J. agric. Sci., Camb. (1980), 94, 675-689 6 7 5With 5 text-figuresPrinted in Great Britain

Genetic improvements in winter wheat yields since 1900 andassociated physiological changes

BY R. B. AUSTIN, J. BINGHAM, R. D. BLACKWELL, L. T. EVANS*, M. A. FORD,C. L. MORGAN AND M. TAYLOR

Plant Breeding Institute, Trumpington, Cambridge

(Revised MS. received 6 December 1979)

SUMMARYExperiments were carried out to assess the increase in yield potential of winter

wheat in the U.K. due to variety improvement since the early years of this century.The effects of other genetic changes were minimized by applying fungicide to controleyespot and foliar diseases, and by using nets to prevent lodging. The experiments werecarried out in 1978 at Cambridge. One, on soil of high fertility in Camp Field, received104 kg N/ha and the other, on soil of lower fertility in Paternoster Field, received38 kg N/ha. Twelve genotypes were tested. Eight were varieties which formed achronological series beginning with Little Joss, introduced in 1908. The remaininggenotypes were recently developed selections from the Plant Breeding Institute anda line bred by the French breeders, Benoist.

The average yield of the 12 varieties and lines tested was 3-96 t/ha in PaternosterField and 6-40 t/ha in Camp Field. In both fields the two highest yielding entries,Hobbit and the advanced breeding line 989/10, outyielded Little Joss by close to 40 %.Benoist 10483 was the only entry for which the percentage yield advantage dependedon high soil fertility.

The newer, high yielding, varieties were shorter and reached anthesis earlier thanthe older varieties. They had lower stem weights/m2 than the older varieties but similarmaximum leaf area indices and leaf weights/ma. Within each experiment the totaldry-matter production of the varieties was similar, the increase in grain yield due tovariety improvement being associated mainly with greater harvest index (ratio ofgrain yield to grain + straw yield).

It is argued that by a continuation of the trend towards reduced stem length, withno change in above-ground biomass, breeders may be able to increase harvest index,from the present value of about 50% to about 60%, achieving a genetic gain in yieldof some 25%. As the limit to harvest index is approached, genetic gain in yield willdepend on detecting and exploiting genetic variation in biomass production.

improvement in the timeliness of cultivation opera-turns, and an increase in the use of herbicides and

The yield of wheat in the United Kingdom has crop protection chemicals, all of which have beenincreased by an average of almost 70 kg/ha/year claimed to increase yield. On the other hand theover the last 30 years (Austin, 1978) or by about increase in the intensity of cereal cultivation and80%. During these years a succession of new its extension to less suitable soils and climaticvarieties has been introduced, each new one regions will have tended to reduce nationaloutyielding its predecessor. These new varieties, average yields.having shorter and stiffer straw, have permitted Elliott (1962) and Silvey (1979) have summarizedthe use of increased amounts of nitrogen fertilizer, data from variety trials carried out by the Nationalrates of application having more than doubled Institute of Agricultural Botany showing the per-over the period. In addition there has been an centages by which successively introduced new

Present address: Division of Plant Industry, varieties of winter wheat have outyielded the onesCSIRO, P.O. Box 1600, Canberra City, A.C.T. 2601, they replaced. From their figures, Maris HuntsmanAustralia. would be expected to yield 47 % more than Hold-

0021-8596/80/2828-3700 $01.00 1980 Cambridge University Press

676 R. B. AUSTIN AND OTHERSfast, one of the most widely grown varieties in1947. In these trials lodging and disease are notcontrolled, and varieties being outclassed arelikely to be more susceptible than those whichreplace them. Therefore the yield improvementscalculated from variety trial data include a com-ponent due to decreased susceptibility to lodgingand disease. Additionally, as the new varietieshave been selected on soils of progressively increas-ing fertility, their increased yield may depend onhigh soil fertility.

No direct comparisons of the yields of a seriesof old and modern varieties of winter wheat appearto have been made in England since the introduc-tion of effective cereal fungicides. In Holland, VanDobben (1962) compared old Dutch varieties withsome modern ones then in use and noted that themodern ones yielded more grain, less straw but asimilar amount of total above-ground dry matter.This trend was confirmed by results of Watson,Thome & French (1963) who compared the oldvariety Squareheads Master with Cappelle-Desprez.More recently, similar results have been reportedby Jain & Kulshrestha (1976), Syme (1970) andothers, and these studies have included varietieshaving major dwarfing genes. In none of theserecent studies was it ascertained whether thechanges in height and dry matter distribution werethe only differences consistently associated withincreased yield.

