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Application of the ideotype concept in breeding for higher yield in the oilseed brassicas
Field Crops Research, 26 ( 1991 ) 201-219
201
Elsevier Science Publishers B.V., Amsterdam
Application of the ideotype concept in breeding for higher yield in the oilseed brassicas
N.
Thurling
Crop and Pasture Sciences, School of Agriculture, The University of Western Australia, Nedlands, W.A. 6009, Australia
ABSTRACT Thurling, N., 1991. Application of the ideotype concept in breeding for higher yield in the oilseed brassicas. Field Crops Res., 26:201-219. Approaches to yield-improvement breeding methodology which utilize information from biometrical genetic studies and from plant physiological studies aimed at developing ideotype models are reviewed here, with special emphasis on yield improvement in the oilseed brassicas. Major difficulties involved in yield-improvement programmes are the accurate selection of parents from which to generate superior breeding populations, and the selection of high-yielding genotypes from early-segregating generations. In both cases, the complex polygenic inheritance of yield and the magnitude of genotype × environment interactions affecting yield greatly reduce the chances of isolating superior genotypes. A wide array of biometrical procedures have been proposed in developing yield-improvement programmes. Some have been utilized successfully. However, their use in analyzing a character such as yield has been generally limited because of the need to conduct analyses with populations sown at much lower densities than commercial crops. Development of the ideotype concept has focussed the attention of plant physiologists on identification of simple morphological characters which have some influence on physiological processes determining the yield of the economic organs. Characters such as leaf inclination and leaf shape, for example, are often simply inherited and can greatly influence crop canopy structure and radiation interception. Such characters could be rapidly modified by selection to increase crop photosynthesis and yield. Ideotype definitions integrate information on these relationships and provide plant breeders with a clear blueprint of the characteristics of a high-yielding cultivar in a specified environment. Research with oilseed Brassica species in Western Australia and Tasmania has identified a number of highly heritable characters which provide a basic framework for the definition of a yield ideotype for spring rape (B. napus). These include an optimal time of flowering, apetalous flowers and long upright pods to improve light penetration of the dense pod canopy, and pod shattering resistance to allow for direct harvesting of the crop. A major advantage in breeding for this ideotype is that genes for certain characters can be easily introgressed from the related species B. campestris and B. jut,cea. It is also possible, using microspore cultures, to generate large numbers ofB. napus pure tioes directly from plants selected in F2 populations on the basis of ideotype characteristics. This wouM allow for large-scale yield testing of selections within two to three years of making the original crosses.
0378-4290/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
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N. THURLING
INTRODUCTION
Yield improvement has always been a major objective of modern plant breeding. In Australia, and other countries with high-input crop production systems, yield improvement must be maintained to ensure that returns to farmers will be greater than the costs of production. However, maintenance of a steady rate of yield improvement could become more difficult with a depletion of genetic variability to breeding programs (Frey, 1971 ). Recognition of this decline in genetic diversity within domesticated plant species has prompted a major world-wide effort to conserve the genetic resources of these species surviving as land races and/or wild relatives in centres of origin (Holden and Williams, 1984). Given this enhancement of the gene pools of domesticated species, there would still be a need to further improve the efficiency with which the greater genetic variability could be utilized by selection to increase yield. Approaches to improving the methodology of breeding for higher yield have been the subject of considerable research (Jensen, 1988). For some time it was thought that biometrical genetics would offer an effective means of yieldimprovement breeding strategies by providing a better understanding of its complex polygenic inheritance. Unfortunately, early hopes of a major contribution of biometrical genetics to the improvement of yield-breeding methodology in self-pollinating species have not been realized. More recently, with a rapid expansion in our understanding of physiological determinants of yield, the ideotype concept has evolved to provide breeders with specific models upon which to base their yield-improvement strategies (Donald, 1968). In this case, the desired high-yield phenotype may be defined in terms of several relatively simple plant characters. Breeders should then have a greater chance of significantly improving yield by selecting more efficiently for higher heritability characters they intend combining in a superior cultivar. The main purpose of this paper is to show how the definition of a yield ideotype might be best utilized to improve the efficiency of breeding for higher oilseed Brassica yields. Some emphasis will be given to the application of information from biometrical genetic analyses to improving yield-breeding methodology. The problems encountered with the effective application of such analyses will serve to highlight the greater benefits expected from appropriate manipulations of more simply inherited yield determinants. BREEDING FOR HIGHER YIELD IN SELF-POLLINATING PLANT SPECIES
Bulk selection and pedigree selection have been the methods most commonly used in breeding for higher yield in self-pollinating plant species with populations derived from hybridization between selected parents. Although it is time-consuming, the pedigree method is normally preferred because se-
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
