Plant Physiol. (1983) 72, 679-6840032-0889/83/72/0679/06/$00.50/0

Growth Characteristics, Grain Filling, and Assimiilate Transport ina Shrunken Endosperm Mutant of Barley1

Received for publication January 31, 1983

FREDERICK C. FELKER, DAVID M. PETERSON, AND OLIVER E. NELSONDepartments ofAgronomy (F. C. F., D. M. P.) and Genetics (0. E. N.), and United States Department ofAgriculture, Agricultural Research Service (D. M. P.), University of Wisconsin, Madison, Wisconsin 53706

ABSTRACT

The reported inheritance pattern of the segl shrunken endospermmutant of barley (Hordeum vulgare L. cv Betzes) suggests that somedefective process in the maternal plant tissues, and not in the endosperm,prevents normal grain filling in the mutant. To identify the physiologicalmechanism of the mutation, we compared growth, carbon exchange, andassimilate transport of Betzes and segl plants. Betzes and segl plants didnot differ in mean relative growth rate, mean net assimilation rate, orcarbon exchange rate. The rate and duration of grain growth of segl waslower than Betzes on intact plants and on detached, cultured spikes.Increasing the supply of sucrose in culture media up to 300 mm sucrose didnot eliminate differences between normal and mutant grain growth. Trans-location of "C-labeled assimilates into segl grains ceased by 21 days afteranthesis, and assimilates were diverted to lower plant parts. In contrast,assimilates were still entering Betzes grains at 29 days after anthesis.Evidence suggests that some maternal spike or grain tissue is affected bythe mutation after the onset of grain filling. Identification of the specificsegl defect may provide information about the cessation of normal grainfiling.

The segl mutant of barley (Hordeum vulgare L.) is one ofseveral shrunken endosperm mutants described by Jarvi andEslick (6) that have an unusual inheritance pattern. Heterozygousplants produce only normal seeds, and one fourth of the F2 plantsproduce only shrunken seeds regardless of pollen source. Thus,the phenotype of the seed is not dependent on the seed's genotypebut rather that of the female parent. The homozygous recessivemutant segl/segl (hereafter designated segl ), which is the subjectof this study, produces viable seeds of about 35 to 55% of normalweight on normal appearing plants. This suggests that somephysiological defect exists in mutant plants that prevents forma-tion of normal sized seed.

Grain filling is governed by the interdependent processes ofphotosynthesis, phloem loading, assimilate transport, phloem un-loading, and utilization of sugars in starch synthesis. While theinheritance pattern ofsegl suggests that endosperm starch synthe-sis is not involved, the normal appearance and vigor of mutantplants does not suggest a general metabolic or source-relateddefect. Identification of the specific physiological defect in seglmay provide information about what causes the cessation of grain

' Research supported by the College of Agricultural and Life Sciences,University of Wisconsin, Madison, the United States Department ofAgriculture, Agricultural Research Service, and by grant number 59-2177-1-1-609-0 from the Competitive Research Grants Office, Science andEducation Administration, United States Department of Agriculture.

growth in normal barley. Nelson (13) suggested that if suchmutants affected transport of sugars to the spike, they wouldprovide useful experimental probes for studying transport phe-nomena. Therefore, we compared segl and normal Betzes barleywith respect to growth and apparent photosynthesis of plantsbefore anthesis, growth ofgrains on intact plants and on detached,cultured spikes, and transfer of "C-labeled flag leaf assimilatesduring grain filling.