The two experiments reported here were carried

out to measure the component of the increase inyield due to variety improvement since the intro-duction of Little Joss in 1908 and associated phy-siological changes. Lodging was prevented andchemicals were applied to prevent severe infectionsof disease, both of which would have complicatedthe interpretation of the results and probablyexaggerated differences in yield. To ascertainwhether the yield advantages of the new varietiesdepended on high soil fertility, one experiment wascarried out on a low fertility site and the other ona high fertility site.

MATERIALS AND METHODSBoth experiments were carried out at Trumping -

ton in the growing season 1977-8. The low fertilitysite, in Paternoster Field, was on a sandy loamoverlying a gravel terrace. It had been under grasssince 1970; during this time it had not receivednitrogenous fertilizer and the grass had been cutbut not removed. The high fertility site was inCamp Field where the soil was a clay loam over-lying a chalky marl. It had been cropped withcereals and field beans and had received fertilizerin accordance with modern farm practice.

Each experiment comprised four randomizedblocks of 12 plots of varieties and advanced breed-ing lines (Table 1). Each plot consisted of threeadjacent subplots, referred to as A, B and C, Bbeing the centre one. Each subplot, 4-2 m long,

Table 1.

Variety* i1 Little Joss2 Holdfastf3 Cappelle-Desprez4 Maris Widgeonf5 Maris Huntsman6 HobbitJ7 MardlerJ8 370/500J9 730/3637$

10 989/10J11 Armada12 Benoist 10483

S.E.

Mean

Yields, heightsand harvest indicesYield of dry grain

, moisture)(t/ha)

A

Year of Paternoster Campntroduction

1908193519531964197219771978

1978

Field3 3 02-913-743-574-044-634'074-254-354-594-243-800 1 13-96

Field5-224-965-865-686-547-306-216-347-077-576-867-230 1 3640

of varieties ir,t, two fieldsHeight tobase of ear

(cm)Paternoster

Field1129887968664576160647667

0-977

Camp ]Field142126110127106

80777878849787

1-399

Harvestindex(%)

PaternosterField

343442394650474950494545

0-544

i

CampField

363642394648494848514348

0-944

* The numbers 370/500, etc., refer to advanced breeding lines from the winter wheat breeding programme ofthe Plant Breeding Institute. The numbers 1-12 are used to identify the varieties in Figs 4 and 5.

f These varieties are of breadmaking quality,j Homozygous for the major dwarfing gene Rht2.

Genetic improvements in winter wheat 677contained seven rows drilled 16-8 cm apart. Thedistance between the centre rows of adjacent sub-plots was 1-5 m, and there was a path approxi-mately 1 m wide separating the ends of the plots.Both experiments were surrounded by at least oneline of plots of winter wheat.

Seed dressed with a benomyl/thiram mixturewas sown on 25 October 1977 at the rate of 160 kg/ha. In Paternoster Field fertilizer was appliedbefore sowing to give 50 kg K2O/ha and 50 kgP2O6/ha. No fertilizer was applied before sowingin Camp Field. The experiments were sprayedwith methabenzthiazuron before emergence tocontrol weeds, and with benomyl on 28 April,triadimefon on 12 May and benodanil on 17 Mayto control eyespot and foliar diseases. All chemicalswere applied at the manufacturers' recommendedrates. Nitrogenous fertilizer was applied on 1 Aprilat 38 kg N/ha in Paternoster Field and at 104 kgN/ha in Camp Field. To prevent lodging in CampField, coarse netting (10 x 10 cm mesh) was placedover the plots in May and held in position withstakes. The shoots readily grew through the netting,which did not appear to have any adverse effecton growth or yield. At the beginning of June,following a period of high evapotranspiration theplants in Paternoster Field 'showed signs of waterstress, and the experiment was irrigated to restorethe soil to field capacity.

Sampling and harvestingIn each subplot the number of plants in one

50 cm length of row was recorded on 1 February.On 7 March the total number of shoots in a marked50 cm length of row in each A and B subplot werecounted. Further counts of the shoots in theselengths were made at fortnightly intervals until 27July.

On 22 and 23 May when the penultimate leaveswere fully expanded the shoots in two 10 cm widestrips across the centre five rows of each C subplotwere cut at ground level. Previous experience(Austin et al. 1976; Austin & Jones, 1975) indicatedthat leaf area index (LAI) was likely to be at orclose to its maximum at this stage. The area anddry weight of the leaf laminae, and the dry weightsof the stems and sheaths, were determined. Subse-quently the plots were inspected each day and thedates when 50% of the shoots had reached earemergence and anthesis were recorded. At anthesis,and at weekly intervals thereafter until maturity,samples of ten ear-bearing shoots were taken atrandom from each C subplot. From these samples,leaf lamina area and dry weight and the dry weightsof the stems and sheaths and of the ears weredetermined. The grains from florets 1, 2 and 3(1 = basal) of two central spikelets of each earwere dissected out, dried and weighed, providing

data on the growth of the grains in these positions.For the harvests 3 weeks after anthesis to maturity,the remaining grains were threshed out of the 10-ear samples and the total grain dry weight persample determined.