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lection can be commenced as early as the F 2 generation to isolate superior plants on the basis of progeny performance. This type of selection is repeated in the succeeding generations to isolate relatively few superior and genetically uniform lines for advanced testing over a range of environments. Selection for yield itself is normally delayed until the F4 generation, when sufficient seeds can be harvested from remnant plants of selected F3 lines to establish large replicated plots in the following generation. The first step in implementing pedigree selection for higher yield is selection of the parents which will be crossed to generate the breeding population. If yield itself is the only criterion on which parent selection is based, simply selecting the two highest-yielding lines from a germplasm collection is unlikely to isolate the best possible cross, i.e. the one generating a population in which transgressive segregation for high yield is greatest. Generally, because of the complex inheritance of yield, identification of superior crosses cannot be based solely on the yields of potential parents. Some evaluation of crossbred populations, such as bulk F2 or F3 populations, has been advocated as being a more reliable basis for cross-prediction (Harrington, 1940). However, the results from different experiments revealed inconsistencies, casting some doubt on the effectiveness of this approach to parent selection. Irrespective of the deficiencies in various methods used in selecting parents, the ability to accurately identify a clearly superior cross would obviously improve the efficiency of pedigree selection by allowing the breeder to concentrate on a much larger elite population. The selection of desirable phenotypes from a crossbred population can first be conducted in the F2 generation which, according to Jensen (1988) is a pivotal generation "when the population numbers are smallest, when selection or rejection has the greatest impact, and when the most information is present." Direct selection for yield in early generations of a pedigree selection scheme has invariably been ineffective when selection was based on either grain weight measurements (Whan et al., 1981 ) or visual criteria (TownleySmith and Hurd, 1973). However, it is interesting to note that experienced breeders were better able to visually identify high-yielding lines of Triticale than inexperienced breeders (Salmon and Larter, 1978 ). The general lack of success in identifying high-yielding genotypes in the F2 or F3 generations is not unexpected in that selections made in these generations are based on a plant population totally different to the commercial crop. Selection among widely-spaced F2 plants, for example, makes no allowance for the highly competitive environment of a crop population. In fact, selection for high yield at low density tends to isolate genotypes poorly adapted to crop densities (Hamblin, 1975). The difficulties of selecting accurately for yield in early generations of a pedigree selection program has prompted a search for highly-heritable yieldrelated characters to serve as alternative selection criteria. Time of flowering,
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N. THURLING
for example, is particularly relevant to improving yield in short-season environments such as occur in the West Australian wheatbelt. Studies with lupins (Lupinus angustifolius) in this environment (Ratinam and Thurling, 1989) showed that flowering times of ten cultivars in a low-density mixture were negatively correlated with their yields in large, crop density monoculture plots ( r = - 0 . 8 0 ) . An F4 population derived from the earliest-flowering plants in an F2 of a late × early lupin cross had a 40% greater yield than one derived from a random selection of F2 plants (Thurling and Ratinam, 1989 ). Grainyield components (ears m - 2, spikelet number, grains per ear and grain weight) have also been extensively studied in respect of their value as alternative criteria of selection for yield in early generations. As they are simple to measure and usually have higher heritabilities than yield, these characters seemed to offer considerable scope as selection criteria. However, their use in breeding programs has been limited because strong negative correlations between some components means that selection for one component will be compensated for by a decrease in the mean value of another, and yield will not be affected (Adams, 1967). The difficulty of improving the efficiency of selection for yield, either directly or indirectly, in Fz o r F 3 populations has also prompted the development of rapid generation advance techniques such as single-seed descent (SSD; Goulden, 1939) which involve a rapid turnover of generations (until F5 o r F 6 ) during which time selection is largely or wholly ignored. A major aim of this procedure is to produce uniform lines as quickly as possible and concentrate selection for yield among large plots of these lines. This type of selection would be far more efficient than selecting for yield between plants and single rows in early generations, and could be commenced as soon as two years after producing F1 seeds. Despite the fact that no selection may be practiced during the development of SSD lines, they appear to include as many superior genotypes as are found amongst pedigree lines (Park et al., 1976 ). Thus, with the shorter time required to reach the stage of advanced testing, single-seed descent is now considered an effective alternative to pedigree selection in improving the yield of self-pollinating species. The various difficulties encountered in pedigree selection with procedures such as parent selection and early-generation selection for yield have provided a focus for much biometrical genetic research. In fact, the need to improve methods of breeding for higher yield in all types of domesticated species has stimulated the development of a wide range of analytical techniques for studying the inheritance of polygenic characters in these different plant species. Some basic aspects of the application of biometrical genetic principles to improving pedigree selection procedures are considered in the following section.
