Field Crops Research, 6 (1983) 219--238 219 Elsevier Scient if ic Publishing Company , Amste rdam -- Printed in The Nether lands

SOME PHYSIOLOGICAL ASPECTS OF THE DOMESTICATION AND IMPROVEMENT OF RICE (OR YZA spp.)

M.G. COOK and L.T. EVANS

CSIRO, Division o f Plant Industry, P.O. Box 1600, Canberra City, A.C.T. 2601 (Australia)

(Accepted 4 Oc tober 1982)

ABSTRACT

Cook, M.G. and Evans, L.T., 1983. Some physiological aspects o f the domest ica t ion and improvement of rice (Oryza spp.). Field Crops Res., 6: 219--238.

Fo r ty -one lines represent ing seven species involved in both Asian and African domestica- t ion and improvement o f rice were grown and compared under control led env i ronment condi t ions at 27/22°C. Kernel size varied over a four-fold range, being smallest in the wild Asian species, but there was no consistent increase in ei ther grain or leaf size with domesti- cat ion and improvement . Pho tosyn the t i c rate per uni t leaf area tended to be lower in the African and wild Asian species than in O. sativa cultivars. Flag leaf photosynthes is re- mained high for longer in O. sativa than in o ther species. Leaf pho tosyn the t i c rate was posit ively correlated with leaf ni t rogen con ten t and wi th specific leaf weight across lines. The p ropor t ion o f dry ma t t e r present as leaf varied lit t le, bu t greater differences in leaf area ratio and specific leaf weight occurred. These affected variat ion in relative growth rate, which tended to be greatest at high leaf area ratios but least at high rates of photosynthesis . Percentage seed set and harvest index of the main shoo t bo th increased with domest ica t ion and improvement . These findings are compared with those for o ther domest icated species.

I N T R O D U C T I O N

Physiological aspects of crop improvement, such as changes in rates of photosynthesis or growth, have often been examined by comparing older and newer varieties, but the changes found have been neither striking nor consistent. For several crops the comparisons have therefore been broadened to include also the wild relatives or progenitors of the crop plant, to gain a wider perspec- tive on the physiological changes that have accompanied domestication and improvement.

Wheat, cowpea, maize, pearl millet, sorghum, sugar cane and cot ton have all been examined to some extent in this way (Evans, 1976; Gifford and Evans, 1981), wheat by far the most extensively (Evans and Dunstone, 1970; Evans et al., 1970; Khan and Tsunoda, 1970a, b; Dunstone et al., 1973; Dunstone and Evans, 1974). One of the more unexpected findings with wheat has been that the maximum photosynthet ic rate per unit leaf area appears to have fallen in the course of domestication and improvement. This could, however, be due to

0378-4290 /83 /$03 .00 © 1983 Elsevier Science Publishers B.V.

220

changes in leaf and cell size associated with the increase in ploidy during the evolution of wheat, and for that reason we decided to examine another cereal with the C3 pathway of photosynthesis. Rice was selected because all the cultivated rices and their wild progenitors are diploid, because domestication has occurred independently in both Asian and African species, and because we could take advantage of the remarkably comprehensive collection of rice genetic resources maintained by the International Rice Research Institute (IRRI).

Fig. 1 indicates Chang's (1976a) view of the relations between the species used in the present study, including the weedy forms which have developed by introgressive hybridization between the wild and cultivated species. Com- pared with O. glaberrima, O. sativa has undergone far more diversification and improvement through its spread to different climatic regions and the develop- ment of agronomic practices. Of the two major races, the indica race is essentially tropical while japonica is temperate, although both are grown in subtropical areas such as Taiwan, and some recent varieties have both indica and japonica parentage. The advanced indica dwarf and semi-dwarf cultivars were derived from crosses between dwarf indica varieties from Taiwan and tall tropical indica varieties. The third race, ]avanica, is of minor importance

COMMON ANCESTOR

SOUTH & SOUTHEAST ASIA TROPICAL AFRICA

I I Wild perennial O. rufipogon ~ x O. /ongis tominata

| I Oniora Weedy annual * O. stapf17

Cultivated annual O. sa t i re O. glabarriroa x

indica japonica javanica

E]~INI @0 0

indica x japon ica •

Fig. 1. Re l a t i onsh ip s b e t w e e n the t w o cu l t iva ted species of rice and the i r wild relatives. The symbo l s used in Figs. 2- -6 and 9 are shown .

and largely restricted to the Indonesian area. O. rufipogon is adapted to permanent and of ten deep water, while O. nivara is found in shallow water. O. sativa, although grown predominantly on shallowly f looded land, also has varieties adapted to upland and to deep water conditions. O. glaberrima evolved in West Africa and has not spread widely beyond that region; it is grown only under traditional farming systems (Chang, 1976a, b).