MATERIALS AND METHODS

Seeds of Hordeum vulgare L. cv Betzes and segl were originallyobtained from Dr. R. F. Eslick, Montana State University, Boze-man, MT, and increased in the field at Madison, WI. For two ofthree spike culture experiments and the "C-assimilate transferexperiment, plants were grown in a greenhouse maintained at 18+ 2°C with supplemental lighting for 16 h/d in 15-cm-diameterclay pots containing a 2:1 loam:sand mixture. Granular fertilizer(6:24:24) was mixed in the soil at the rate of 10 g/pot beforeplanting.Growth Analysis. Plants were grown in an environmental cham-

ber in a 1:1 peat:vermiculite mixture (three plants per pot) andirrigated with a one-quarter strength modified Hoagland solution(14) containing Sequestrene (Fe 330, CIBA-GEIGY)2 as the ironsource. Cool white fluorescent and incandescent lights providinga PPFD3 of 600 ILE m-2 s-' at the plant tops were used for 16 h/d. In the first of two growth experiments, the temperature of thechamber was maintained at 18/15°C day/night, and five harvestswere made from 17 to 42 d after planting. There were fourreplications of three pots each in a randomized complete blockdesign, and the leaf area and shoot dry weight of each plant weredetermined. In the second experiment, plants were maintained at23/18°C day/night. Eight harvests from 18 to 42 d after plantingwere made, and only shoot dry weights were recorded. In anattempt to reduce experimental error caused by plant variability,pots were randomly repositioned after each sampling, and acompletely randomized design was used.

Detached Spike Culture. Spikes from greenhouse-grown plantswere cultured beginning 9 and 17 d after anthesis in separateexperiments. Spikes were cut below the flag leaf node, and theflag leaves (blades and sheaths) were removed. The stems weresurface-sterilized with 0.525% (w/v) NaOCl, rinsed three timeswith sterile distilled H20, recut to 9 cm under sterile water, fittedwith a cotton plug, and placed in 125-ml Erlenmeyer flasks

2 Mention of a trademark or proprietary product does not constitute aguarantee or warranty of the product by the United States Department ofAgriculture and does not imply its approval to the exclusion of otherproducts that may also be suitable.

'Abbreviations: PPFD, photosynthetic photon flux density; RGR, rel-ative growth rate; NAR, net assimilation rate.

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Plant Physiol. Vol. 72, 1983

containing 50 ml of medium. The culture medium contained 58mm sucrose, inorganic salts, vitamins, and an amino acid mixtureas described by Donovan and Lee (1) and was adjusted to pH 5.5and filter-sterilized. All transfers were done under a laminar flowhood. The flasks were placed in a water bath at 2°C to minimizecontamination. The air temperature around the spikes was ap-proximately 20°C. Warm-white fluorescent lights provided aPPFD of 100 ,tE m-2 S-1 for 16 h/d. At the beginning of theexperiments, 10 grains were removed from one side of the centralportion of each spike for determination of their initial dry weight.At each harvest time, 10 grains from the remaining half ofpreviously designated spikes were removed, dried, and weighed.In a third experiment, spikes from field-grown plants were pre-pared in a similar manner at 9 d after anthesis and cultured for 10d in nutrient media containing 0, 100, 200, or 300 mm sucrose.Net Carbon Exchange. A laminated acrylic plastic leafchamber

similar to that described by Nelson et al. (12) was constructed withan inlet port connected to an electric air pump, an outlet portleading to an air flow meter, and with a thermocouple in the leafchamber. The outlet of the flow meter was covered with a rubberserum vial septum through which was inserted an 18-gauge needleconnected to a 50-ml plastic syringe. A vent hole just beyond the50-ml mark permitted continuous flushing of the syringe when theplunger was withdrawn past it but still engaged in the syringe. Todetermine net carbon exchange, a leaf was inserted into thechamber and the air flow was adjusted to 100 ml/min. After 3min of equilibration, the plunger was closed to the 50-ml mark,the syringe withdrawn, and the needle inserted into a rubberstopper to seal it. The CO2 concentrations of these gas samplesand ambient air samples were determined with an IR gas analyzer.Images of sampled leaves made with photosensitive paper werecut out for leaf area determinations. Three experiments wereperformed on successive cloudless days using the youngest fullyexpanded leaves of intact field plants at the four- to five-leaf stage.The narrow leaf tips were cut off to permit insertion of 15 to 20cm2 leaf area into the chamber. Although the 100 ml/min flowrate was suboptimal for maximum carbon exchange rates, itprovided much larger differences in CO2 concentration betweensamples and ambient air than did faster flow rates. A fourthexperiment using a flow rate of 500 ml/min resulted in highercarbon exchange rates at the expense of sensitivity.