At maturity, two further 10 cm strips of plantswere harvested from the C subplots, and leaf,stem and sheath, chaff and grain dry weightsdetermined. Further strips of row were removedfrom some subplots to enable edge and neighboureffects to be estimated (Austin & Blackwell, 1980).The remaining plants were harvested with a plotcombine. The total yield of dry grain (0 % moisturecontent) was determined for each subplot. Else-where (Austin & Blackwell, 1980) it has been shownthat yields calculated on the basis of the harvestedarea (4*2 x 1-18 m) overestimate the true yield by24 %. Therefore the yields given in this paper havebeen reduced by 24% to correct for edge effects.

The plant material harvested on 22 and 23 Mayand at anthesis was milled and analysed for nitro-gen by a Kjeldahl procedure. Milled samples ofthe grain and of the straw from the samples takenat maturity were also analysed. From these data,and from the dry weights at the times of samplingthe amount of nitrogen present in the crops on theseoccasions was calculated.

RESULTSPlant establishment, numbers of shoots and earsThe average number of plants/m2 determined

from the counts in February was 272. In bothfields, plant densities of nine of the varieties variedlittle from this average, though the densities ofMardler and Holdfast were 310 and 300 and ofHobbit, 230. In neither field, however, was thenumber of ears/m2 correlated with plant density.Numbers of shoots, counted on the 2 x 50 cm rowlengths, were plotted against time to provideestimates for each variety in each field of themaximum shoot number. Shoot survival was cal-culated as the number of fertile ears expressed asa percentage of the maximum number of shoots(Table 2). Number of ears/m2 was estimated bydividing the corrected grain yield per plot by themean grain weight per ear determined at harvestin August. The accuracy of these calculated valueswas verified from counts of the stubble at harvest,but the latter data are not presented because theywere based on only a small subsample of the har-vested area. Maximum number of shoots/m2 wascalculated from the number of ears/m2 at maturity(Table 4) and the percentage survival of shoots(Table 2). As expected, fewer shoots were producedin Paternoster Field than in Camp Field, maximumnumbers being 701 and 937/m2 respectively. Inboth fields, Little Joss and Holdfast produced

678 R. B. AUSTIN AND OTHEES

Table 2. Maximum numbers of shoots, number of ears and percentage survival ofshoots of varieties in two fields

VarietyLittle JossHoldfastCappelle -DesprezMaris WidgeonMaris HuntsmanHobbitMardler370/500730/3637989/10ArmadaBenoist 10483

S.E.Mean

Maximum number ofshoots/m2

PaternosterField870865664637673598700741584745558773

50701

CampField11141200925922947794821837922813753

117235

937

No. of ears/m2

PaternosterField296346279274249257308289257283296379

15292

CampField366468435415379429386385424366535715

22442

Survival ofshoots (%)

PaternosterField

344042433743443944385349

42

CampField

323947454054474646457161

48

more shoots/ma than all the varieties exceptBenoist 10483. Benoist 10483 produced a similarnumber of shoots to Little Joss and Holdfast inCamp Field, but only a little more than theaverage number in Paternoster Field. A smallerpercentage of the shoots survived in Paternosterthan in Camp Field (Table 2), but there was astrong correlation between the shoot survival ofthe varieties in the two fields. In both fields amuch greater percentage of shoots survived inArmada and in Benoist 10483 than average, whilea lower percentage of those produced by LittleJoss survived.

Numbers of ears/m2 were considerably greaterin Camp than in Paternoster Field. Benoist 10483produced many more ears than average in bothfields and Armada produced more than averagenumbers in Camp Field. With these exceptionsthere was no consistent trend in number of ears/m2with year of introduction.

Dates of anthesis and maturityDefining anthesis as the date when anthers

were visible on an estimated 50% of the ears,Little Joss was the latest variety in both fields,and Benoist 10483 the earliest. For the oldervarieties, for Huntsman and for 730/3637 anthesisoccurred 5-7 days later in Camp than in Pater-noster Field. For all the other varieties the differ-ence was only 3 days (Table 3). Although dates ofmaturity could not be denned with precision,judged by the date when no further green leaf waspresent, all varieties were earlier in Paternosterthan in Camp Field. Benoist 10483 was earlier and

Little Joss and Holdfast were later than the othervarieties in both fields. The plots were combine-harvested on 8 August in Paternoster Field and on15 August in Camp Field. For practical reasons allplots in an experiment had to be harvested on thesame date.