Two physiological characteristics that have already been studied in detail within the genus Oryza are photoperiodic response (see Vergara and Chang,

221

1976) and photosynthesis in relation to leaf nitrogen contents (e.g. Takano and Tsunoda, 1970, 1971; Yoshida and Coronel, 1976).

In this paper we compare aspects of seedling growth -- such as leaf and tiller production, distribution of dry matter, photosynthet ic rate and relative growth rate -- and also of grain production, including flag leaf photosynthesis, grain yield components and harvest index in perennial, wild annual, cultivated and weedy accessions of both the Asian and African series of species grown under controlled environment conditions.

MATERIALS AND METHODS

Plant material

Seed samples of representatives of all taxa shown in Fig. 1 were obtained from the International Rice Research Insti tute (IRRI), Los Bafios, the Philippines and multiplied under quarantine for one or two generations in a 27/22°C glasshouse of the Canberra phy to t ron . Precautions were taken to prevent cross pollination, prevalent in the wild species (Oka and Morishima, 1967). Several lines were lost through failure to pass Australian quarantine regulations, including all material of O. longistaminata and three O. sativa cultivars, Dular, Kinandang Patong and Cina. Forty-one lines, listed in Table I, were finally used in the experiments.

Cultural conditions

The use of the standard phyto t ron regime for plant growth led to high mortali ty rates amongst the seedlings of many of the African and wild Asian lines and to a lesser extent of the primitive indica varieties. Following a series of experiments on the mineral nutrition of seedlings (Cook and Evans, 1983), a set of modified procedures was adopted which resulted in good seedling growth in all lines. Seeds were germinated on filter paper in petri dishes and kept in the shade in a 27/22°C glasshouse. They were planted out after 4 days into 12.5 cm diameter pots filled with vermiculite. The pots were placed in trays which were 4 cm deep and filled with a modified nutrient solution (Cook and Evans, 1983): these were topped up twice daily with the solution and were drained and refilled twice weekly. The plants were grown in a naturally illuminated glasshouse with the photoper iod extended to 16 h by incandescent lamps. The air temperature was 27°C for 8 h during the day and 22°C at night.

Seedling experiment

As seeds of some of the wild lines were dormant, seeds of all lines were heat treated at 52°C for 5 days, after which the dormant lines were dehulled and their pericarp scratched near the embryo (cf. Chang, 1976c), before sowing

222

T A B L E I

Details o f l ines and cul t ivars e x a m i n e d

Species or g roup Cul t ivar IRRI Accession No.

Origin

O. rufipogon

O. nivara

O. nivara-like

O. spontanea O. sativa f spontanea

O. sativa indica Primi t ive

Upland

I n t e r m e d i a t e

" T a i w a n "

A d v a n c e d

india X japonwa, advanced

japon&a Old

Advanced

javanica

O. barthii

O. stapfii

( Up land )

O. glaberrima

1 0 0 9 6 0 1 0 2 1 5 8 1 0 2 1 8 6

1 0 0 1 9 6 1 0 1 5 0 8 1 0 2 1 8 5 101147

1 0 2 1 6 2 1 0 1 4 4 8

JBS 377B 9836 Molagasamba G18 6114 Madael 15426 B e t i c h i k o n 26303 R a y a d a 16-02 27588

Kaw-pi-pang 2 8 5 0 2 Khao Lo 12904

JP5 13382 E m a t a Yin 4140 N am Sagui 19 11462 Latisail 8340

I-Geo-Tze 120 Dee-Geo-Woo-Gen 123 Ta ichung Nat ive 1 105

IR5 9926 IR8 * Pel i ta I /1 1 4 5 6 0

C e n t u r y P a t n a 231 1814

Asahi 2536 U n n a m e d Shens i var. 28482

Hu-hsuan 19 28477 H o y o k u 10797 Ta inan 3 92

R i k u t o Nor in 21 7697

Belanak Kesambi 17263

1 0 1 9 3 7 1 0 2 1 1 3

1 0 0 9 2 1 1 0 0 9 3 4

1 0 0 1 4 4 1 0 0 1 4 9 1 0 2 2 5 3

Cuba Thailand India

Burma India Ganga Plains N. Australia

Ganga Plains Assam

India Sri Lanka Sri Lanka Bangladesh Bangladesh

Thailand Laos

India Burma Thailand India

Taiwan Taiwan Taiwan

Philippines Philippines Indonesia

U.S.A.