1'C-Labeled Assimilate Transfer. An acrylic plastic leaf cham-ber was designed to allow simultaneous exposure of the flag leavesof four plants to "CO2. Plastic tubing connected the air pumpoutlet to a vial covered with a serum vial septum, and hence tothe chamber inlet. The chamber outlet was connected directly tothe pump inlet, forming a closed loop. For each of six experimentsusing plants of different maturities from 2 to 29 d after anthesis,two shoots each of Betzes and segl were selected from greenhouse-grown plants. Flag leaf blades were fitted into the chamber eitherthe night before or 6 h before an experiment to allow the plantstime to recover from possible stresses of manipulation. During thistime, the system was open with air flowing through the chamber.After 6 h of illumination with either a 400-w high pressure sodiumlamp or two cool-white fluorescent lamps providing a PPFD ofapproximately 100 ,uE m-2 s-', the system was closed, and 2 ml of85% lactic acid was injected into the vial, which contained 100ACi of Ba"4CO3. After circulating the "CO2 for 15 min, the systemwas opened and allowed to circulate air for 105 min. Then eachshoot was separated into the following parts: flag leaf blade andflag leaf sheath (later grouped as the source); first (top) internode,rachis, grains, and awns (grouped as the upward sink); and second,third, and fourth internodes and their associated leaves (groupedas the downward sink). Each part was immediately chopped andplaced in 80%o (v/v) ethanol and later ground with a mortar andpestle. After heating to 80°C for 2 h, the extracts were cooled,filtered with Whatman No. 1 filter paper in a Buchner funnel,

with rinsing, and brought to volume. Aliquots of the extracts wereadded to Aquasol liquid scintillation cocktail (New England Nu-clear) and their radioactivity determined with a Beckman liquidscintillation spectrometer.

RESULTS

Growth Analysis. Dry weight (Fig. 1) and leaf area (Fig. 2)increase of Betzes and segl plants up to 42 d after planting showedalmost identical patterns. Data from each replication were fittedto power curves (y = axb), and from these equations the meanRGR and mean NAR were calculated for the growth period of 15to 40 d after planting (15). Betzes and segl exhibited mean RGRvalues of 0.160 and 0.189 g g-1 d-', respectively, and mean NARvalues of 5.48 and 5.73 g m-2 d-', respectively. The values corre-sponding to the two genotypes did not differ significantly foreither mean RGR or mean NAR at the 5% level based on standard

--* Betzes

3 c>_-- seg

aL 2 P3

X

0~~~~~~~~

0/,

0 10 20 30 40DAYS

FIG. 1. Dry weight increase of Betzes and segl shoots up to 42 d afterplanting (experiment 1). Points are averages of four replicates and thecurves represent the power curve (y = ax') regression lines obtained usingdata from all four replications.

800 * Betzes0_ seg

600_

E

400

IL. <: / ~~~~~~~~~~~0,w

200

0 K,

0 10 20 30 40DAYS

FIG. 2. Leaf area increase of Betzes and seg I plants up to 42 d afterplanting (experiment 1). Points and curves determined as in Figure 1.

680 FELKER ET AL.

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SHRUNKEN ENDOSPERM MUTANT OF BARLEY

0.6* - B

0.4 ---- 5

CD

3

cra

0'

0

0.21

0

-0.2F

-0.4_

-0.61_

letzes 0ysegI~~~~~~~I

/'//

/,'

y- 3.817x-5.696 /

Rz 0.99 /,

f,00 /

' y- 3.671x-5.561

/,' R = 0.99

/,

,1-0.8F

1.2 1.3 1.4 1.5 1.6log 10DAYS

FIG. 3. Linear regression of logarithmically transformed data showingdry weight increase of Betzes and segl plants up to 42 d after planting(experiment 2). Points are averages of three pots.