Growth of vegetative organs and grainData are presented graphically for only four

varieties: Little Joss, Huntsman, 989/10 andBenoist 10483. These genotypes represent therange observed in pattern of growth.

On 22 and 23 May when leaf area indices (LAI)were likely to have been close to their maximum,there were significant differences between varietiesin LAI (Table 3), though no consistent trend withdate of introduction, or correlation with yield.The two highest-yielding varieties, Hobbit and989/10, had lower than average LAIs in bothfields. On 22-23 May the mean LAI in PaternosterField was less than half that in Camp Field, where-as maximum number of shoots/m2 was only 25 %less (Table 2).

The changes with time in LAI between anthesisand maturity are shown in Fig. 1 (a) and (6). FromTable 3 and Fig. 1 it can be seen that there was aconsiderable decrease in LAI between 22-23 Mayand anthesis, this being most marked for LittleJoss, the latest variety, in Camp Field. In Pater-noster Field there was little difference betweenvarieties in LAI (Fig. la). Mainly for this reasonthere was little variation in leaf area duration(LAD, the integral of leaf area index betweenanthesis and maturity with respect to time), much

Genetic improvemevts in winter wheat 679

Table 3. Leaf area indices on 22-23 May, dates of anthesis and loss of dry matter fromvegetative organs between anthesis and maturity for 12 varieties in two fields

VarietyLittle JossHoldfastCappolle-DesprezMaris WidgeonMaria HuntsmanHobbitMardler370/500730/3637989/10ArmadaBenoist 10483

S.E.Mean

Leaf area indexon

22-22

PaternosterField3-552-673073103 1 33043-323-653072-823123140-20314

(MayCampField6-906647-378108 1 86-277-648-848145-887-587-460-687-42

Date of anthesis(date in June)

PaternosterField

151212121199

10111094

10-3

CampField

20191817161212131713127

14-7

Loss of dry matterfrom vegetativeorgans between

anthesis andmaturity (g/ma)

PaternosterField146599179

15616023513616817358

14330

134

CampField10983

172156200158137140253203166227

46167

of the variation being associated with later anthesisin the older varieties, and no correlation betweenLAD and grain yield. In Camp Field on the otherhand, in spite of the range of 13 days betweenvarieties in the date of anthesis, the decline tolow LAI occurred on about the same date in allvarieties. Consequently, in this field, the earlieranthesis in the newer varieties was associated withgreater LAD, greater post-anthesis increase in cropdry weight and greater grain yield. LAD varied by2-4-fold, and grain yield was strongly positivelycorrelated with LAD (r = +0-79). (Unless other-wise stated correlation coefficients given in thispaper have been calculated using variety means,and have 10 degrees of freedom.)

The total dry matter in the above-ground organsincreased after anthesis in Paternoster Field untilmid July and in Camp Field until late July. Therewere no marked differences between varieties inthe rates of dry-matter accumulation, variationin crop weight on a given occasion during grainfilling reflecting differences in crop weight at an-thesis. The weights of all above-ground vegetativeorgans continued to increase for at least 3 weeksafter anthesis, weights declining thereafter. Themaximum weights of the vegetative organs of thefour oldest varieties; Little Joss, Holdfast,Cappelle-Desprez and Maris Widgeon, were con-siderably greater than those of four of the modernvarieties (Hobbit, 370/500, 730/3637 and 989/10)having the major dwarfing gene, Rht2 (Gale, 1979).

This difference was due mainly to the greater stemand leaf sheath weight of the older varieties: 641and 848 g/m2 in Paternoster Field and Camp Fieldrespectively compared with that of the four semi-dwarf ones, 510 and 668 g/m2 (S.E. 14 and 19).Leaf weights were almost identical for the twotypes.

In Paternoster Field the vegetative organs ofthese four semi-dwarf varieties gained less in dryweight after anthesis and maximum weights wereattained earlier than in the four older varieties(Fig. 2 a). No such differences were evident for thesame sets of varieties in Camp Field (Fig. 26). Themean loss in weight from the vegetative organsbetween the time of attainment of their maximumweight and maturity was the same for both sets ofvarieties (Table 4). Because the maximum weightsof the vegetative organs were greater in Camp thanin Paternoster Field these losses in weight expressedas a percentage of the maximum weights were 29and 33 % respectively. There were only minor butconsistent differences between the four modern andthe four older varieties in this respect (Table 4).Loss in weight from the leaf laminae may partlyhave resulted from leaf-fall, though it has beenargued (Austin et al. 1977) that a substantial partof the loss in leaf weight over this period is likelyto be due to the export of nitrogenous substances.

