, C. B., C. Glienke, and G. O. C. Leguizamen. 1994. Inheritance of apomixis inBrachiaria, a tropical forage grass. Apomixis Newsl. 1994(7):42-43.

do Valle, C. B., and Y. Savidan. 1996. Genetics, cytogenetics, and reproductive biology ofBrachiaria. p. 147-163. In: J. W. Miles, B. 1. Maass, and C. B. do Valle (eds.), Brachiaria:Biology, agronomy, and improvement. CIAT-EMBRAPA, Cali, Colombia.

Vijayakumar, C. H. M., M. 1. Ahmed, B. C. Viraktamath, and M. S. Ramesha. 1997. Selection of parents and prediction of heterosis in rice. p. 18-19. In: J. G. Coors, S. Pandey,M. V. Grinkel, A. R. Hallauer, D. C. Hess, K. R. Lamkey, A. E. Melchinger, G. Srinivasan,and C. W. Stuber (eds.), The genetics and exploitation of heterosis in crops. Proc. Int.CIMMYT Symp., 17-22 August 1997. ASA, CSSA, SSSA, Madison, WI, CIMMYT. Mexico. (AbstL)

Vijayakumar, C. H. M., M. S. Ramesha, B. C. Viraktamath, S. Singh, and M. 1. Ahmed. 1998.Analyzing leaf number in parental lines for hybrid rice seed production. Int. Rice Res.Notes 1998(2):27.

Viraktamath, B. C., C. H. M. Vijayakumar, M. 1. Ahmed, S. Singh, and M. S. Ramesha. 1998.Leaf number: a reliahle parameter for determining seeding intervals between parentallines in hybrid rice seed production. Int. Rice Res. Notes 1998(2):25-26.

Virmani, S. S. 1993. Progress in hybrid rice research in India. Int. Rice Comm. News!.42:35-38.

Virmani, S. S. 1994a. Heterosis and hybrid rice breeding. Springer-Verlag, Berlin and Heidelberg.

Virmani, S. S. 1994b. Prospects of hybrid rice in the tropics and subtropics. p. 7-19. In:S. S. Virmani (ed.), Hybrid rice: New developments and future prospects. Int. Rice Res.Inst., P.O. Box 933, Manila 1099, Philippines.

Virmani, S. S. 1996. Hybrid rice. Adv. Agron. 57::~77-462.

Virmani, S. S., and R. D. Dalmacio. HJ87. Cytogenic relationship between two cytoplasmic male sterile lines. Int. Rice Res. Newsl. 1987(1):14.

Virmani, S. S., and 1. B. Edwards. 1983. Current status and future prospects for breedinghybrid rice and wheat. Adv. Agron. 36:145-214.

Virmani, S. S., R. K. Govinda, C. Casal, R. D. Dalmacio, and P. A. Aurin. 1986. Currentknowledge of and outlook on cytoplasmic-genetic male sterility and fertility restorationin rice. p. 633-647. In: Rice genetics. Proc. Int. Rice Genetics Symp. Int. Rice Res. Inst.,P.O. Box 9~B, Manila 1099, Philippines.

Virmani, S. S., R. V. Kumar, and C. CasaL 1989. Cytoplasmic diversity among CMS linesof rice. Rice Genet. Newsl. 6:127-129.

Virmani, S. S., and H. L. Sharma. HJ93. Manual for hybrid rice seed production. Int. RiceRes. Inst., P.O. Box 933, 1099 Manila, Philippines.


Virmani, S. S., and C. Shinjyo. 1988. Current status of analysis and symbols for male sterile cytoplasms and fertility restoring genes. Rice Genet. Newsl. 5:9-15.

Virmani, S. S., B. C. Viraktamath, and M. T. Lopez. 1997. Nucleus and breeder seedproduction of thermosensitive genic male sterile lines. Int. Rice Res. Notes 1997(3):26-27.

Virmani, S. S., and B. H. Wan. 1988. Developments ofCMS lines in hybrid rice breeding.p. 103-114. In: Hybrid rice (Proc. Int. Symp. on Hybrid Rice, October 6-10,1986, Changsha, China). Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Virmani, S. S., J. B. Young, H. P. Moon, I. Kumar, and J. C. Flinn. 1991. Increasing riceyields through exploitation of heterosis. IRRI Res. Pap. Ser. 156.

Wagner, R. P. 1969. Genetics and phenogenetics of mitochondria. Science 163:1026-1031.Wan, B. H., X. H. Chen, Y. P. Lu, and K. Q. Liang. 1997. Studies and utilization of rice

germplasm with short photoperiod and low temperature inducing male sterility. p.59-63. In: Proc. Int. Symp. on Two-line System Heterosis Breed. in Crops, Changsha,Hunan, China, September 6-8,1997.

Wan, B. H., D. M. Li, and Q. L. Xuan. 1988. Classification of male sterile cytoplasms inrice by their nuclear-cytoplasm interactions. p. 262-263, In: Hybrid rice (Proc. Int.Symp. on Hybrid Rice, October 6-10,1986, Changsha, China). Int. Rice Res. Inst., P.O.Box 933, Manila 1099, Philippines.

Wan, B. H., and H. F. Ma. 1996. Genetical study on three T(P)GMS germplasms in rice. p.127-131. In: China National Hybrid Rice Research and Development Center (ed.), Proc.Annual Hainan Meeting China's National Two-line Hybrid Rice Program Plan, Hainan,China.

Wan, B. H., and H. F. Ma. 1997. Studies on the inheritance of three types germplasm inphoto-thermo sensitive genic male sterile rice. J. South China Agr. Univ. 1997(2):1-6.