50

cn

C]E

01

40

30

20

I0

24

221

cxCK

CP

E

0'

a:

201

8-

1 6

14

2

10

8

0 5 10 15DAYS IN CULTURE

FIG. 5. Growth of Betzes and segl grains on detached spikes placed inculture 9 d after anthesis. Bars represent + I SE.

1-CE

(9

a:r0

0 10 20 30 60

DAYS AFTER ANTHESISFIG. 4. Growth of Betzes and segl grains under greenhouse conditions.

Grains were mature and almost completely dried down by 60 d afteranthesis. Bars represent + 1 SE.

t tests. However, dry weights and leaf areas on a given day wereusually significantly different which reflected a temporal shift ofabout 5 d in the growth curves. When allowance was made for the5-d lag period such that the growth period of 10 to 35 d of Betzeswas compared with the 15- to 40-d period of segl, mean RGRvalues were closer (0.204 and 0.189 g g-1 d-'). Plants of bothgenotypes maintained in the growth chamber reached anthesissimultaneously. By this time, segl plants were not noticeablysmaller than normal Betzes plants, suggesting that the differencebetween the plant sizes eventually diminished. Final straw weightsof Betzes and segl plants grown in the greenhouse in 1980 werenot significantly different. There was typically no apparent differ-ence between Betzes and segl plants with regard to grain set,number of spikes per plant, or flowering date.A second growth experiment was performed at higher temper-

atures (Fig. 3). To test for a difference in growth rate betweenBetzes and segl in the absence of blocks, data points for triplicatesamples were transformed logarithmically and analyzed by linear

401

35-

301

25

20

0 5 10 15DAYS IN CULTURE"

FIG. 6. Growth of Betzes and segl grains on detached spikes placed inculture 17 d after anthesis. Bars represent + I SE.

regression. The slopes of the regression lines calculated from thetransformed data (Fig. 3) were not significantly different, andmean RGR values over the period of 15 to 40 d based on theregression lines were 0.149 and 0.144 g g-1 d-l for Betzes and segl,

respectively. In this experiment, the temporal shift of the seglgrowth curve was approximately 2 d.Dry weight increases of Betzes and segl grains of greenhouse-

grown plants are shown in Figure 4. Both Betzes and segl grainsachieved their maximum dry weight by 30 d after anthesis, al-though segl grains grew little after 17 d and only reached 56% ofthe dry weight of Betzes. Under field conditions, segl grainsattained 44% (1980), 43% (1981), and 35% (1982) of Betzes dryweight. The mutant seed was conspicuously flattened and oftenhad a depression on the lemma side of the grain. The growthpatterns suggest that during the first 10 d after anthesis, Betzesand segl did not differ greatly in the rate ofgrain filling. However,

Betzes

seg,ItIA---

Betzes

se

--

- 1-

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Plant Physiol. Vol. 72, 1983

E

0100

cc 8D i ___iH

z 6cc

-2

w

<S 0 100 200 3006 mM SUCROSE IN MEDIUM

FIG. 7. Effect of medium sucrose concentration on growth of Betzesand segl grains on detached spikes placed in culture 9 d after anthesis.Bars represent ± I SE.

Table I. Carbon Exchange Rate (CER, ± 1 SE) of the Youngest FullyExpanded Leaves of Betzes and segi Barley Plants Prior to AnthesisExperiments were performed on successive cloudless days with chamber

air flow rates of 100 ml/min (experiments 1-3) or 500 ml/min (experiment4).