In all varieties there was a net loss in dry weightfrom the vegetative organs between anthesis andmaturity (Table 3). Generally, the loss was smaller

680 E. B. AUSTIN AND OTHERS

7.vi 7. viiDate

17. vii 27. vii 6. viii

Fig. 1. Time course of changes in crop components in two fields, (a) and (6): leaf area index; (c) and(d): grain dry weight. Lines represent four varieties: Little Joss () , Maris Huntsman () , 989/10 (A),and Benoist 10483 () . The first point on each line is the value of the given component at anthesis. (a)and (c) Paternoster Field, (6) and (d) Camp Field.

in Paternoster than in Camp Field, and Holdfastlost the least dry weight in both fields. In CampField grain yield was negatively correlated withthis loss in dry weight (r = 0-77). In PaternosterField the correlation (r = 0-50) was not signi-ficant.

Growth in grain weight/m2 (Fig. lc and d)followed the usual pattern and was essentially linear

between 20 and 45 days after anthesis, the meanrates being 11-5 and 17-5g/ma/day in Paternosterand Camp Fields (S.E. 0-3 and 0-5 respectively).(The grain and crop growth rates given in thisparagraph are the means for all varieties except theearly maturing Benoist 10483. Mean rates werecalculated from joint regression analysis.) In bothfields three varieties, Huntsman, 730/3637 and

Genetic improvements in winter wheat 681

9. vi 19. vi 29. vi 9. vii 19. vii 29. viiDate

800

7. vi 17. vi 27. vi 7. vii 17. vii 27. vii 6. viii 16. viii

Fig. l(c) and l(d). For legend see opposite.

989/10, had the highest rates, differences betweenthe remaining varieties being smaller and incon-sistent between fields. In Camp Field, rates forHuntsman, 730/3637 and 989/10 were 19-7, 19-8and 22-1 g/ma/day respectively (S.B. 1-56). Cropgrowth rates between 20 and 45 days afteran thesis were much lower than grain growth rates

over the same period. Varietal differences in cropgrowth rates were not significant in either field,the mean rates being 5-5 g/ma/day in PaternosterField and 9-6 g/m2/day in Camp Field (S.E. 0-6and 1-0 respectively). Samples were not takenfrequently enough to be able to determine accu-rately dates of cessation of grain dry-matter

682 R. B. AUSTIN AND OTHERS

600

400 -

S 200 -

6

-20010 20 30 40

Days after anthesis50 60 70

-20030 40

Days after anthesis50 60 70

Fig. 2. Changes in dry weight/m2 of grain ( ) and straw ( ), i.e. stems, leaves and chaff, afteranthesis in two fields: (a) Paternoster Field and (6) Camp Field. , Mean of the four oldest varieties:Little Joss, Holdfast, Cappelle-Desprez and Widgeon; %, mean of four semi-dwarf varieties, Hobbit,370/500, 730/3637 and 989/10.

Genetic improvements in winter wheat 683

Table 4. Less in dry weight of various organs from the four oldest and from four newer, semi-dwarf varieties,between the time of attainment of maximum weight of the vegetative organs and maturity

Paternoster Field Camp Field

Oldest varietiesSemi-dwarf varieties

S.E.

Loss inOldest varietiesSemi-dwarf varieties

Allvegetative

organs

266275

23dry weight as

Stemsand

sheathsLoss

227220

19

Leaflaminae

Earstructures

in dry weight (g/m225353

a percentage of the8581

99

14202

' total loss5

10

Allvegetative

organs

31434430

Stemsand

sheaths

2272209

Leaflaminae

56713

from the vegetative organs6565

1816

Earstructures

53554

1719

Loss in dry weight of the organs as a percentage of their weight at the time of maximum vegetative weightOldest varieties 32 36 25 14 26 24 30 33Semi-dwarf varieties 35 41 22 23 32 33 34 28

accumulation, though it appeared that these re-flected dates of anthesis, the earlier varietiesceasing grain growth sooner than the later ones.

Growth curves of the grains in the first, secondand third florets of the central spikelets of the earsshowed that in these grains there were varietaldifferences in growth rates in both fields (see Fig.3 for examples). The grains in florets 1 and 2 hadthe fastest average growth rate. The mean ratesof growth of the grains in the three florets weresimilar in the two fields. In both fields, averagingover florets, Maris Huntsman and 989/10 had thefastest rates, and Holdfast, Mardler and Benoist10483 the slowest rates. When the mean rates ofgrowth of the grains in florets 13 of the centralspikelets were multiplied by the number of grains/ma, the resulting rates of growth in g/m2/day werestrongly correlated with the rates estimated fromwhole-ear data. Although the sampling frequencywas insufficient to quantify varietal differences inthe duration of grain growth, the curves given inFig. 3, as well as those for the other varieties,indicate that the duration was less in Little Jossand Holdfast than in most modern varieties.