Wan, C. C. 1989. Techniques on hybrid rice seed production and cultivation. Agr. Press.Beijing, China.

Wan, J. M., and H. Ikehashi. 1997. Identification of two types of differentiation in cultivated rice (Oryza sativa L.) detected by polymorphism of isozymes and hybrid sterility. Euphytica 94:151-161.

Wan, J. M., S. Yanagihara, H. Kato, and H. Ikehashi. 1993. Multiple alleles at a new locuscausing hybrid sterility between a Korean indica variety and a javanica variety in rice(Oryza sativa 1..). Japan. J. Breed. 43:507-516.

Wang, B., Y. N. Li, X. W. Zhang, L. Hu, and J. Z. Wang. 1990. Double-stranded RNA andmale sterility in rice. Theor. Appl. Genet. 79:556-560.

Wang, B., W. W. Xu, J. Z. Wang, M. L. Weng, M. Y. Li, R. C. Yang, and H. T. Nguyen. 1996.Tagging and mapping the thermosensitive genic male-sterile gene of rice with molecular markers. p. 596-600. In: G. S. Khush (ed.), Rice genetics III. Int. Rice Res. Inst., P.O.Box 933, 1099 Manila, Philippines.

Wang, B., W. W. Xu, J. Z. Wang, W. Wu, H. G. Zheng, Z. Y. Yang, J. D. Ray, and H. T.Nguyen. 1995.a. Tagging and mapping the thermo-sensitive genic male-sterile gene inrice (Oryza sativa 1..) with molecular markers. Theor. Appl. Genet. 91:1111-1114.

Wang, C. 1.., and Y. G. Tang. 1990. Studies on the genetic effects of sterile cytoplasm inhybrid rice (Oryza sativa L. subsp. sinica). Acta Agron. Sinica 1990(4):335-341.

Wang, F. P., M. H. MeL C. G. Xu, and Q. F. Zhang. 1997a. PMS1 is not the locus relevantto fertility difference between the photoperiod-sensitive male sterile rice Nongken 58sand normal rice Nongken 58. Acta Bot. Sinica. 39:922-925.

Wang, J. J" Y. B. Xu, and Z. T. Shen. 1991. Exploring the major restraints to the utilization of indica-japonica hybrids in rice. Sci. Agr. Sinica 1991(1):27-33.

Wang, J. X., L. Zou, L. P. Xu, Y. T. Cai, and L. S. Liu. 1994. Preliminary identification ofWCGin rice. p. 78-79, National Seminar Two-line System Hybrid Rice, Yangzhou, China.


Wang, J. Z., B. Wang, Q. F. Xu, D. C. Yang, and Y. G. Zhu. 1995b. Tagging rice photoperiodsensitive genic male sterile gene by RAPD analysis. Acta Genet. Sinica 22:53-58.

Wang, K. X. 1983a. Preliminary study of the genotypic model of the maintainer andrestorer of WA type male sterile cytoplasm. Fujian Agr. Sci. Tech. 1983(4):22-24.

Wang, S. H., C. Q. Li, K. N. Xu, Y. C. Luo and D. Z. Wang. 1994. The effects of ecologicalconditions on the fertility of photoperiod (thermo)-sensitive genic male sterile rice. J.Anhui Agr. Sci. 1994(2):119-123.

Wang, S. L. 1980. Inheritance of R genes in rice and methods for selection of new R lines.Hunan Agr. Sci. Tech. 1980(4):1-4.

Wang, S. L. 1983b. The development of R lines: Testcross breeding method in early generations. Hunan Agr. Sci. 1983(2):1-4.

Wang, S. L. 1996. Summary on China's three-line hybrid rice breeding during 1991-1995.p. 219-220. In: China National Hybrid Rice Research and Development Center (ed.),Proc. Annual Hainan Meeting China's National Two-line Hybrid Rice Program Plan,Hainan, China.

Wang, T., and Z. Tong. 1992. Changes in anther microstructure of the photoperiodsensitive genic male sterile rice. Acta Agron. Sinica 1992(2):132-136.

Wang, T., Z. Tong, T. Y. Kuang, and P. S. Tang. 1997b. Analysis of N-terminal sequenceof a 41kDa protein from a photoperiod-sensitive genic male-sterile mutant of rice. ActaBot. Sinica 39:979-982.

Wang, T., Y. H. Xiao, and W. F. Liu. 1990. Study on changes of leaf protein in HPGMRduring photoperiodic induction. J. Huazhong Agr. Univ. 9:369-374.

Wang, X., R. F. Que, Z. R. Jiang, and M. F. Xu. 1981. Physiological effects of Ethrel on theinduction of male sterility of rice. Acta Plant Physiol. Sinica 7:11-18.

Wang, X. M., M. Q. Wang, G. Z. Mei, H. Y. Wu, W. J. Duan, and W. J. Wang. 1991a.Photoperiod-conditioned male-sterility and its inheritance in rice. p. 217-226. In: RiceGenetics II. Int. Rice Res. Inst., P.O. Box 933,1099 Manila, Philippines.

Wang, X., M. Y. Yu, L. X. Tao, and F. D. Yao. 1991b. The male sterility in rice plantsinduced by CRMS. I. Effect ofCRMS on male sterility in rice plants. Chinese J. Rice Sci.1991(2):49-55.

Wang, X., M. Y. Yu, L. X. Tao, and F. D. Yao. 1991c. The male sterility in rice plantsinduced by CRMS. II. Physiological effects of CRMS on the inducing male sterility inrice. Chinese J. Rice Sci. 1991(3):104-108.