CER

Exp. I Exp. 2 Exp. 3 Exp. 4

mg CO2 dm-2 h-'Betzes 11.57 + 0.74 10.15 + 0.97 9.62 + 1.00 19.06 + 1.50segl 10.51 + 0.39 9.92 ± 0.69 9.71 ± 0.55 17.88 ± 1.56na 6 11 8 9a Number of plants of each genotype in each experiment.

between 10 and 20 d, the rate of assimilate movement into seglgrains decreased significantly as compared to the earlier stage,whereas assimilate movement into Betzes grains continued at ahigher rate up to 30 d after anthesis.Detached Spike Culture. Increase in dry weight of grains is

shown for spikes placed in culture beginning 9 d (Fig. 5) and 17d (Fig. 6) after anthesis. In general, the growth pattern of grain oncultured spikes paralleled that of intact plants (Fig. 4). After 15 dof culture in both experiments, the awns, paleas, and lemmas ofspikes of both genotypes began to yellow and senesce. Comparedwith intact grains on greenhouse plants, at the end of the secondexperiment Betzes and segl grains had reached 84% and 92% ofthe dry weight of intact grains of the same developmental age,respectively. In the first experiment, from 9 to 24 d after anthesis,both Betzes and segl grains gained dry weight, with the growthrates being equal for 5 d and Betzes exceeding segl in growth ratefor the next 10 d (Fig. 5). However, in the second experiment,from 17 to 32 d after anthesis, Betzes grains gained in dry weightfor the duration of the culture period, while segl grains grewrelatively slightly for only 5 d, showing no significant growth from22 to 32 d after anthesis (Fig. 6). These results along with thegrowth pattern of intact grains (Fig. 4) suggest three phases ofgrain development in segl: a first phase (0-15 d after anthesis)during which growth parallels that of normal giains, a secondphase (15-22 d after anthesis) during which the growth rate is lessthan normal, and a third phase (22 d to maturity) during whichgrain filling ceases in the mutant but continues in normal grains.

>- 2 days after 9 days 17 daysH anthesis> 800

0

o 40z

w

a-0 ABC ABC ABC ABC ABC ABC

Betzes seg Betzes seg Betzes seg,

FIG.8.D 21 days 25 days 29 days

>80-0

60-

o 40-zwo 20-wa-

E

0 ABC ABC ABC ABC ABC ABCBetzes seq, Betzes seg , Betzes segq

FIG. 8. Distribution of radioactivity 2 h after expo'sing flag leaves ofBetzes and segl to equal amounts of "'CO2- Six experiments using shootsof different developmental stages are shown. Each group of three barsrepresents the average of two plants. Data were combined as follows:A = flag leaf including blade and sheath; B = first internode, awns, rachis,and grains (black), and rachis; C = second, third, and fourth intemodes,leaves, and sheaths.

Table II. Radioactivity Recoveredfrom Shoots Whose Flag Leaves WereExposed to "'CO2for 15 Minutes (Average of Two Plants)

For each maturity level, similar areas of flag leaf were simultaneouslyexposed on two plants of each genotype.

Time after Anthesis Betzes segl segl/Betzes

d cpm x 1o-6 %2 11.85 13.54 1149 5.22 4.30 82

17 7.41 5.68 7721 7.10 5.36 7525 4.37 3.12 7129 4.96 5.63 114

The lack of significant dry weight increase of Betzes grains andslight loss of dry weight of segl grains after 10 d in mediumwithout sucrose (Fig. 7) suggests that dry weight increases weredue to uptake of sucrose from the medium and not photosynthesisor mobilization of reserves in the stem or rachis. At either 100,200, or 300 mm sucrose, change in dry weight for both Betzes andsegi grains was similar to that in experiments using 58 mm-sucrosemedium. Growth of Betzes grains always exceeded that of seglgrains regardless of increased sugar concentration, suggesting thatsegl spikes in culture cannot use additional sucrose to achieve anormal grain growth rate.