Japan N. China

Yangtze Valley Japan Taiwan

Japan

Indonesia

W. Africa W. Africa

W. Africa W. Africa

W. Africa W. Afr ica W. Afr ica

*Seed o b t a i n e d locally.

223

in January 1980. The six seedlings planted in each of 15 pots per line were subsequently thinned to four per pot. The trays were repositioned in the glass- house twice weekly.

Four harvests of 15 plants per line were made, the first by removing one seedling from each pot and subsequent ones by using five pots of three seed- lings. Harvests, which took 3 days to complete, were taken at weekly intervals between 18--20 and 39--41 days after sowing. The areas of individual main stem leaves and, except in the final harvest, the total leaf area per plant were measured. Plants were divided into three portions (leaf laminae, stems including leaf sheaths, and roots), dried in a forced-draught oven at 75°C and weighed.

Observations of main stem leaf and tiller numbers were made at regular inter- vals on a sample of 15 plants. Leaf interval was calculated by regression over the period from the first appearance of a fully extended leaf until the final count at 38 days. Measurements of photosynthesis were made as described in the following section and the leaf portions used for these were then dried to obtain their specific leaf weight (dry weight per unit leaf area, SLW) after which the leaf tissue was digested by a modified Kjeldahl method and assayed for N in an automatic analyser as described by Williams and Twine (1967).

Photosynthesis measurements

Measurements were made using an infrared gas analyser (Analytical Develop- ment Co. Series 225) on 5 cm long central portions of leaf blades enclosed in a perspex chamber 30 cm × 5 cm X 1.8 cm. Air which had been drawn from the external atmosphere through a mixing tank was pumped through the cham- ber at rates of from 4 to 6 1 min- ' . The lower surface of the leaf blades was maintained at 27 + 1°C.

Seedling leaf photosynthet ic rates were measured under fluorescent and incandescent lamps supplemented by a mercury vapour lamp to boost the photon flux density within the chamber to 1200 pE m -2 s -1. Measurements were made in the period between the final two harvests on the uppermost fully extruded leaf blades, which varied from leaf 6 to leaf 8. In each line, two samples of four leaves were measured, 2 days apart.

Flag leaf photosynthet ic rates were measured under fluorescent and in- candescent lamps only which produced a photon flux density of up to 775 pE m -2 s -1 in the assimilation chamber. Measurements were made on a single sam- ple of four leaves at weekly intervals from soon after full extrusion until harvest when SLW was determined. Using the same flag leaves, light response curves were obtained by successive reduction in the photon flux density from 775 ~E m -2 s -~ to darkness, between 1 and 2 weeks after anthesis.

Photorespiration measurements were made on flag leaves of first generation plants after anthesis, using a photon flux density of 750 p E m -2 s -~. Measure- ment at atmospheric 02 concentration was followed by that at 1.5% 02 using a gas mixture with 300 ppm CO2.

224

Grain yield components

Fifteen seedlings per line, selected for evenness, were grown for 62 days in trays in the conditions described above, at a density of about 60 plants m -~. Normal watering was then begun with standard nutrient solution once and water twice daily. At the same time (February 1980) all plants were transferred from long days to an 8 h photoper iod achieved by wheeling them into a dark- room at 22°C each night. Each line was returned to long days when the flag leaf on the main shoot of the most advanced plant was fully extruded. Except for two cultivars, this lay between 28 and 52 days after the first short day. It was intended that the t ime at which short day induction began would precede floral induction in the daylength-insensitive lines yet be late enough for all other lines to have entered the photoper iod sensitive phase. However floral development had already begun in Rikuto Norin 21, while Belanak Kesambi presumably remained vegetative for several weeks after the beginning of treat- ment.