Wang, Z. Q., Y. H. Xiao, and W. F. Liu. 1991. Study on the changes in leaf protein in NongKen 58s induced by photoperiod. Hereditas (Beijing) 1991(5):1-3.

Watanabe, T., S. Sakaguchi, and M. Kudo. 1968. Male-sterile rice with the cytoplasm ofLead rice from Burma. Japan. J. Breed. 18 (Suppl 2):77-78.

Watanabe, Y. 1971. Establishment of cytoplasmic and genetic male-sterile lines by meansof indica-japonica cross. Oryza. 8(Suppl. 2):9-16.

Weerarathe, H. 1954. Hybridization technique in rice. Trop. Agr. (Colombo) 110:93-97.Wei, B. G., G. C. Sun, and J. W. Chang. 1997. Photoperiod sensitivity of a photoperiod-sen

sitive male sterile line in soybean. p. 114-120. In: Proc. Int. Symp. on Two-line SystemHeterosis Breed. in Crops, Changsha, Hunan, China, September 6-8,1997.

Wei, J. c., R. L. Wang, and G. Y. Cheng. 1994. Studies on the kinetic properties of ribulose1, 5-bisphosphate carboxylase from F1 hybrid rice. Acta Phytophysiol. Sinica. 20:55-60.

Wu, B. J., M. T. Xie, Y. P. Chen, H. Jiang, and X. X. Guo. 1991. Cytologic and embryological studies on apomicts discovered from male sterility line CI001 in rice (Oryza sativaL.). Sci. China. Ser. B. 34:823-831.

Wu, C. M., J. M. Cao, and X. L. Fu. 1993. Fertility alteration and flowering behaviors ofphotoperiod sensitive genic male-sterile rice (PGMR) in the cold regions. Chinese J. RiceSci. 1993(2):113-116.


Wu, H. Y., and X. M. Wang. 1990. Effects of length of photoperiod on microsporogenesisin Nongken 58s. J. Huazhong Agr. Univ. 9:464-465.

Wu, X. J., and H. Q. Yin. 1992. Genetics and stability of thermo-sensitive genic male sterile rice. Chinese J. Rice Sci. 1992(2):63-69.

Xi, D. W., W. J. Cheng, D. 1. Yi, D. R. Yu, Z. 1. Ning, X. X. Deng, and M. Li. 1997. Geneticanalysis on the TGMS line in rape of B. napus Xiangyou 91s. p. 215-220. In: Proc. Int.Symp. on Two-line System Heterosis Breed. in Crops, Changsha, Hunan, China, September 6-8, 1997.

Xiao, G. Y., and 1. P. Yuan. 1997. Effects of water temperature on male sterility of thethermo-sensitive genic male sterile (TGMS) rice lines under tbe simulated low air temperature conditions appeared occasionally in high summer. Chinese J. Rice Sci.1997(4):241-244.

Xiao, J. H., S. Grandillo, S. N. Ahn, S. R. McCouch, S. D. Tanksley, J. M. Li, and 1. P. Yuan.1996b. Genes from wild rice improve yield. Nature 384:223-224.

Xiao, J. B., J. M. Li, S. Grandillo, S. N. Ahn, 1. P. Yuan, S. D. Tanksley, and S. R. McCouch.1998. Identification oftrait-improving QTL alleles from a wild rice relative, Oryza rufipagan. Genetics 150:899-909.

Xiao, J. H., J. M. Li, 1. P. Yuan, S. R. McCouch, and S. D. Tanksley. 1996a. Genetic diversity and its relationship to hybrid performance and heterosis in rice as revealed by PCRbased markers. Theor. Appl. Genet. 92:637-643.

Xiao, J. H., J. M. Li, 1. P. Yuan, and S. D. Tanksley. 1995. Dominance is the major geneticbasis of heterosis in rice as revealed by QTL analysis using molecular markers. Genetics 140:745-754.

Xiao, Y. H., P. Chen, and W. F. Liu. 1987. A comparative analysis of free amino acids ofHPGMR during its abortion of anthers. J. Wuhan Univ. (HPGMR):7-16.

Xiao, Y. H., and W. F. Liu. 1981. Studies on esterase isozymes of hybrid rice and its threeparental lines. Hubei Agr. Sci. 1981(11):1-4.

Xiao, Y. H., G. F. Sun, and W. F. Liu. 1991. Research on prediction of rice heterosis andscreening for elite rice hybrids on the basis of the activity of nitrate reductase. p.261-267. In: Papers on basic research of rice breeding techniques. China Sci. Tech.Press, Beijing, China.

Xie, F. M. 1993. Hybrid performance and genetic diversity as revealed by RFLPs in rice(Oryza sativa 1.). Ph.D. Diss. Texas A & M Uniy., College Station.

Xie, H. A., J. M. Luo, J. T. Zhang, S. G. Zhang, and J. X. Zhang. 1996. Progress on the development of restorer lines and elite hybrids in rice. p. 221-227. In: China National HybridRice Research and Development Center (ed.). Proc. Annual Hainan Meeting China'sNational Two-line Hybrid Rice Program Plan, Hainan, China.

Xiong, Z. M., S. H. Cheng, and 1. Y. Cao. 1989. The status and prospects in the researchand utilization of heterosis in crosses between indica and japanico in rice. Rice Abstr.1989(5):1-4.

Xiong, Z. M., S. K. Min, S. H. Cheng, and L. Y. Cao. 1990. Identification and evaluationof wide compatibility in rice cultivars T984 and Pecos. Chinese J. Rice Sci. 1990(1):9-14.