Grain yield, yield components, height andharvest index

Holdfast, a wheat of breadmaking quality, gavethe lowest yield in both fields, the yields of LittleJoss being somewhat greater. The highest-yieldingvarieties were 989/10 and Hobbit (Table 1).Taking Little Joss as the standard against whichto measure the yield improvement achieved bybreeding for similar use, the mean yields of thesetwo highest yielding varieties were 139 and 142%in Paternoster and Camp Fields, respectively.Benoist 10483 yielded relatively less than the other

varieties in Paternoster Field, the lower fertility,lower-yielding site, than in Camp Field (Fig. 4).

The greater yields of the modern varieties werenot associated with changes in any one yieldcomponent (Table 5). Thus, 989/10 produced nomore fertile ears/m2 than Little Joss, and its yieldadvantage over the latter depended on its havingheavier and more grains per ear. This held true forboth fields. In Paternoster Field, Armada had asimilar number of ears/m2 to Little Joss, and itsyield advantage depended on the production ofmore and heavier grains per ear. In Camp Fieldhowever, Armada had more ears/m2 but fewer andlighter grains. Benoist 10483 produced many moreears/m2 than Little Joss in both fields, but was asmall-eared variety with grains of similar weightto those of Little Joss. Its yield advantage overLittle Joss (15% and 38% in Paternoster andCamp Field respectively) appeared to depend onits ability to produce more fertile ears/m2, andthis was expressed most strongly in Camp Field.

The total number of spikelets per ear was lessin Paternoster than in Camp Field in all varieties(19-0 as compared with 20-2). Benoist had thefewest, 730/3637 the most and there was no cleartrend with year of introduction, Little Joss beingsimilar to 989/10 and Armada. Number of grainsper spikelet in the central spikelets was greater inthe varieties having the RM2 gene (3-18) than in thefour old varieties lacking it (2-50). Number of grainsper central spikelet was slightly lower in Paternosterthan in Camp Field (2-77 and 2-95 respectively).

For all varieties, mature plant height to thebase of the ear was 23-35 % greater in Camp thanin Paternoster Field, the average increase being22 cm or 29 % of the height in Paternoster Field(Table 1). Weight per stem, both maximum and

684 R. B. AUSTIN AND OTHERS

10 20 30 40Days after anthesis

SO 60 70

10 20 30 40Days after anthesis

60 70

Fig. 3. Time course of changes in mean grain dry weight in floret 1 for four varieties: Little Joss (#),Maris Huntsman () , 989/10 (A) and Benoist 10483 () , in two fields: (a) Paternoster Field, (6) CampField, S.E. of a value is 6%.

final, bore a reasonably close, though not propor-tional, relation to stem height. Stem weight perunit height was greater in Paternoster than inCamp Field. Reduction in stem height by one half,the varietal range, was accompanied by a reductionin stem weight by about one third. The loss instem weight from the time of maximum vegetativedry weight bore no relation with height. Harvestindex (the ratio of grain yield to grain + strawyield at maturity) varied from 34-36 % for LittleJoss and Holdfast to 48-51% for 989/10 and

Hobbit. Harvest index was strongly negativelycorrelated with plant height, in both fields.

Nitrogen offtakeThe amount of nitrogen in the entire above-

ground biomass changed little between 22-23 Mayand maturity. As expected, the nitrogen concentra-tions in the various organs were greater in Campthan in Paternoster Field and this resulted in thenitrogen offtake by the crop being 2-76 times

Genetic improvements in winter wheat 685

500 -

500300 350 400 450Grain yield in Paternoster Field (g/m2)

Fig. 4. Yields of varieties in Camp Field plotted against those in Paternoster Field. The numbersrefer to those alongside the variety names in Table 1. iji, S.E. of a value.

VarietyLittle JossHoldfastCappelle-DesprezMaris WidgeonMaris HuntsmanHobbitMardler370/500730/3637989/10ArmadaBenoist 10483

S.E.

Mean

Table 5. Characteristics of theMean grainweight (mg)

PaternosterField3 7 031-948-145-74 6 14 6 141-843-639448

686 R. B. AUSTIN AND OTHERS

Table 6. Nitrogen present in the crop at maturity, nitrogen harvest, indexand grain nitrogen concentration

VarietyLittle JossHoldfastCappelle-DesprezMaris WidgeonMaris HuntsmanHobbitMardler370/500730/3637989/10ArmadaBenoist 10483

S.E.