Dates of flag leaf extrusion, panicle emergence (first visible spikelets) and first anthesis were recorded for the main shoot of each plant together with the number of tillers at anthesis. At harvest, about 5 weeks after anthesis when the main panicle was mature, the tiller number was again counted, including both basal and nodal tillers where appropriate, and various features of the main shoot were measured or counted such as the area of the top three leaves, stem height, panicle size, the number of grains and the total number of spikelets. The main shoot was separated from the remainder of the plant before drying to obtain its grain (minus husk) and biological yields and harvest index.

An additional experiment was sown in February 1981 using all Asian lines excepting thejavanica cultivar. The three seedlings sown per po t were thinned to one, providing nine plants for each line or cultivar. Pots were removed from the trays of nutrient solution 35 days after sowing and transferred to naturally lit cabinets, under a photoper iod of 11.5 h. By restricting the size of the ex- periment it was possible to utilize cabinets which could provide a more typical photoper iod throughout panicle development and grain growth. Measurements of flag leaf photosynthesis were made 1--2 weeks after anthesis, and plants were harvested in the same manner as before.

RESULTS

For the tabular presentation of results, the 41 lines have been grouped according to species, race and stage of domesticat ion as shown in Table I. All linear regressions, however, were calculated on values for individual lines.

Seedling establishment

Kernel weights averaged over each group of lines, listed in Table II, column 1, ranged from 10.4 mg in the wild Asian perennial O. rufipogon to 22.9 mg in

225

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226

the intermediate indicas. However, the smallest kernels (7.7 mg) belonged to one of the O. nivara accessions. Amongst the O. sativa indica cultivars, the primitive lines generally had smaller grains, but the upland cultivar Khao Lo had the largest (29.3 mg). Of the African lines we used, O. barthii had the largest grains. After selective dehulling, germination rates were fairly uniform. There was a close relation between kernel weight and seedling weight at first harvest when plants had three to four leaves on the main shoot and a maximum of two tillers, but this relation became weaker in succeeding harvests.

Kernel size was not reflected in the area of the first leaf (Table II, columns 1 and 2; r = 0.04 n.s.). The most noticeable features were the extremely small leaves produced by the ]aponica cultivars and the relatively large leaves produced by the small-grained O. rufipogon and by the African lines. The relative increase in area of successive leaves differed between lines and groups: it was greatest in the ]avanica and the advanced japonica cultivars but tended to decrease with improvement in indica cultivars. A comparison of the areas of leaves 1 and 7 (Table II) gives an indication of these trends. Leaf 7, the largest leaf to have expanded in all lines before the final harvest, was generally similar in area to the flag leaves of plants in the other experiment, and was largest in upland indica cultivars and in O. glaberrima.

The interval between successive leaves (Table II, column 4) was shortest in the japonica cultivars, the advanced indicas and O. nivara.

Considerable variation was observed in seedling tiller product ion (Table II, column 5). Among indica cultivars, but not in japonica, there seems to have been an increase in the rate of tillering, but profuse tillering was also apparent in some lines of the wild annual O. nivara and in its African counterpart O. barthii. Groups having the most tillers also tended to begin tillering earlier.

Seedling photosynthesis

Although the youngest fully expanded leaf, used for these photosynthesis measurements, ranged from leaf 6 to leaf 8, no differences were evident be- tween consecutive leaves within lines in either photosynthet ic rate, SLW or N content . The rates given in Table II, column 6 suggest that the photosynthet ic rates of the wild Asian species, at least of O. nivara, are slightly lower than those of O. sativa, and that among the cultivars of that species rates tend to be lower in the primitive and upland indica cultivars and the javanica than in the remainder. The highest rates were measured in Nam Sagui 19, Tainan 3 and Century Patna 231. The three African species all had low rates of photo- synthesis. Although the rates of photosynthesis for flag leaves (Table III, col- umn 1) were somewhat lower, due at least in part to the lower irradiance used, there was significant agreement in the relative values obtained in the two ex- periments despite some differences in ranking, such as with O. rufipogon and within O. sativa. These measurements, together with those on first generation plants and on a number of lines in two preliminary experiments confirmed the differences described above.