Xiong, Z. M., S. K. Min, X. D. Zhu, J. 1. Wang, and 1. Y. Cao. 1993. Discussions on genetics of wide compatibility by pedigree analysis. Chinese J. Rice Sci. 1993(2):101-104.

XU, J., and 1. Wang. 1980. A preliminary study on heterosis and combining ability of rice.Yichuan (Hereditas, Beijing) 2:17-19.

XU, J. F., and 1. Y. Wang. 1981. The genetic divergence and the heterosis in rice. AnhuiAgr. Sci. (Special issue, Rice Quantitative Genetics). p. 65-77.

Xu, M. L. and G. Q. Zhou. 1996. Studies on thermo-sensitive part of Pei'ai 64s relating toits fertility expression. Hybrid Rice. 1996(2):28-30.


Xu, R. L., X. H. Jiang, S. Y. Shi, A. M. Liu, and Q. H. Yi. 1992. Comparative studies on theconstitutions of protein polypeptides between a WA type cytoplasmic male sterile riceand its maintainer. Acta Genet. Sinica 19:446-452.

Xu, S. H. 1979. Cytological studies on pollen abortion of Honglian Hua-ai male sterile linesin rice. Nat. Sci. J. 1979(2):79-85.

Xu, S. J., and B. H. Li. 1988. Managing hybrid rice seed production. p. 157-163. In: Hybridrice (Proc. lnt. Symp. on Hybrid Rice, October 6-10,1986, Changsha, China). Int. RiceRes. Inst., P.O. Box 933, 1099 Manila, Philippines.

Xu, Y. B., and Z. T. Shen. 1988. Receptivity of exserted stigmas. Int. Rice Res. Newsl.1988(3):7-8.

Xu, Y. B., J. J. Wang, and Z. T. Shen. 1989. Study on intersubspecific compatibility in rice.Chinese J. Rice Sci. 1989(3):113-118.

Xue, G. X. and P. Chen. 1992. The effects of temperature on the fertility-transition ofPGMSrices induced by different photoperiods. Acta Agron. Sinica 1992(6):429-438.

Xue, G. X., and J. Y. Deng. 1991. Studies on the Hubei photoperiodic sensitive nuclear malesterile rice Oryza-sativa-ssp-japonica: modification of minor genes on the phenotypic values of progenies of photoperiodic sensitive male sterile rice. Acta Genet Sinica. 18:59-66.

Xue, G. X., and J. Z. Zhao. 1990. A preliminary study on the critical daylength evokingthe photoperiodic sensitive male sterility of rice and their responses to other environmental factors. Acta Agron Sinica 1990(2):112-122.

Yab, T. c., and S. C. Chang. 1976. Heterosis of rice hybrids under wetland and drylandconditions. SABRAO J. 1976(1):35-40.

Yabuno, T. 1977. Genetic studies on the interspecific cytoplasm substitution lines ofjaponica varieties of Oryza sativa L. and O. glaberrima Steud. Euphytica 26:451-463.

Yamaguchi, Y., R. Ikeda, H. Hirasawa, M. Minami, and A. Ujihara. 1997. Linkage analysis of thermosensitive genic male sterility gene, tms-2 in rice (Oryza sativa L.). Breed.Sci. 47:371-373.

Yan, J. Q, and Q. Z. Xue. 1995. Genetic analysis of wide-compatibility and fertility restoration ofcytoplasmic male sterility in rice (Oryza sativa L.). Acta Genet. Sinica. 22 :361-371.

Yan, Q. C. 1987. Identification of the genuineness and the purity of hybrid rice (Oryzasativa ssp. indica) and its parental lines by electrophoretic analysis of esterases. SeedSci. Tech. 15:645-649.

Yan, S. G., Y. Song, and J. Zhang. 1991. Preliminary report on Chengdu apomictic rice. p.101-105. In: K. D. Zhou (ed.), Study on rice apomixis. Chengdu, Sichuan Sci. Tech.Press, China.

Yanagihara, S. H., H. Kata, and H. Ikehashi. 1992. A new locus for multiple alleles causing hybrid sterility between an Aus variety and Javanica varieties in rice (Oryza sativaL.). Japan. J. Breed. 42:793-801.

Yang, D. c., and Y. G. Zhu. 1990. The photoreversibility ofred/far-red light for the pollenfertility of HPGMR (Nongken 58s). J. Huazhong Agr. Univ. 18:461-463.

Yang, D. c., Y. G. Zhu, and L. J. Tang. 1990. The contents of four hormones in the leavesof HPGMR and the fertility conversion. J. Huazhong Agr. Univ. 9:394-399.

Yang, D. H. and F. Y. Chen. 1990. A study on heredity of hybrid rice with wild abortivecytoplasm male sterility and restoring characteristics. J. Guangxi Agr. College1990(2):61-69.

Yang, F. Y. 1991a. The prediction of heterosis via homogenate complementation methodin hybrid rice. p. 251-255. In: Papers on basic research of rice breeding techniques.China Sci. Tech. Press, Beijing, China.

Yang, F. Y. 1991b. Rice heterosis and the ATP content in the yellowish tips ofits parentallines. p. 246-250, In: Papers on basic research of rice breeding techniques. China Sci.Tech. Press, Beijing, China.

154 J. 11 AND 1. YUAN

Yang, F. Y., Q. R. Xing, W. W. Chen, and J. P. Zhang. 1990. ATP concentration in parentalseedlings in relation to heterosis of hybrid rice. Acta Bot. Sinica 32:622-625.

Yang, J. D. 1990a. The pollen fertility transformation of a temperature-sensitive male sterile line, 5460TS, under natural conditions in Fuzhou. J. Fujian Agr. College1990(3):245-251.