Mean

N in crop(g/m2)

Paternoster CampField6-46 06-86-56-87 16-67-26-77 07-57-20-276-8

DISCUSSION

Field16119018-318-318019-817117120020-120-421-50 6 318-8

Nitrogenharvest index (%)

Paternoster CampField

747577787979788282788180

78

Field716968687272717171726773

71

Genetic improvements in winter wheat 687

800 -

700 -

4 600 -x:eo

I 500 -

I 400.6

300

200 -

-

-

1

ft'10

-i-H 2

7 " 8

1

". n912

i

11

3 '\"4

'" 1 2

i i

400 500 600 700 800Straw dry weight (g/m2)

900 1000

Fig. 5. Grain dry weight plotted against straw dry weight at maturity for Camp Field ( ) and PaternosterField (#) . The lines show a slope of 1-0. The numbers refer to those alongside the variety names inTable 1. |-iJ-l, S.E. of a value.

both low and high fertility in our experiments, asis often found in trials with varieties carried out indifferent years and sites.

As found in other studies (e.g. Van Dobben,1962) our experiments also show that in a givenenvironment shorter varieties had a greater harvestindex and grain yield than the older, taller ones(Table 1). Fig. 5 shows that the straw yield wasnegatively correlated with grain yield, the regres-sion coefficients being 0-69 (S.E. 0-16) inPaternoster and 0-44 (S.E. 0-26) in CampField. The poor correlation in Camp Field waspartly due to Mardler, the grain yield of which waslower than expected on the basis of results fromother trials, and to 370/500 and Armada. Theregression coefficient for the Camp Field data,omitting these varieties, was 1-28 (S.E. 0'25).None of these coefficients was significantly differentfrom 1-0. Thus in each field there was a ceilingto total dry-matter production (8-97 and 14-40 t/hain Paternoster and Camp Field respectively) withlittle variation among varieties. Within an experi-ment, variation in grain yield was strongly asso-ciated with changed distribution of dry matter, inparticular with reduced stem length and weight/m2.However, there were other differences betweenhigh- and low-yielding varieties and no singleattribute or yield component was uniquely asso-ciated with high yield.

Benoist 10483 was the exception to the generali-zation that variety yields in Camp Field werestrongly correlated with those in Paternoster Field.

Benoist 10483 was the only variety or advancedbreeding line tested which had not been selectedfor high yield in the U.K. and it was includedmainly because it was known to be freely tillering.It had the highest number of ears in both fields,partly because it produced more shoots, and partlybecause more of these shoots survived than inmost other varieties. Armada also had a highpercentage shoot survival, and this variety andBenoist 10483 were clearly different in this respectfrom the varieties and breeding lines originatingfrom the Plant Breeding Institute which were usedin these experiments.

Although the straw weight at maturity of modernvarieties was less than that of the older varieties(Fig. 5), the stems and leaf sheaths at this stage com-prised 30-35 % of the straw weight in the modern,but 45-50% of it in the older varieties. As a result,at maturity the leaf weights of the two lowestyielding varieties, Little Joss and Holdfast, werevery little different from those of the two highestyielding ones, Hobbit and 989/10. The same heldtrue for leaf area index on 22-23 May (Table 3),and there was no relationship between leaf areaindex at this time and grain yield.

Grain growth rates between 20 and 45 days afteranthesis exceeded crop growth rates by an averageof 6-0 g/m2/day in Paternoster and 7-9 g/m2/dayin Camp Field. This does not necessarily imply thatthe requirements for carbon for grain growth couldnot be met from current photosynthesis, becauserespiratory losses, particularly from the stems, are

688 R. B. AUSTIN AND OTHERSknown to be high during this period (Rawson &Evans, 1971; Austin et al. 1977) and they wouldreduce net photosynthetic and hence crop growthrates. However, some of the carbon assimilatedduring early grain growth and temporarily storedas soluble carbohydrate in the stems is subse-quently relocated to the grains (Rawson & Evans,1971; Austin et al. 1977). As the loss in weightfrom the vegetative organs between anthesis andmaturity was greater in the high- than in the low-yielding varieties it seems that the margin be-tween the demand for carbon by the grains andthe supply from current assimilation during thelinear phase of grain growth may be smaller inthe newer varieties.