227

The relationship found in an earlier experiment between leaf N content and photosynthetic rate for several lines and over a wide range of N concentrations in the nutrient solution (Cook and Evans, 1983) was shown to hold when all o f the lines were compared under a single nutrient regime (Fig. 2). In spite of more scatter than before, a substantial proportion of the variance was attributable to the regression (r = 0.66, P < 0.001). Lines of O. nivara, O. glaberrima and O. stapfii in particular were low in both N content dm -2 and net photosynthetic rate. O. rufipogon tended to have a high N content dm -2 relative to its photosynthetic rate. Less of the variance was attributable to the regression in the relation between seedling photosynthetic rate and specific leaf weight (r = 0.53, P < 0.001). SLW's (i.e. of measured leaves) are listed in Table II, column 7.

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100 2: I -

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N I T R O G E N C O N T E N T OF LEAF (rag N d m 2)

Fig. 2. Relation between leaf nitrogen content and photosynthetic rate in the lines and cultivars examined; O. rufipogon (a), O. nivara (V), O. spontanea (o), primitive (a), upland (~), intermediate (m), "Taiwan" (B) and advanced (m) indicas, old (Q) and advanced (o) japonicas, indica × japonica ( . ), javanica (o), O. barthii (+), O. stapfii (*) and O. glaber- rima (x ) .

Relative growth rate

The relative growth rates (RGR's) for dry weight of seedlings were calculated for all lines between harvests 2 and 4, during which interval growth was found

228

to be approximately exponential in all lines in spite of the seedlings being quite large. For the interval between harvests 2 and 3, mean RGR was 0.139, and for harvests 3 and 4, mean RGR was 0.129. RGR's during the exponential phase of growth provide a useful basis for comparing seedling growth across many lines differing substantially in initial seedling size, and were used for this purpose in previous comparative studies on wheat and cowpeas. The interval over which RGR's are given encompassed that during which the photosynthet ic measurements were taken.

Although the relative growth rate over this interval showed a tendency to be lower in those lines with the greater initial dry weight at harvest 2 (Fig. 3, r = - 0 . 5 1 , P< 0.001), comparisons between groups can be made. All three African species and O. nivara had high RGR's, while the more advanced indicas had lower rates, relative to initial plant size, than the older cultivars.

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Fig. 3. The relat ion be tween relative growth rate o f seedlings be tween harvests 2 and 4 and dry weight at the start of the measuremen t period. Symbols are as in Fig. 2.

To determine which factors were associated with the differences in RGR, these values were compared with both photosynthet ic rate and leaf area. Fig. 4 shows that, far from there being a positive relationship between photosynthet ic rate per unit leaf area and RGR, there was in fact a significant negative correla- tion (r = - 0 . 5 5 , P < 0.001). In Fig. 5 RGR is plotted against leaf area ratio (LAR; leaf area relative to total plant dry weight). RGR was positively related to LAR (r = 0 .50 ,P < 0.001) when lines of all species were compared.

A larger leaf surface area can be generated either by reducing the specific leaf weight or by increasing investment in leaf growth relative to the remainder of the plant. The correlation between RGR and the percentage of plant dry

229

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NET P H O T O S Y N T H E S I S (mg CO 2 dm 2 h 1 )

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Fig. 4. The relation be tween relative growth rate o f seedlings and photosynthet i c rate o f uppermost ful ly extruded leaves. S y m b o l s are as in Fig. 2.

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weight in the leaves was significant (r = 0.45, P < 0.01) although the range in the latter was small: at harvest 3 (day 3 2 34) the lowest group mean was 44.5% (old japonica) and the highest 49.8% (O. glaberrima). On the other hand there was a wide range in SLW values for whole seedlings, group means varying from

2 3 0

2.37 to 3.09 mg cm -2, while the correlation with RGR was similar (r = 0.40, P < 0.01). In Fig. 6 SLW is plotted against LAR, illustrating the close relation between them. Deviations from the inverse curve of LAR on SLW, such as the curve drawn in Fig. 6 for 46.5% leaf (the mean value obtained), are due to differences in the proportion of plant dry matter in the leaves. Those on the upper (concave) side of the curve have a relatively larger investment in leaf growth.

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Species such as O. glaberrima and O. nivara whose relative growth rates were amongst the highest had low SLW combined with the highest percentage dry matter in the leaves to give the highest LAR's. The proportions of dry matter invested in the stems and roots varied more widely than that in the leaves, the ranges in group means being 31.3--37.3% and 15.5--22.6% respectively. The two wild annual species, the primitive indicas and the javanica cultivar had the lowest % root while the Taiwan and advanced indicas had the highest (cf. Cook and Evans, 1983).