Yang, K. H. and Z. B. Li. 1989. A preliminary study on rice wide compatibility and its utilization in hybrid rice breeding. J. Huazhong Agr. Univ. 17:302-310.

Yang, R. c., K. Liang, N. Wang, and S. Chen. 1992. A recessive gene in indica rice 5460sfor thermosensitive genic male-sterility. Rice Genet. Newsl. 9:56-57.

Yang, R. C., and H. R. Lu. 1984. Preliminary analysis of restorer genes in the restorer lineIR24. Acta Agron. Sinica 1984(2):81-86.

Yang, R. c., and H. R. Lu. 1989. Study on effects of nucleo-cytoplasmic interaction in Oryza1. Sci. Agr. Sinica 1989(2):56-63.

Yang, S. R. 1959. Research on the indica/japonica crossing. Agr. J. 1959(4):3-6.Yang, S. R. 1987. New trends in breeding rice for very high yields-combination of ideal

plant morphology with favorable vigor. J. Shenyang Agr. Univ. 1987(1):1-5.Yang, Z. P. 1997. Inheritance of photoperiod sensitive genic male sterility and breeding

of photoperiod sensitive genic male-sterile lines in rice (OIyza sativa 1.) through antherculture. Euphytica 94:93-99.

Yang, Z. P., M. H. Chen, X. M. Deng, and Y. F. Wang. 1997. Rice anther culture: a tool forbreeding of widely compatible restorer lines for cytoplasmic male sterility. Acta Agr.Shanghai 1997(1):1-6.

Yang, Z. Y. 1990b. Evaluation and utilization of usable and unusable heterosis in F I

hybrids between indica and japonica rice subspecies. Chinese J. Rice Sci.1990(2):49-55.

Yao, J. 1., D. T. Cai, P. F. Ma, and H. Zhu. 1997. Embryological studies on apospory in riceHDAR. Chinese J. Rice Sci. 1997(2):113-117.

Yao, S. Y., M. T. Henderson, and N. E. Jodon. 1958. Cryptic structural hybridity as a probable cause of sterility in intervarietal hybrids of cultivated rice, Oryza sativa 1. Cytologia 23:46-55.

Yasuki, U., O. Hisatoshi, S. Hiroyuki, and O. Hiroshi. 1997. Utilization of environmentsensitive genic male sterility in hybrid rice breeding. p. 193-197. In: Proc. Int. Symp.on Two-line System Heterosis Breed. in Crops, Changsha, Hunan, China, September6-8,1997.

Ye, X. 1., Z. 1. Chen, and Y. Q. Li. 1995. The characteristics of apomictic PDER rice variety and development of diploid embryo sac. J. Trap. Subtrop. Bot. 1995(3):36-42.

Yin, H. Q. 1993. Progress and recommendations in hybrid rice in northern Vietnam. Int.Rice Comm. Newsl. 42:39-41.

Ying, C. S. 1994. Rice germplasm resources in China. Chinese Rice Res. Newsl.1994(2):9-10.

Yogeesha, H. S., and M. Mahadevappa. 1995. Histological and histochemical causes ofmale sterility in two cytoplasmic-genic male sterile lines of rice (Oryza sativa L.).Indian J. Genet. Plant Breed. 55:58-64.

Yogeesha, H. S., and M. Mahadevappa. 1996. Adaptability of CMS and maintainer linesof rice (Oryza sativa 1.). Seed Res. 1996(2):98-103.

Yokozeki, Y., A. Nakamura, A. Inagaki, N. Ichikawa, Y. Koshino, H. Araki, H. Shimada,T. Tsuchiya, C. Shinjyo, and T. Fujimura. 1996. DNA markers of a fertility restorationgene Rf-1 in rice, Oryza sativa 1. Breeding Science 46(suppl. 1):73.

Yoshida, H., and H. Fujimaki. 1985. Development of male sterile-restoring system for hybridrice production. 4. Standard heterosis appearing in yield components of half-sib F I

hybrids with a common female parent "Akihikari". Japan. J. Breed. 36(suppl.1): 98-99.


Young, J. B., and S. S. Virmani. 1990. Heterosis in rice ov~r environments. Euphytica51:87-93.

Yu,1. Q., and Q. Lin. 1962. A study on the relationship of cutivated rice varieties in China.Acta Agron. Sinica 1:233-258.

Yu, M. Y., X. Wang, 1. X. Tao, and F. D. Yao. 1991. The male sterility in rice plant inducedby CRMS: III. Effect of CRMS on free proline content in anthers of rice and its possiblerelation to abortion of anther. Chinese J. Rice Sci. 1991(4):169-174.

Yu, S. B., J. X. Li, C. G. Xu, Y. F. Tan, Y. J. Gao, X. H. Li, Q. F. Zhang and M. A. Maroo£.1997. Importance of epistasis as the genetic basis of heterosis in an elite rice hybrid.Proc. Nat. Acad. Sci. (USA). 94:9226-9231.

Yu, Z. H., S. R. McCouch, T. Kinoshita, S. Sato, and S. D. Tanksley. 1995. Association ofmorphological and RFLP markers in rice (Oryza sativa 1.). Genome. 38:566-574.

Yuan, 1. P. 1966. A preliminary report on male sterility in rice (in Chinese). Sci. Bul.1966(4):32-34.