APPLICATIONOF THE IDEOTYPECONCEPTOILSEED BRASSICAS
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A P P L I C A T I O N S O F B I O M E T R I C A L G E N E T I C S TO P L A N T B R E E D I N G P R O B L E M S
Selection of parents for yield improvement in selfpollinating species Diallel analysis was the first biometrical technique to be used extensively in improving the efficiency of plant-breeding programs. Its main attractions were that relevant information could be obtained without going beyond the F~ generation and information on both genetic relationships among a group of potential parents and their potential as parents of a yield-improvement program could be obtained from the one experiment. The analytical method developed by Griffing ( 1956 ) measures the genetic potential of parental line in terms of combining ability and has the advantage of making no assumptions regarding genetic relationships among parents. By contrast, the other major analysis developed by Jinks ( 1954 ) and Hayman ( 1954 ) is primarily aimed at investigating genetic relationships among the parents, and its accuracy depends on the experimental materials conforming to several different genetic assumptions. Generally, no real advantage in accuracy of parent selection has been gained from the use of the Jinks-Hayman analysis (Lupton, 1961 ), probably because the assumptions are rarely satisfied in actual experiments (Baker, 1978 ). Also, since most diallel experiments with cereal crops are conducted at plant densities much lower than that of the commercial crop, it is not surprising that this technique has been of limited value in predicting cross potential. Genetic information from segregating generations of crosses between selected parental combinations has also been utilized in predicting cross potential through estimation of the number of transgressive lines likely to be obtained through single-seed descent (Jinks and Pooni, 1976). This prediction is based on a ratio of the difference between parental means to the additive genetic variance obtained from measurements of early-generation populations - F2 and first backcrosses. The accuracy of predictions based on this method has been confirmed from measurements of plant heights and flowering times of advanced lines, but these characters are genetically less complex than yield. Ideally, parent selection should be based on comparisons among groups of advanced lines derived from different crosses, particularly where such lines can be generated within a short period of time. The latter requirement is now possible in plant species where relatively large numbers of haploid plants can be obtained directly from F~ hybrids through chromosome elimination techniques or anther culture. Doubling of the chromosome numbers of haploid plants by colchicine treatment will result in each chromosome having an identical partner, and the seed produced will then constitute a genuine pure line (dihaploid line). Lines derived from an F~ hybrid in this way will be
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lq. THURLING
representative of the genetic variation released at meiosis in the hybrid, and can be generated in a period as short as six months. A major advantage of this technique, apart from the rapid development of experimental materials, is that lines can be tested at crop densities to obtain relevant yield assessments. This is normally not possible with biometrical analyses based on early-generation populations such as F~'s, F2's and backcross populations. Studies with dihaploid lines of barley (Reinbergs et al., 1976) showed that only twenty of these would be required for predicting the relative yield potential of a cross, so many crosses may be evaluated at the same time.