Net Carbon Exchange. There was no significant difference atthe 5% level based on standard t tests in net carbon exchange ratebetween Betzes and segl leaves in any of four experiments (TableI). Rates were based on mean differences between ambient andsample CO2 concentrations of about 188 ,ul/l for experiments I to3 and 47 ,il/l for experiment 4. As a test of the ability of ourtechnique to detect differences in net carbon exchange rate, an-

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SHRUNKEN ENDOSPERM MUTANT OF BARLEY

other barley cultivar, Karl, was compared with Betzes. In thisexperiment, using seven plants of each genotype, Betzes and Karlexhibited carbon exchange rates of 10.84 and 8.21 mg CO2 dm 2h-1, respectively, and this difference was significant at the 5% levelbased on a t test.

"'C-Labeled Assimilate Transfer. The interval of 2 h from theonset of "'CO2 labeling was chosen for the experimental transporttime as a compromise between a longer time which would allowa larger percentage of radioactive assimilates to reach the grainsand a shorter time which would minimize the conversion ofradioactive assimilates into alcohol-insoluble substances. The dis-tribution of "4C-labeled flag leaf assimilates in Betzes and seglplants in six experiments with culms bearing spikes from 2 to 29d after anthesis is shown in Figure 8. Whereas most of theradioactivity was always recovered from the flag leaves, variationsin the percent of radioactivity recovered from the grains (blacksegment of bar B) and that portion of the shoot below the flag leafnode (bar C) reflect the relative proportions of translocated assim-ilate moving toward these sinks. The percent of radioactivity inthe first internode, rachis, and awns (white portion of bar B)closely paralleled the percent found in the grains. At 2 d afteranthesis, little radioactivity entered the grains, and most trans-ported radioactivity entered the lower internodes, suggesting thatat this time the sink strength of the roots and lower stems farexceeded that of the young spike tissues. At 9 d after anthesis, theupper sinks contained a greater percentage of radioactivity thanthe lower sinks for both Betzes and segl. The distribution ofradioactivity among both sinks was similar for Betzes and segl atthis stage. At 17 d after anthesis, a greater percentage of radioac-tivity was recovered in the grains of Betzes than in the grains ofsegl, although the total proportions of radioactivity recoveredfrom the first internode upward and the second internode down-ward were the same in both genotypes. At 21 d after anthesis,most of the radioactivity leaving the flag leaf blade and sheath ofBetzes plants entered the grains, whereas essentially no radioactiv-ity was found in the first internode, awns, grains, or rachis of seglplants. The proportion of label recovered from the lower sinks wasmuch higher in segl plants than in Betzes. At 25 d after anthesis,distribution of radioactivity in Betzes plants resembled that of 17-d Betzes plants, whereas no radioactivity entered the upper sinktissues of segl. At 29 d, transport of assimilates from the flagleaves to the grains of Betzes plants was reduced, and no radio-activity moved toward segl grains. Rather, a larger proportion ofradioactivity was found in the lower sink tissues of segl plants.The relative amount of radioactivity recovered from each plant

within an experiment provides an estimate of the assimilation ratesince equal areas of flag leaf were labeled simultaneously (TableII). In the first and last experiments, when both genotypes hadvery weak grain sinks, segl flag leaves fixed slightly more 14CO2than Betzes flag leaves. However, from 9 to 25 d after anthesis,during which time Betzes grains were active sinks, segl leavesfixed less 14CO2 than did Betzes leaves and the ratio ofsegl/Betzesassimilation decreased as the sink strength of segl grains de-creased.

DISCUSSION

Evidence from these experiments suggests that the mechanismof the segl mutation involves tissues of the grain and manifestsitself sometime after the growth of the endosperm begins. Theonly difference between normal and mutant plants before anthesiswas a delay in the onset of growth, which is probably attributableto the reduced endosperm reserves of the smaller segl seeds. Seedsof mutant plants germinated more slowly than did normal seeds.However, our data show that once growth began, the growth rateand assimilation rate were not lower in segl plants. This issupported by the similarity in final straw weight, flowering date,and net carbon exchange rate between Betzes and segl.