Yuan, L. P. 1985. A concise course in hybrid rice. Hunan Sci. Tech. Press, Changsha, China.Yuan, L. P. 1987. The strategy on hybrid rice breeding. Hybrid Rice 1987(1):1-3.Yuan, L. P. 1989. Hybrid rice US Patent-4827664 (May 9 1989).Yuan, L. P. 1990. Progress of two-line system hybrid rice breeding. Sci. Agr. Sinica

1990(3):1-6.Yuan, L. P. 1991a. Breeding for intersubspecific heterosis in rice. Program Report. China

National Hybrid Rice Res. Develop. Center.Yuan, L. P. 1991b. Outlook on the development of hybrid rice breeding. p. 205-211. In:

Prospects of rice farming for 2000. Zhejiang Pub!. House Sci. Tech., Hangzhou, China.Yuan, L. P. (ed.). 1992a. Current status of two-line hybrid rice research. Agr. Press, Bei

jing, China.Yuan, 1. P. 1992b. The technological strategy for the development of T(P)GMS lines.

Hybrid Rice 1992(1):1-4.Yuan, 1. P. 1992c. Recent breakthroughs in hybrid rice research and development in

China. Int. Rice Commission News!. 41:7-13.Yuan,1. P. 1994a. Purification of T(P)GMS lines of rice and their production of founda

tion seeds. Hybrid Rice. 1994(6):1-3.Yuan, 1. P. 1994b. Purification and foundation seed production of T(P)GMS lines in rice.

National Seminar on Two-line System Hybrid Rice, Yangzhou, China. p. 1. (Abstr.)Yuan, L. P. E}94c. Purification and production of foundation seed of rice PGMS and

TGMS line. Hunan Agr. Res. Newsl. 1994(2):2-3.Yuan, L. P. 1994d. Increasing yield potential in rice by exploitation of heterosis. p. 1-6.

In: S. S. Virmani (ed.), Hybrid rice: New developments and future prospects. Int. RiceRes. Inst., P.O. Box 933, Manila 1099, Philippines.

Yuan, L. P. 1995. Current status of hybrid rice in China and future strategies for 21st century. p. 31-33. In: M. I. Ahmed, and B. C. Virakatamath (eds.), Hybrid rice seed production technology: Theory and practice. Hyderabad, India.

Yuan, 1. P. 1996. Prospects for hybrid rice breeding in China. October 29-November 1,1996. Grupo Tecnico de Trabajo de Hibridos de Arroz Para America Latina y el Caribe(GRUTHA), Ibaqu8, Columbia.

Yuan, L. P. 1997a. Exploiting crop heterosis by two-line system hybrids: Current status andfuture prospects. p. 1-7. In: Proc. Int. Symp. on Two-line System Heterosis Breed. inCrops, Changsha, Hunan, China, September 6-8,1997.

Yuan, L. P. 1997b. Strategy for the development of super-high-yielding hybrid rice. HybridRice 1997(6):1-6.

Yuan, L. P. 1998a. Hybrid rice breeding for super high yield. XVIIIth Int. Congr. Genet.,Beijing, China, August 10-15, 1998.


Yuan,1. P. 1998b. Hybrid rice breeding in China. In: S. S. Virmani, E. A. Siddiq, and K.Muralidharan (eds.), Advances in hybrid rice technology (Proc. 3rd lnt. Symp. HybridRice, Hyderabad, India, October 1996). lnt. Rice Res. lnst., P.O. Box 933,1099 Manila,Philippines.

Yuan, 1. P., and H. X. Chen. 1988. Breeding and cultivation of hybrid rice. Hunan Sci.Tech. Press, Changsha, China.

Yuan, 1. P., Y. C. Li, and H. D. Deng. 1990. Progress of studies in rice twin seedlings. The4th annual meeting ofthe Rockefeller Found. lnt. Program Rice Biotechnol., May 9-12,1990. lnt. Rice Res. lnst., P.O. Box 933,1099 Manila, Philippines.

Yuan, 1. P., and S. S. Virmani. 1988. Status of hybrid rice research and development. p.7-24. In: Hybrid rice (Proc. lnt. Symp. on Hybrid Rice, October 6-10,1986, Changsha,China). lnt. Rice Res. lnst., P.o. Box 933, Manila 1099, Philippines.

Yuan, 1. P., X. J. Wu, Y. C. Yan, and X. H. Luo. 1997. A strategy for developing wide spectrum compatibility rice line. Sci. Agr. Sinica 1997(4):1-8.

Yuan, S. c., Z. G. Zhang, H. H. He, H. L Zen, K. Y. Lu, J. H. Liang, and B. X. Wang. 1993.Two photoperiod-reactions in photoperiod-sensitive genic male-sterile rice. Crop Sci.33:651-660.

Yuan, S. c., Z. G. Zhang, and C. Z. Xu. 1988. Studies on the critical stage offertility changeinduced by light and its phase development in HPGMR. Acta Agron. Sinica 1988(1):7-13.

Zeng. S. J.. C. Y. Liang, J. K. Liang, and B. Y. Chen. 1997. Studies on the heterosis of twoline hybrid rice. 1. Performance of main agronomic characters. J. Trop. Subtrop. Bot.1997(4):56-61.

Zeng, S. X.. Z. W. Lu, and X. Q. Yang. 1979. Studies on the heterosis ofF j hybrids in riceand its relation to the parents. Acta Agron. Sinica 1979(3):23-24.

Zhang, A. F., B. S. Shen, X. K. Dai, M. H. Mei, M. A. Saghai Maroof, and Z.B. Li. 1993a.An RFLP-based genetic analysis of photoperiod sensitive male-sterility in rice. RiceGenet. News!. 10:94-97.

Zhang. D. P., X. N. Deng, G. X. Yu. X. H. Lin. Y. F. Xie, and Z. B. Li. 1990. Chromosomallocation of the photoperiod sensitive male sterile gene in Nongken 58s. J. Huazhong Agr.Univ. 18:407-419.