Biometrical analyses of yieM component relationships in self-pollinating plant species As indicated earlier, grain-yield components were considered ideal candidates as alternative criteria of selection for high yield in early generations of pedigree selection programs. Despite their higher heritabilities and simplicity of measurement, they have not proved to be widely successful because of the common tendency for them to be negatively correlated. This so-called yieldcomponent compensation was seen as a manifestation of the sequential development of yield components, supported either by a limited constant input of metabolites or an oscillatory input which is limiting at critical stages of the developmental sequence (Adams, 1967). Such sequential development was thought to compromise information on the genetic control of yield components coming from conventional biometrical analyses. Accordingly, it was suggested that raw yield-component data should be transformed in such a way as to minimize the developmental influence and thereby enable the direct genetic control of components at the end of sequence to be more accurately defined (Thomas et al., 197 l; Driscoll and Abel, 1976). Following transformation of data on yield components from an experiment with oats (Grafius and Thomas, 1971 ), it was found that genotypes producing similar numbers of heads per unit area would be either high- or low-yielding, depending on whether they produced high or low numbers of seeds per head, respectively. Once these components have been determined, seed size will be largely determined by the amount of metabolites remaining to support this final stage of the developmental sequence. These results suggested that the best strategy for yield improvement would be to select primarily for a larger number of seeds per head. MORPHO-PHYSIOLOGICAL DETERMINANTS OF YIELD
The previous sections have considered some avenues for using biometrical genetics to improve the efficiency of breeding for higher yield in self-pollinating plant species. Overall, this approach has not been particularly successful,
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
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largely because of the complex inheritance of yield and the necessity to use low-density populations in obtaining estimates of genetic parameters. A more productive approach to improving the efficiency of yield improvement is likely to come from manipulation of relatively simple morphological characters found to have a significant influence on physiological processes determining seed yield. Enunciation of the ideotype concept (Donald, 1968 ) and pleas for greater collaboration between plant breeders and physiologists (Fischer, 1977 ) has encouraged a more rational approach to yield improvement based on combining specific physiological yield determinants in the superior cultivar. At the simplest level, this could involve crosses between a cultivar with a high biological yield and one with a high harvest index, to combine these characters and increase yield (Donald and Hamblin, 1976). Many of the characters which have been included in ideotype definitions for different plant species are, in fact, more simply inherited than biological yield or harvest index. Some may be determined by a single major gene, in which case introgression of the gene into a commercial cultivar through backcrossing affords a simple avenue to yield improvement. This procedure has often been used to develop isogenic or near-isogenic lines which provide the most effective basis for assessing the effects of a simply inherited character on seed yield. In this case, it is best to utilize a number of recipient parents, as the effects of a major gene can vary depending on genetic background. This was apparent in studies with soybean (Clawson et al., 1986 ) in which the incorporation ofa gene for dense leaf pubescence had a significant effect on growth rate in the cultivar Harosoy, but no effect in the cultivar Clark. More recently, leaf morphological variants have been studied more extensively with particular reference to their effects on crop canopy structure and the efficient interception of incident radiation. In peas, for example, three genes have been located which, singly or in combination, have profound effects on leaf morphology (Wehner and Gritton, 1981 ). Gene afreplaces leaflets with tendrils, tl replaces tendrils with leaflets and st reduces the size of the large stipules. Leafless peas ( a f a f s t st) have been the subject of considerable research in which modifications to existing crop management such as substantial increases in seeding rate appear necessary to achieve a leaf-area index of sufficient magnitude to ensure a high yield (Hedley and Ambrose, 1981 ). A series of alleles at a single gene locus are known to greatly modify the leaf shape of cotton and thereby influence light penetration of the canopy. Studies with isogenic lines (Wells et al., 1986) showed that alleles determining the okra and super okra leaf types were associated with reductions in leaf area index, but improved light penetration of the crop canopy. Generally, the okra leaf type is associated with about a 5% yield disadvantage relative to the normal leaf type. However, incorporation of the okra leaf gene into an appro-
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N. THURLING
pilate genetic background can lead to a significant yield improvement over a corresponding normal leaf line (Meredith and Wells, 1986 ). DEVELOPMENT OF A YIELD IDEOTYPE FOR THE OILSEED BRASSICAS
Genetic diversity in the oilseed brassicas
Rapeseed is obtained from two closely related species ofBrassica, B. napus and B. campestris. Although grown primarily as a source of mustard, B. juncea is an important oilseed crop of the Indian sub-continent which also has a potential in lower-rainfall areas of southern Australia (Kirk and Oram, 1978 ). All three species are cytogenetically related, as shown in Fig. 1, and genes may be transferred among these and the other Brassica species included in this diagram. Improvement of the more important rapeseed species B. napus can therefore utilize an enormous reservoir of genetic diversity residing in its own primary gene pool and those of the related species. Two of the diploid Brassica species - B. campestris and B. oleracea - are also highly diverse and each comprises a range of distinct sub-species which have evolved in response to human domestication. Overall, the genetic variation within each of the six domesticated and semi-domesticated Brassica species may be utilized in the improvement of any of the amphidiploid species. However, the gene pools of B. campestris and B. juncea can be accessed most easily for the improvement ofB. napus. Phenological development
Time of flowering is a major criterion of selection for higher yield in grain crops grown throughout southern Australia. Although the length of the growing-season varies widely throughout this region, it is usually necessary to develop cultivars which will flower at a time allowing for the completion of seed development before the onset of drought stress at the end of the growingseason, as well as the accumulation of sufficient biomass by the crop before flowering to support this seed development. An optimal time of flowering has been identified for wheat crops grown in southeastern Australia such that higher-yieldingcrops are most likely to be obtained with cultivars which flower after early spring frosts, but are still early enough for seed development to be completed before the onset of drought stress (Syme, 1968 ). As rapeseed yields are not as susceptible to frost effects as cereals because of their indeterminate developmental pattern, an earlier time of flowering could be considered as optimal from the point of view of allowing for a longer period of reproductive development. Data collected from an experiment with oilseed brassicas in a relatively low-rainfall environment in Western Australia (own unpublished data, 1989 ).