It is generally accepted that flower number, fertility, and grainset are regulated in part by the level of assimilate production or,more specifically, by the amount of assimilates reaching the spike(2, 3). Therefore, it is reasonable to expect that if photosynthesis,phloem loading, or phloem translocation were directly affected bythe mutation, either fewer spikes and flowers would be initiated,or a lower percentage of flowers would be fertile, or both. The'fact that mutant plants bear the usual number of fertile seedswhich are of half the normal weight thus serves as evidence thatthe mutation is expressed after grain set has occurred.No grain growth resulted from culturing detached spikes in

medium without sucrose (Fig. 7), which is consistent with resultsobtained with wheat (1, 7) and oats (11). Therefore, we assumethat the grains were dependent on externally supplied sucrose forgrowth rather than photosynthesis or mobilization of reserves inspike tissues, and culturing detached spikes eliminated possibledifferences in leaf photosynthesis, phloem loading, and long-dis-tance phloem transport between Betzes and segl plants. The factthat the pattern of grain growth on cultured spikes paralleled thatof intact grains supports the view that photosynthesis and phloemloading and long-distance transport are not directly affected bythe mutation.The sucrose requirement of grains on cultured spikes was

apparently saturated below 100 mK, since (a) the amount of dryweight increase of grains on spikes cultured 10 d in 58 mm sucrosemedium (Fig. 5) was approximately the same as amounts on spikescultured at 100, 200, or 300 mm sucrose (Fig. 7); and (b) thegrowth rate of 17-d-old spikes in 58 mm sucrose for the first 10 d(Fig. 6) exceeded that of 9-d-old spikes cultured in the samemedium for 10 d (Fig. 5). Grains on detached wheat ears, whencultured in sucrose solutions supplying above normal levels ofsucrose, did not take up higher levels of sucrose or synthesizemore starch than normal (8). Similarly, growth of segl grains didnot increase significantly when supplied with higher levels ofsucrose (Fig. 7). This suggests that some process in the grain itself,rather than the supply of assimilates to the spike, limits growth ofsegl grains.

During the normal grain filling period, less "'CO2 was fixed insegl flag leaves than in Betzes flag leaves (Table II), possiblyreflecting an inhibition of photosynthesis by reduced sink demandin segl. Ear removal reduced flag leaf photosynthesis in wheat byabout 50%o in 3 to 15 h (10). However, work by Geiger (4) suggeststhat photosynthesis is not controlled by a direct feedback inhibi-tion by leaf sugars, but rather that the regulation of photosynthesisis more complex and requires 2 or 3 d of adjustment. Since filingof segl grains slows down before 10 d after anthesis, even amechanism requiring a period of several days to depress photo-synthesis may have been operating in segl plants in this experi-ment.The movement of "'C-labeled flag leaf assimilates into upper

sink tissues (Fig. 8) is consistent with other observations regardinggrowth of segl grains. Identical proportions of radioactivity re-covered from grains, awns, rachis, and first internode of bothBetzes and segl plants at 9 d after anthesis correspond with thesimilar grain growth rate between these genotypes on intact plantsand on cultured spikes. At 17 d after anthesis, when rate of graingrowth was less in segl than in Betzes, a slightly lower proportionof radioactivity entered segl grains than Betzes grains, while thetotal percent of radioactive assimilates moving into upward anddownward sinks (bars B and C) was the same in both genotypes.The lack of radioactivity entering any upper sink tissues of seglat 21 through 29 d after flowering corresponds to that period ofdevelopment when grain growth was very slow or had ceasedentirely. Although this experimental technique would not accountfor assimilates entering the grains from sources other than the flagleaf (ie. other leaves or stem reserves), the recovery of significantamounts of radioactivity from lower sink tissues in all experiments

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Plant Physiol. Vol. 72, 1983

suggests that carbon flow was proceeding downward from the flagleaf into these other tissues.