Although grain yield was positively correlatedwith leaf area duration in Camp Field, the lack ofsuch correlation in Paternoster Field suggests thatthe relationship in Camp Field was not causal, asthe highest yielding varieties outyielded Little Jossby a similar percentage in both fields. The greaterLAD of the higher-yielding varieties in Camp Fieldwas associated to a considerable extent with theirearlier anthesis, since they reached maturity atthe same time as the lower-yielding varieties. Asgrain dry matter originates mainly from photo-synthesis after anthesis, when leaf area is decliningrapidly, a further increase in post-anthesis photo-synthesis may be obtained by selecting genotypesin which anthesis is closer to the time of maximumLAI: our results show that there has been a trendin this direction.

As noted earlier, the yield advantage of themodern varieties did not depend on a change inone, or mainly one, yield component. Varietiespossessing the major dwarfing gene Rht2 (Hobbit,Mardler, 370/500, 730/3637 and 989/10) had, asa group, more grains per ear but fewer ears/m2than the other varieties, though their mean grainweight was close to average. The greater numberof grains per ear was achieved because there weremore grains per spikelet in the Rht2 dwarf varietiesthan in the other ones.

These effects of the Rht2 gene are now wellknown (Gale, 1979) and may be the consequenceof reduced competition between the stem and theear for a limited supply of assimilate, as suggestedby Friend (1965), Rawson & Hofstra (1969),Bingham (1969) and Patrick (1972). It is evident,however, from Tables 1, 2 and 5, that there wasvariation in yield and its components among thefive varieties having the Rht2 gene. It is not knownwhether the negative association between heightand grain yield is a direct result of competition forassimilates and holds irrespective of which geneor genes affect stem growth and plant height.

Despite the generally lower nitrogen concen-tration in the grain of the newer as compared with

the older varieties, the nitrogen offtake by thegrain and by the crop was greater for these varie-ties than for the older ones (Table 6). In CampField this amounted to an average of 25 kg N/hafor each 1 t/ha increase in grain yield. This figureprovides an indication of the minimum increasein fertilizer nitrogen application that would beneeded to sustain an increased yield of 1 t/ha inintensive wheat growing.

Although the genetic gain in grain yield in winterwheat during this century has been associated witha corresponding decrease in straw yield, it is clearthat there must be a limit to the improvement inyield which can be achieved by continuing thistrend. A further reduction in non-grain biomass maybe achievable by selecting still shorter genotypes,but because of the need for leaves to be arrangedin the canopy to maximize the interception oflight and its uniformity of distribution over theirarea, and to provide adequate mechanical supportfor the ears, it is unlikely that stem and sheathdry weights could be reduced much further. Theaverage stem and sheath dry weight at maturityof Hobbit, 370/500, 730/3637 and 989/10 in CampField was 453 g/ma. The dry weights of the othercomponents at harvest were, in g/m2, grain 707,chaff (rachis, glumes etc.) 139, and leaf laminae 143.This dry-weight distribution gives a harvest indexof 49%. Assuming constant biomass yield, a re-duction in stem and leaf sheath dry weight to halfthe observed value, and a pro-rata increase inchaff weight to accommodate the extra grain, theorgan weights would become: grain 895, chaff 178,leaf laminae 143, stem and leaf sheath 226. Thesevalues give a harvest index of 62 %, for an increasein grain yield over that observed for the genotypesof 26%. It may prove impossible to producegenotypes having these characteristics: if so breed-ers are close to exhausting the opportunity forincreasing yield by modifying dry-matter distri-bution.

This argument suggests that breeders will needto detect and exploit genetic differences in totaldry-matter production if there is to be a continuedgenetic gain in yield. Straw strength and numberof ears will have to be increased, or at least main-tained, implying a need to retain a distribution ofdry matter between grain and straw similar to thatin the best of the present varieties. Attempts toincrease dry-matter production by means otherthan increasing photosynthetic rate per unit leafarea may be unproductive because they are likelyto result in an increased water requirement. Asshortage of water frequently limits growth andyield at present, genotypes with substantiallygreater leaf area expansion rates and LAI wouldbe more prone to drought, particularly duringgrain filling. The obvious alternative, an increase in

Genetic improvements in winter wheat 689the rate of photosynthesis per unit leaf area, mayprovide more dry matter for the same, or evenlower, water use, but there is no evidence thatmodern varieties of wheat have this characteristic,as primitive forms have higher photosyntheticrates, though much smaller leaves (Evans &Dunstone, 1970). It has yet to be ascertained

whether increases in photosynthetic rate can beachieved without compensating reduction in leafsize or longevity, and clearly this is a major chal-lenge for wheat breeders and physiologists.

We are grateful to Mrs C. Starr for analysing thesamples of grain and straw for nitrogen.

Note added in proofThe advanced breeding line 989/10 has now been added to the varieties on the National List and

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