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
209
B.nigra 2n = 16 8 BB
B.carinata 2n = 34 1 8 BB + 9 CC
I
J
B.oleracea 2n = 18 9 CC
~ ~/ ~
B.napus 2n = 38 10 AA + g CC
I
ssp.gemmifera Brussels sprout)
Issp.oleifer a
ssp.acephala kale)
~swede)
i(ra pe )
issp.rapifer a
~sp.gongyloides kohl rabi) ~sp.capitata cabbage) ;sp.italica broccoli) ~sp.botrytis cauliflower)
B.juncea 2n = 36 10 AA + 8 BB
B.campestris 2n = 20 10 AA
I OILSEEDS ~sp.oleifera iturnip rape)
;sp.dichotoma itoria) ~spArilocularis sarson) .EAF VEGETABLES ~sp.pekinensis Chinese cabbage
~sp.chinensis Chinese mustar¢ :lOOT VEGETABLE ;ap.rapifera turnip)
Fig. 1. The Brassica gene pools.
reveals the importance of early flowering as a determinant of high yield (Table 1 ). Genes for early flowering had been introgressed into a leading B. napus cultivar (Wesbrook) from an early-flowering B. napus line (RU2) and an early-flowering B. campestris population (Chinoli C42). Introgression was achieved through three successive generations of backcrossing to Wesbrook and three subsequent generations of selfing, without any selection. The earliest-flowering B3 $3 lines from each cross were selected for field testing. Incorporation of the early-flowering trait into the Wesbrook background had a markedly beneficial effect on yield, although only the Wesbrook X RU2 line had a significantly higher yield than Wesbrook. The higher yield of this line was primarily associated with a much greater biological yield, and particularly with a greater increase in dry-matter between anthesis and maturity. Earlier flowering provided for a longer period of development before matu-
210
N. THURLING
TABLE 1 Agronomic performance of oilseed Brassica populations and their hybrid derivatives at a low-rainfall site in the Western Australian wheatbelt Time to 50% flowering (days)
Seed yield (g m - 2)
Harvest index
B. napus
84
225
(RU2) Best inbred backcross line
80
Dry-weight ( g m -2 ) Total (maturity)
Increment (anthesis to maturity )
0.26
858
646
251
0.26
965
721
94
151
0.23
656
404
78
173
0.27
650
390
75
157
0.29
560
368
4
41
0.04
184
197
(a3 S3) B. napus
(Wesbrook) Best inbred backcross line (B3 $3) B. campestris
(Chinoli C42 ) LSD (0.05)
rity, but it appeared that RU2 also contributed genes for a higher growth rate during the reproductive period. Variation in flowering time among spring cultivars of B. napus in the field is essentially continuous, but distinct differences in response to photoperiod and vernalization in controlled environments are detected among the same cultivars (Thurling and Vijendra Das, 1977). Differences in vernalization response were particularly marked, and it was possible to distinguish cultivars with little or no vernalization requirement (e.g., Target) from those which had a relatively high vernalization requirement (e.g., Isuzu and Bronowski ). Cultivars having a relatively high vernalization requirement also differed in their responses to increasing duration of the seedling vernalization treatment. Genetic analyses of populations derived from crosses among the cultivars Target, Isuzu and Bronowski showed that duplicate recessive genes determined the vernalization requirement of Bronowski, but two independent recessive genes (one having a markedly greater effect than the other) determined the vernalization requirement of Isuzu (Thurling and Vijendra Das, 1979). Target carried the dominant alleles of all four genes controlling vernalization requirement. Further analyses with inbred-backcross lines derived from the crosses Target)< Isuzu and Target XBronowski confirmed earlier analyses by identifying lines carrying specific major genes for a vernalization requirement, as well as major genes donated by Bronowski and Isuzu which caused flowering to occur earlier than the recurrent parent Target (Fig. 2 ).