Zhang, G., T. S. Bharaj, Y. Lu, S. S. Virmani, and N. Huang. 1997a. Mapping of the Rf-3nuclear fertility-restoring gene for WA cytoplasmic male sterility in rice using RAPDand RFLP markers. Theor. Appl. Genet. 94:27-33.

Zhang, G., Y. Lu, and N. Huang. 1996. Molecular mapping of fertility-restoring gene Rf-3in rice. lnt. Rice Res. Notes 1996(2-3):16-17.

Zhang, G. Q., and Y. G. Lu. 1989. Genetic studies on the hybrid sterility in cultivated rice:1. Diallel analysis of the hybrid sterility among isogenic F j sterile lines. Chinese J. RiceSci. 1989(3):97-101.

Zhang, H. L., and Y. D. Deng. 1990. Progress in the development of three-line system intersubspecifc hybrid rice between indica and japonica. Hybrid Rice 1990(2):29-31.

Zhang, J. H., Z. Tong, Z. K. Chen. and F. Zhou. 1994a. Ultrastructural studies onmicrosporogenesis in the photoperiod-sensitive genic male-sterile rice. Acta Bot. Sinica.6:129-133.

Zhang, J. Z., 1. J. Fang, and Y. W. Yuan. 1987. Study on the application of genetic distanceto the selection of Honglian type restorers in hybrid rice. Guangdong Agr. Sci. 1987(1):1-3.

Zhang, N. G., and X. Zhou. 1992. Relation between endogenous acidic phytohormones(IAA. GA1+4. ABA) and fertile transformation of photoperiod sensitive genic malesterile rice (PGMR)-Nongken 58s. J. Nanjing Agr. Univ. 1992(3):7-12.


Zhang, P. J., S. Ming, C. w. Xu, L. S. Yang, Y. S. Bai, C. Q. Sun, and X. K. Wang. 1997b.Heterosis and combining ability of hybrid rice and its relation to japonica-indica indexof parents. Rice Genet. Newsl. 14:34-36.

Zhang, Q. F., B. Z. Shen, X. K. Dai, M. H. MeL M. A. S. Maroof, and Z. B. Li. 1994b. Usingbulked extremes and recessive class to map genes for photoperiod-sensitive genic malesterility in rice. Proc. Nat. Acad. Sci. (USA). 91:8675-8679.

Zhang, Q. F., K. D. Liu, G. P. Yang, M. A. A. Shaghai, C. G. Xu, and Z. Q. Zhou. 1997c.Molecular marker diversity and hybrid sterility in indica-japonica rice crosses. Theor.Appl. Genet. 95:112-118.

Zhang, S. Q., S. H. Cheng, and L. Y. Cao. 1988. The compatibility of F1 hybrid betweenindica and japonica. Chinese J. Rice Sci. 1988(2):94-96.

Zhang, S. Q., X. B. Xie, and H. F. Chen. 1992. A preliminary research and progress in thedevelopment of three-line system intersubspecific crosses. Presentation Annual Meeting of Zhejiang "8812" Research Plan, Hangzhou, China.

Zhang, T. B. 1985. Male sterility and cytoplasmic regulation of gene action in rice: A briefaccount of studies. SABRAO J. 1985(1):87-90.

Zhang, X., and B. Wang. 1990. A newly discovered double stranded RNA associated withcytoplasmic male-sterile rice. Acta Genet. Sinica 17:289-293.

Zhang, X. G., and Y. G. Zhu. 1991. The inheritance of sterility in Hubei photoperiod sensitive nuclear male-sterile rice. Heriditas (Beijing) 1991(3):1-3,37.

Zhang, Z. G., X. G. Lu, and L. P. Yuan. 1992. Reflections on identification of light andtemperature properties of fertility transformation of PGMR. Hybrid Rice 1992(6):29-32.

Zhang, Z. G., S. C. Yuan, and C. Z. Xu. 1987. The influence of photoperiod on the fertility changes of Hubei photoperiod-sensitive genic male-sterile rice (HPGMR). ChineseJ. Rice Sci. 1987(3):137-144.

Zhang, Z. G., H. L. Zeng, Y. Z. Li, S. C. Yuan, and D. P. Zhang. 1992. Influence of photoand thermo-conditions during vegetative growth of PGMS on its fertility alteration.Hybrid Rice 1992(5):34-36.

Zhang, Z. G., H. L. Zeng, Y. Z. Li. S. C. Yuan, and D. P. Zhang. 1993h. Influence on thefertility alteration of photo- and thermoconditions during vegetative growth phase inPGMR J. Huazhong Agr. Univ. 1993(1):84-86.

Zhang, Z. G., H. L. Zeng, J. Yang, S. C. Yuan, and D. P. Zhang. 1994c. Conditions inducing fertility alteration and ecological adaptation of photoperiod-sensitive genic malesterile rice. Field Crops Res. 38:111-120.

Zhangxing, Z., and H. Chunnong. 1980. A study of male sterility induced rice (Dryza sativaL.) by 3-(p-chlorophenyl) 6-methoxy-s-triazine-2,4-(1H,3H) Dione. J. Hangzhou Univ.(Zhejiang, China) 1980(4):81-87.

Zhao, B. R, H. D. Deng, and Y. Q. Li. 1992. Preliminary report on the embryological studyof apomixis in rice twin seedlings. Wuhan Bot. Res. 1992(3):213-218.

Zhao, S. X. 1977. How to fix heterosis'? Genetics & Breed. 1977(:~):21-22.