No significant relation was found between the rate of tiller production and RGR. When RGR's were calculated over a constant dry weight range similar relationships were found between them and the characteristics described above.

231

Flag leaf photosynthesis

Although the SLW of the flag leaves was considerably higher than that of the seedlings, the ranking from highest to lowest changed little and the SLW's of seedling leaves and flag leaves were significantly correlated (r = 0.40, P < 0.01). Similar results were found in the additional experiment using tiller flag leaves harvested 1 to 2 weeks after anthesis. Photosynthetic rate in all species and groups increased progressively with increased irradiance up to 775 t~E m -2 s -1 (Fig. 7), but the results of a preliminary experiment using several lines suggested that leaves were approaching light saturation at 1000 pE m -2 s -1. In Fig. 7 the response curves for all seven African lines have been averaged, as have those for all indica and alljaponica cultivars. The curve of the Taiwan group, which had the highest maximum rates, has been dotted in to indicate the range found amongst the indica cultivars. The rates of the African lines were probably abnormally low due to a red spider mite infestation which affected them more severely than the Asian lines.

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The lines within each group showed quite consistent time courses for net photosynthesis from flag leaf emergence to harvest. Photosynthesis in virtually all lines increased after the first measurement, to reach a maximum at anthesis or later. It is evident in Fig. 8 that the photosynthetic rate in the two wild Asian species and in all the African species began to decline soon after anthesis,

232

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whereas in O. sativa cultivars it was at least as high at the time of harvest as at anthesis. These patterns confirmed the results of measurements made on first generation plants. At that t ime several of the japonica cultivars whose flag leaf photosynthesis was monitored for 14 weeks after anthesis showed little decline until 12 weeks or more, long after grain maturation. There was no evidence that the photosynthet ic rate fell faster in lines with higher maximum rates, as was the case with wheat (Evans and Dunstone, 1970).

Photorespiration measurements taken mostly on flag leaves after anthesis in the first generation indicated a 29% enhancement of net photosynthetic rate at 1.5% O: over that at atmospheric levels with no differences evident be- tween species or groups of cultivars.

Grain yield components

Examination of yield components was restricted to the main shoot. Even so, spikelet numbers per panicle varied widely according to environmental con- ditions, affecting comparisons between species and types of cultivars. The time of year, growth duration and photoperiod conditions provided for floral in- duction were different in each of the three major experiments (first generation, 1980 and 1981), and spikelet numbers in nearly half of the lines varied by more than two-fold.

Grain set tended to be highest in the japonica cultivars and low in the wild species in all experiments (e.g. Table III, column 3). For other components, discussion is based mainly on the 1981 experiment.

Grain yield of the main panicle (Table III, column 4) was least in O. rufi- pogon and O. nivara and highest in advanced japonica cultivars. Within the indica race however the intermediate cultivars produced the greatest grain yield and advanced cultivars the least. Grain yield per panicle was closely related to spikelet number (r = 0.88, P < 0.001) and weakly to kernel weight

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(r = 0.45, P < 0.05). Grain yield per panicle was also correlated with the photosynthet ic rate of the flag leaf, expressed on a whole leaf basis and mea- sured between 1 and 2 weeks after anthesis (r = 0 . 7 9 , P < 0.001). In Fig. 9, which illustrates this relationship, it is noticeable that every wild or weedy line falls below the regression line of grain yield on photosynthesis, whereas every advanced cultivar lies above it.

z

w a.

n-

z

248' 2 0

1.6 B I ~

12 @ @ II

[] 0

0.8 • •

0.4 z&

~7 0 ~ I I I I I I ; ) I

2 4 6 8 10 12 14 16 18 20

NET PHOTOSYNTHESIS OF FLAG LEAF (rag CO 2 leaf - 1 h - 1 )

Fig. 9. Relationship between grain yield per panicle and flag leaf photosynthesis 1--2 weeks af te r an thes i s for l ines and cul t ivars used in the 1981 e x p e r i m e n t . Symbo l s are as in Fig. 2.

The harvest indices {H.I.) {Table III, column 5) are lower than many published values partly because they are based on kernel weight only, and exclude the husks and panicle included in some previous estimates. Harvest index increased through the Asian evolutionary and domesticat ion sequence from O. rufipogon to O. sativa.