211
APPLICATION O F THE IDEOTYPE C O N C E P T OILSEED BRASSICAS
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Such genes are apparently not expressed in the parents in the presence of genes for a vernalization requirement, but obviously have a marked effect in a different genetic background. These studies highlight the importance of appropriate genetic analyses in detecting useful major genes which could be easily manipulated in breeding B. napus cultivars closely adapted to a particular environment. Canopy structure
Early post-anthesis development of a B. napus crop is characterized by the rapid formation of a dense canopy of bright yellow flowers which reflects a significant portion of the incident radiation. At a later stage, the dense canopy of pods will tend to shade earlier-formed pods, with the result that seed development in the latter is adversely affected and only a small number of seeds are produced in these pods. Although the production of a large number of pods is generally favourable to high seed yield, pod production can often be excessive and seed yield is limited by the production of few seeds in heavily shaded lower pods (Mendham et al., 1981 ). The importance to seed yield in B. napus of the production of adequate numbers of seeds per pod can be seen from the results of experiments (Table 2) conducted in Tasmania (Mendham et al., 1984). A much greater seed survival per pod was the main factor contributing to the marked yield advantage of the two Victorian lines which had Japanese cultivars among their parents. Seed survival in B. napus could be significantly improved by increasing light penetration of the canopy. One way this might be achieved is through introgression of two major genes for the apetalous character into a commercial cultivar (Buzza, 1983 ). These genes inhibit petal development, and would therefore favour the penetration of the canopy by a greater proportion of the incident radiation. Comparisonsbetween normal and apetalous lines (Srinivasa Rao and Mendham, 1985 ) showed light penetration to the base of the inflorescence in the latter at peak flowering to be 30% greater than in the normal line (Fig. 3 ). The same study also showed that the apetalous line had TABLE 2 B. napus cultivar means for yield and yield components in an autumn-sown experiment in Tasmania
(adapted from Medham et al., 1984) Cultivar/ breeding line
Midas Marnoo RT2 Wesreo S.E.
Pods m - 2 ( × 103)
7.42 7.16 6.50 5.65 1.13
Seeds (n p o d - t )
Mean weight (rag)
Yield (t ha -~ )
3.2 10.4 11.3 6.5 0.9
4.73 3.07 3.73 4.30 0.23
0.72 2.08 2.16 0.72 0.25
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
213
120
Apetalous
lOO 8o 60 ~
CE
40
20
• 0
•
I 20
•
•
i 40
•
•
i
•
60
,
i 80
•
•
i
•
100
•
i 120
Crop heightabove ground(cm.) Fig. 3. Difference between a standard cultivar and an apetalous line in light penetration of the canopy (Srinivasa Rao and Mendham, 1985 ). TABLE3
Yield and yield component means for B.napuscv. Marnoo and an apetalous line (Srinivasa Rao and Medham, 1985) Line/cultivar
Plant part
Pods m - 2 Prod.
Seeds Unprod.
( n p o d - J)
Yield ( g i n -2)
Apetalous
Mainstem Branches Total
1391 3633 5024
250 1526 1776
22.3 23.5 23.1
118 355 473
Marnoo
Mainstem Branches Total
2420 5076 7496
150 812 961
19.0 17.1 17.6
150 288 438
a slightly greater yield primarily because of the greater number of seeds produced by each pod (Table 3 ). An increase in seed number per pod or seed weight per pod might also be achieved through selection for longer pods, which could provide a better environment for a higher proportion of ovules surviving as mature seeds. Considerable variation in pod length has been detected in the cultivar China A, and long and short pod lines selected from this cultivar have been compared over a range of managerial treatments (Chay and Thuding, 1989a). Data from a June sowing in an experiment in which long- and short-pod lines were compared over a range of sowing dates and seeding rates are given in Table 4. The difference in pod length between lines remained large over a range of plant densities, with the long-pod line consistently producing at least five more seeds per pod than the short-pod line. This greater seed number per pod of the long pod line was associated with a higher seed yield only at plant densities lower
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N. THURLING
TABLE4 Means ~rseedyieldanditscomponents,~rlong-andsho~-podlinesofB. nap~ ~ownatdifferent plantdensities(Chayand Thuding, 1989a) Plant density ( n m -2) 25 50 100
Line
LP SP LP SP LP SP
SE
Pod length (mm)
Pods per plant
87 53 89 53 87 54 1.2
211 250 153 173 69 121 12.3
Seeds ( n pod- l )
Wt ( × 103 ) (g)
Yield (kg m -2)
18.4 12.8 17.3 12.1 17.1 12.5 0.43
4.55 4.95 4.97 5.09 5.17 5.07 0.11
0.44 0.39 0.66 0.54 0.61 0.76 0.05
TABLE 5 Variation in seed yield and yield component means between and within groups of full-sib families separated on the basis of pod length Pod-length group (mm) > 64 (15) a 65-74 (33) 75-84 (49) > 84 (15)
Pods (n plant- ~)
Mean Range Mean Range Mean Range Mean Range
127 47-162 125 58-164 123 80-176 124 92-149
Seeds ( n p o d -t )
W t ( × 103) (g)
Wt (mg pod-1 )
Yield(g)
18.5 15.0-25.6 24.3 15.7-30.0 23.1 15.0-36.4 27.7 21.1-46.2
4.28 3.61-5.64 4.41 3.53-5.69 4.61 3.50-5.73 5.05 3.48-5.74
7.3 3.8-10.6 11.0 7.2-20.7 12.6 6.7-20.7 13.8 10.1-21.9
11.3 8.0-17.6 13.8 4.2-27.9 13.6 6.9-26.7 17.0 9.9-26.2
aThe number of families in each pod length group are in parenthesis (Chay and Thurling, 1989b ).