Approximately 80o of the radioactivity recovered from the flagleaves was retained in the blades, which were enclosed in theassimilation chamber, as opposed to the sheaths. Gordon et aL (5)found that a large proportion of assimilates are retained in barleyleaves during the light period, and these reserves are mobilizedand exported during darkness. Therefore, it is unlikely that thehigh proportion of 14C recovered from flag leaves is a result ofabnormally slow translocation caused by plant manipulation.

Khalifa et al. (9) compared assimilate translocation of a com-mercial barley variety, Senta, with low-yielding, primitive Ne-palese varieties. Ears of Senta were increasingly active as sinks upto 3 weeks after anthesis, whereas in the primitive varieties, earsdrew decreasing proportions of assimilates over that period. Therewas no difference in the duration of grain filling. The situation isnot analogous to Betzes and segl, because (a) grain filling ceasedcompletely and abruptly in segl rather than gradually, and (b)values for other plant characteristics such as grain number, leafarea, and straw weight were much lower for the primitive varietiesthan for Senta, whereas Betzes and segl were similar in theserespects. These considerations support the view that the low grainweight ofsegl is the result ofa discrete physiological event causinggrain filling to cease prematurely and not a result of lack of plantvigor or translocation efficiency with a broad genetic basis.

Inasmuch as the inheritance pattern of segl as a maternal plantmonofactorial recessive (6) does not suggest involvement of starchsynthesis or other endosperm events, it is likely that maternaltissues of the spike or grain are affected by the mutation. Physio-logical and histological studies of these tissues are in progress.

LITERATURE CITED

1. DONOVAN GR, JW LEE 1977 The growth of detached wheat heads in liquidculture. Plant Sci Lett 9: 107-113

2. GALLAGHER JN, PV BISCOE 1978 A physiological analysis of cereal yield. II.Partitioning of dry matter. Agric Prog 53: 51-70

3. GALLAGHER JN, PV BISCOE, RK SCOTT 1976 Barley and its environment. VI.Growth and development in relation to yield. J Appl Biol 13: 563-583

4. GEIGER DR 1980 Effects of translocation and assimilate demand on photosyn-thesis. Can J Bot 54: 2337-2345

5. GORDON AJ, GJ RYLE, CE POWELL, D MITCHELL 1980 Export, mobilization,and respiration of assimilates in uniculm barley during light and darkness. JExp Bot 31: 461-473

6. JARVI A, RF ESLICK 1975 Shrunken endosperm mutants in barley. Crop Sci 15:303-366

7. JENNER CF 1968 Synthesis of starch in detached ears of wheat. Aust J Biol Sci21: 597-608

8. JENNER CF, AJ RATHJEN 1972 Limitations to the accumulation of starch in thedeveloping wheat grain. Ann Bot 36: 743-754

9. KHALIFA FM, C MARSHALL, K DANIELS, JR WITCOMBE 1982 Assimilation of'4CO2, assimilate translocation and grain yield in three primitive Nepalesevarieties and a European cultivar (Senta) of six-row barley (Hordeum vulgareL.). Ann Bot 50: 49-56

10. KING RW, IF WARDLAW, LT EVANS 1967 Effect of assimilate utilization onphotosynthetic rate in wheat. Planta 77: 261-276

1 1. LESAR LE, DM PETERSON 1981 Growth and composition of kernels developingon excised oat panicles in liquid culture. Crop Sci 21: 741-747

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13. NELSON OE 1980 Genetic control of polysaccharide and storage protein synthesisin the endosperms ofbarley, maize, and sorghum. InY Pomeranz, ed, Advancesin Cereal Science and Technology, Vol 3. American Association of CerealChemists, Inc., St. Paul, MN, pp 41-71

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