The small H.I. of O. rufipogon lines resulted partly from their considerable stem elongation {Table III, column 6), in contrast for example with O. nivara whose short stems were also slender. Stem height of the dwarf and semi dwarf indica cultivars was only half that of the primitive cultivars.

The proport ion of dry weight in the husk {relative to kernel plus husk) varied widely from 29.2 to 15.2%, as shown in Table III, column 7. The proport ion was influenced by grain size, presence and size of awns and thickness of the husk. O. rufipogon and O. nivara, whose husks represented 29.2 and 24.3% respectively, had the smallest grains together with prominent awns, while the weedy spontanea forms, wi thout awns, had a considerably lower % husk. O. barthii on the other hand, whose larger grains had very thick strong husks and awns, also had a high % husk. O. sativa cultivars had fine husks without awns. The presence of strong husks and barbed awns is advanta- geous to seed dispersal and burial, but uses assimilate which could otherwise be invested in the grain.

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Tillers

Tiller number at anthesis in the 1980 experiment (which ranged for most lines from 92 to 134 days from sowing), was weakly related to that of 38 day old seedlings in the seedling experiment with a correlation coefficient of 0.38 (P < 0.05). Apart from thejavanica cultivar, the group which flowered last was O. rufipogon, and this group had the highest number of tillers at anthesis relative to the seedling number, due to a marked resurgence of tiller production during the short day treatment. Partly as a result of this and further tillering after anthesis, the proport ion of panicle-bearing tillers at main stem harvest in this species was very low, ranging from 0 to 13.4%, panicles being produced by older tillers only. At the other extreme, the three African species all had a high proport ion of panicle-bearing tillers, averaging for each species 63% (O. barthii), 70% (O. stapfii) and 57% (O. glaberrima). For the two wild species, at least, this high proport ion was associated with a small proport ion of tillers produced after the tiUering stage, and the appearance of panicles on nodal tillers. The propor- tion of panicle-bearing tillers in other groups was generally between 30 and 50%, regardless of tiller number. In the 1981 experiment where short day treatment was begun 3 weeks earlier and continued indefinitely, the % of panicle bearing tillers in O. sativa cultivars was higher, although the number of panicles was lower.

DISCUSSION

Our results with the Asian and African species of Oryza reveal both similari- ties and contrasts with physiological changes during the domestication and im- provement of other crops, the differences being most marked between rice and wheat. Both are cereals with the C3 pathway of photosynthesis, but evolu- tion in wheat has been accompanied by increase in ploidy, with associated in- creases in cell and organ sizes.

As in other crop species, seed size has increased during domestication of rice, although the four-fold range in our samples is less than, for example, those in either wheat or cowpea where ranges 20 fold and more were encountered (Evans and Dunstone, 1970; Lush and Wien, 1980). The lack of a close relation between grain and leaf size in rice contrasts with findings on other crops such as wheat (Evans and Dunstone, 1970) and cowpeas (Lush and Wien, 1980).

The proport ion of dry matter directed into leaf tissue varied only slightly among lines, but there was considerable variation in its partitioning between leaf area growth and weight per unit leaf area. Since the rate of photosynthesis per unit area was positively related to SLW, photosynthet ic rate tended to fall as LAR rose (r = - 0 . 5 3 , P < 0.001). The two effects would therefore be inclined to cancel one another out, and in fact RGR varied less than its components. Nevertheless, LAR was positively correlated with RGR (Fig. 5). For example, most lines of the African species and of O. nivara had both high LAR and high RGR. Within O. sativa there was much variation. Intermediate to advanced

236

indicas tended to produce plants of intermediate LAR and SLW, with rates of photosynthesis towards the upper end of the range and with rather low RGR's. Japonica types tended to have low LAR and high SLW and photosynthet ic rate except for the upland cultivar which, together with upland indicas, showed the reverse.

The differences in LAR and rates of photosynthesis could be adaptations to different environments. For example, rapid expansion of the leaf surface could confer a competit ive advantage on seedlings over weeds, or other species in the wild. It would also be beneficial to cultivated rice in hastening the closure of the leaf canopy. But once the canopy is closed, there would be greater advantage in having a higher SLW and photosynthet ic rate. Thus the intermediate properties of many O. sativa cultivars may represent a com- promise between conflicting needs.