than that normally occurring in a commercial crop. Over all densities, any yield advantage accruing from an increase in the number of seeds per pod tended to be compensated for by a lower number of pods per plant. The invariance of pod length over environments suggested that this character had a high heritability, which was confirmed in a later genetic study (Chay and Thurling, 1989b) which showed that the long-pod character was determined by two dominant genes acting in a complementary manner. Families derived from controlled crosses among plants from the F2 of a cross between a short-pod and a long-pod line were also grown in a field experiment for measurements of yield and yield components. The negative association between pod number per plant and seed number per pod was apparently bro-
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
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ken through this intercrossing, and it was evident that seed yield could be improved through selection of genotypes producing large numbers of long pods (Table 5 ). Reduced pod-shattering
The tendency for B. napus pods to shatter spontaneously when mature, and particularly when harvested by direct heading, requires farmers to windrow their crops before ripening. This is an additional cost which has deterred Australian farmers from including rapeseed production in their cropping operations. One obvious approach to overcoming this problem would be the introgression of genes from related species such as B. campestris and B. juncea, which are far less prone to pod-shattering. The development of accurate laboratory testing procedures (Kadkol et al., 1984) provides an efficient means of selection for reduced pod-shattering. Genetic studies with populations derived from crosses between B. campestris subspecies (Kadkol et al., 1986a,b) indicated that relatively few genes interacting epistatically determined pod strength. These results are most encouraging from the point of view of incorporating genes for increased pod strength into B. napus through some backcrossing procedure. BREEDINGSTRATEGIESFOR THE YIELDIMPROVEMENTOF B. NAPUS INVOLVINGSELECTIONFOR IDEOTYPECHARACTERS From the previous section it can be seen that a number of simply inherited characters could be useful as selection criteria in improving the seed yield of B. napus. Current Australian B. napus cultivars should have adequate field resistance to the disease blackleg (Leptosphaeria maculans), and must have Canola quality characteristics. The latter requires the erucic acid content of the oil to be less than 0.5% and the concentration of four specific glucosinolates to be less than 30/zmol g- 1 oil and moisture free meal. Thus, a blacklegresistant commercial cultivar producing Canola-quality seed should be pivotal to any breeding program. Selection amongst early-flowering inbred-backcross lines derived from the Wesbrook X RU2 cross (see Table 1 ) for blackleg resistance and seed quality would provide one of the parents required for the yield-improvement program. The inbred-backcross line derived from the cross of Wesbrook with B. campestris population Chinoli C42 (see Table 1 ) was found to produce strong shattering-tolerant pods, and would therefore be a suitable donor of tolerance to pod-shattering. Uniform apetalous and longpod inbred lines are also readily available at this time. A genetically variable population in which selection can be commenced will be generated from the following crosses:
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Early-flowering (Wesbrook× RU2 ) selection × Shattering-tolerant (Wesbrook × Chinoli C42 ) selection-. F 1 (A) selection Apetalous line × Long-pod line--, F 1 (B) F 1 ( A ) X F 1 ( B ) --. Genetically v a r i a b l e F 2 population There are four possible approaches to selecting higher-yielding lines from this breeding population. ( 1 ) Pedigree selection: not ideal because of deficiencies indicated in Section B. (2) Backcrossing: This would be an appropriate technique for the introgression of the relatively few genes determining apetaly, long pods and podshattering tolerance into an early-flowering selection from the cross Wesbrook × RU2 (recurrent parent ). (3) Sin#e-seed descent: Plants combining blackleg resistance, Canola seed quality, early flowering, apetaly, long pods, and shattering tolerance selected EARLY (WESBROOK x RU2) SELECTIONX SHATTERING TOLERANT(WESBROOKX CHINOLI C42) SELECTION JUNE (YEiR
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Fig. 4. Yield improvement in Brassica napus based on selection for ideotype characters among dihaploid lines.