Zheng, K. L., B. Shen, H. R Qian, and J. L. Wang. 1992. Tagging genes for wide compatibility in rice via linkage to RFLP markers. Chinese J. Rice Sci. 1992(4):145-150.

Zheng, K. L., B. Shen, F. Yu, C. Z. Zhao, X. F. Qi, and X. M. Xu. 1990. Restriction fragment length polymorphism in rice. Chinese J. Rice Sci. 1990(4):145-149.

Zhong, W. G., C. G. Li, Z. D. Chen, and A. N. Yang. 1997. Efficacy of chemical gametocide13 in emasculation ofrice. Jiangsu J. Agr. Sci. 1997(3):191-192.

Zhong, W. G., A. N. Yang, and J. S. Zou. 1992. A preliminary report on the effect of temperature conditions on the chemical emasculation by Emasculator No.2. Jiangsu Agr.Sci. 1992(4):4-5.


Zhou, C. S., J. B. Liu, K. Y. Wu, and X. N. Li. 1993a. Studies of the cold water irrigationtechnique for multiplication of low critical temperature TGMS rice. Hybrid Rice1993(2):15-16.

Zhou, G. F., C. M. Guo, C. Q. Yin, and X. B. Sheng. 1991a. Preliminary observations onthe fertility alteration and genetic behavior of the indica dual-purpose TGMS line,Heng-Nong s-1. Hunan Agr. Sci. 1991(4):10-12.

Zhou, H., J. G. Glaszmann, K. S. Cheng, and X. Q. Shi. 1988. A comparison of classification of cultivated rice. Chinese J. Rice Sci. 1988(1):1-7.

Zhou, K. D., H. W. Li, and Y. Cheng. 1994. Realization of fine grain quality certainly beingthe way to develop hybrid rice. Hybrid Rice 1994(3-4):42-45.

Zhou, K. D., X. D. Wang, M. Luo, K. M. Gao, S. L. Zhou, Z. B. Yan, P. Li, and F. Chen. 1991b.Advance in study on Sichuan apomixis rice. J. Sichuan Univ. 28:382-387.

Zhou, K. D., X. D. Wang, M. Luo, K. M. Gao, S.L. Zhou, Z. B. Yan, P. Li, F. Chen, and G. M.Zhou. 1993b. Initial study on Sichuan apomixis rice (SAR-1). Sci. China. Ser. B.36:420-429.

Zhou, K. L. 1994. Breeding of CMS lines in indica hybrid rice. Hybrid Rice 1994(3-4):22-26.

Zhou, K. L., and F. M. Liao. 1995. Xiangyou 63, a quasi-aromatic hybrid rice with goodquality and high yield. lnt. Rice Res. Notes 1995(4):9-10.

Zhou, K. L., and F. M. Liao. 1997. Xingxiangyou 77, a high-yielding and fine-qualitysemi-aromatic hybrid rice. Int. Rice Res. Notes 1997(3):22-23.

Zhou, M. L., Q. Y. Tang, and J. M. He. 1997. Effects of photoperiod on fertility of ecological male sterile (EMS) wheat. p. 107-113. In: Proc. Int. Symp. on Two-line System Heterosis Breed. in Crops, Changsha, Hunan, China, September 6-8,1997.

Zhou, S. L. 1978. Rice science. Agr. Press, Beijing, China.Zhou, T. L., J. H. Shen, and F. C. Yeo 1983. A genetic analysis on the fertility of hsien type

hybrid rice with wild rice cytoplasm. Acta Agron. Sinica 1983(4):241-247.Zhu, L., C. Lu, P. Li, L. Shen, Y. Xu, P. He, and Y. Chen. 1996. Using double haploid pop

ulations of rice for quantitative trait locus mapping. p. 745-748. In: G. S. Khush (ed.),Rice genetics III. Int. Rice Res. Inst., P.O. Box 933, 1099 Manila, Philippines.

Zhu, Q. S., Z. J. Zhang, J. C. Yang, X. Z. Cao, Y. Z. Lang, and Z. C. Wang. 1997. Sourcesink characteristics related to the yield in intersubspecific hybrid rice. Sci. Agr. Sinica.1997(3):52-59.

Zhu, Y. C., and F. M. Liao. 1990. Research progress on heterosis utilization in two-line system intersubspecific crosses. Hybrid Rice 1990(3):32-34.

Zhu, Y. G. 1979. Studies on male sterile lines of rice with different cytoplasms. Acta Agron.Sinica 1979(4):29-38.

Zhu, Y. G., and D. C. Yang. 1992. Study and application on photoperiod-sensitive genicmale-sterile rice. Wuhan Univ. Press, Wuhan, China.

Zhu, Y. G., and J. H. Yu. 1987. A study on HPGMR breeding in indica. J. Wuhan Univ.(HPGMR):139-144.

Zhu, Y. G., and J. H. Yu. 1989. Stability and inheritance of photoperiod-sensitive genicmale-sterility in rice. p. 745-748. In: S. Iyama and G. Takeda (eds.), 6th Int. Congr.SABRAO. Tsukuba-machi, Japan.

Zhu, Y. G., and W. G. Zhang. 1987. Studies on isozymes with heterosis in the seedlingsof hybrid rice. Acta Agron. Sinica 1987(2):89-96.

Zirkle, C. 1952. Chapter 1. Early ideas on inbreeding and cross breeding. p. 1-13. In: J. W.Gowen (ed.), Heterosis. Iowa State College Press, Ames.

Documents

Plant Breeding Reviews (Janick/Plant Breeding Reviews, Volume 17) || Hybrid Rice: Genetics, Breeding, and Seed Production