The relatively small change in RGR through the evolutionary sequence in rice has parallels in the course of domesticat ion in other crops. For example Lush and Wien (1980) measured higher RGR's amongst the smaller, wild cow- peas than in domest icated lines but no differences were apparent when compar- isons were made at the same seedling size. Evans and Dunstone {1970) and Khan and Tsunoda (1970b) found no substantial differences in RGR between species of wheat, and neither did Duncan and Hesketh (1968) in maize nor Warren Wilson (1972) in tomato.

Photosynthet ic rate in rice has been found to bear a close relationship to nitrogen content per unit leaf area, regardless of whether the variations in the latter were associated with different levels of N supply (Yoshida and Coronel, 1976; Cook and Evans, 1983), cultivars (Ohno, 1976) or cultivars and species (Takano and Tsunoda, 1971). In all these investigations, the photosynthet ic rate increased proport ionally up to N contents of about 15--18 mg N dm -2, but at still higher levels there was a decline in the photosynthet ic response (Takano and Tsunoda, 1971). A positive relation between photosynthet ic rate and leaf N was found in wheat also when strains of wild and cultivated species were examined (Khan and Tsunoda, 1970a); bu t in this case the wild species had the higher values.

The photosynthet ic rates of seedling leaves compared close to light satura- tion, revealed differences between species like those reported by IRRI (1973, pp. 212--213). Photosynthet ic rates of O. rufipogon were higher than those of O. nivara (? O. perennis) and were much the same as the O. sativa cultivars on av- erage. The IRRI s tudy also found that O. glabberrima and O. barthii lines had lower mean photosynthet ic rates than the O. sativa lines. Within O. sativa, there was considerable variation be tween cultivars, particularly among the primitive indicas. Consequently improvement by breeding within O. sativa has not been associated with a consistent increase in photosynthet ic rate, bu t there appears to have been a tendency for photosynthet ic rate to increase through domestication from the annual progenitor, associated with increasing SLW and N content. This is in sharp contrast to the sequence found in wheat, where net photo- synthetic rate has fallen substantially in the course of evolution in association

237

with the increases in leaf size (Evans and Dunstone, 1970) and cell size (Dun- stone and Evans, 1974) which have accompanied increasing ploidy. All the wild and cultivated species of rice are, by contrast, diploid, and therefore more com- parable with other crops such as cowpea (Lush and Rawson, 1979) and maize (Duncan and Hesketh, 1968), where there is little change in photosynthet ic rate on domestication.

However, photosynthet ic duration does appear to have increased during domesticat ion in rice (Fig. 8) as well as in wheat (Evans and Dunstone, 1970) and cowpea (Lush and Rawson, 1979). The flag leaf photosynthet ic rate fell sooner after anthesis in the wild species of rice than in the Asian cultivars where it was still high at the time of harvest.

Increase in the proport ion of dry matter allocated to organs harvested by man is a common phenomenon in crop evolution (Evans, 1976) and is evident in the experiments reported here in spite of the generally low harvest indices. However the marked disparity between experiments in spikelet number per panicle places limitations on our comparison of yield components in species of Oryza. This variability might have served to compress the differences in panicle yields because the lines showing the most variation between experi- ments in spikelet number were predominantly the more advanced O. sativa cultivars.

In conclusion, this s tudy of physiological aspects of the domestication and improvement of rice has revealed some trends common to wheat, rice and cow- peas, and others in which these crops have differed. They agree in showing no increase in relative growth rate (and no relation between RGR and photo- synthetic rate) associated with domestication and improvement, whereas photosynthet ic duration of the flag leaf and harvest index have both increased. Wheat differs from rice and cowpeas in that its increasing ploidy has been associated with a greater increase in leaf and grain size and' with a fall in maxi- mum photosynthet ic rate. In rice, photosynthet ic rate appears to have risen, but only to a limited extent , possibly because of the need for compromise be- tween rapid leaf area growth early in the life cycle and higher photosynthet ic rate at a later stage.

ACKNOWLEDGEMENTS

We are grateful to Kati Bretz for her technical assistance throughout these experiments, to the Phyto t ron staff for their daily care of a very large number of plants, to Dr. T.T. Chang for suggesting a suitable set of accessions and for providing seed of them, and to Drs R.M. Gifford, J.M. Hopkinson, R.W. King and I.F. Wardlaw for helpful discussions and comments.

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