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to initiate single-seed procedure which could generate F 6 lines for extensive yield testing within 12-18 months. This method has some appeal because of the short time in which a superior cultivar could be identified. (4) Doubled haploid method: Since relatively high numbers of haploid plants can be produced quickly using isolated microspore-culture techniques with B. napus (Swanson et al., 1987 ), this method is preferred as genetically uniform dihaploid lines can be generated directly from selected F2 plants in a single step (Fig. 4 ). Efficient selection for yield among dihaploid lines having the required ideotype characteristics could be conducted within two years of making the original crosses, and it would also be possible to undertake an efficient recurrent selection for higher yield using selected dihaploid lines (Griffing, 1975). REFERENCES Adams, M.W., 1967. Basis of yield compensation in crop plants with special reference to field bean, Phaseolus vulgaris. Crop Sci., 7: 505-510. Baker, R.J., 1978. Issues in diallel analysis. Crop Sci. 18, 533-536. Buzza, G.C., 1983. The inheritance of an apetalous character in Canola (Brassica napus). Crucif. Newsl., 8:11-12. Chay, P.M. and Thurling, N., 1989a. Variation in pod length in spring rapeseed (Brassica napus L. ) and its effect on seed yield and yield components. J. Agric. Sci (Camb.), 113:139-147. Chay, P.M. and Thuding, N., 1989b. Identification of genes controlling pod length in spring rapeseed, Brassica napus L., and their utilization for yield improvement. Plant Breed., 103: 54-62. Clawson, K.L., Specht, J.E. and Blad, B.L., 1986. Growth analysis of soybean isolines differing in pubescence density. Agron. J., 78:164-172. Donald, C.M., 1968. The breeding of crop ideotypes. Euphytica, 17: 385-403. Donald, C.M. and Hamblin, J., 1976. The biological yield and harvest index of cereals as agronomic and plant breeding criteria. Adv. Agron., 36: 97-143. Driscoll, G.H. and Abel, M.F., 1976. A correct logarithmic transformation for standardizing multiplicative trait variables. Crop Sci., 16: 302-303. Fischer, R.A., 1977. The physiology of yield improvement - past and future. In: R.W. Downes (Editor), Proc. 3rd Int. Congress, SABRAO, 1977, Canberra. CSIRO, Canberra 3(a), pp. 1-13. Frey, K.J., 1971. Improving crop yields through plant breeding. In" K.D. Eastin and R.D. Munson (Editors), Moving Off The Yield Plateau. ASA, Madison, Wisc., pp. 15-58. Goulden, C.H., 1939. Problems in plant selection. In: R.H. Burnett (Editor), Proc. 7th Int. Genetics Congress, Edinburgh. Cambridge University Press, Cambridge, pp. 132-133. Grafius, J.E. and Thomas, R.L., 1971. The case for indirect genetic control of sequential traits and the strategy of deployment of environmental resources by the plant. Heredity, 26: 422442. Griffing, B., 1956. Concept of general and specific combining ability in relation to diallel crossing systems. Aust. J. Biol. Sci., 9: 465-493. Griffing, B., 1975. Efficiency changes due to use of doubled-haploids in recurrent selection methods. Theor. Appl. Genet., 46: 367-386. H amblin, J.C., 1975. Effect of environment, seed size and competitive ability on yield and survival ofPhaseolus vulgaris L. genotypes in mixtures. Euphytica, 24: 435-445.
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Harrington, J.B., 1940. Yielding capacity of wheat crosses as indicated by bulk hybrid tests. Can. J. Res., C., 18: 578-584. Hayman, B.I., 1954. The theory and analysis of diallel crosses. Genetics, 39: 798-809. Hedley, G.L. and Ambrose, M.J., 1981. Designing 'leafless' plants for improving yields of the dried pea crop. Adv. Agron., 34: 225-277. Holden, J.H.W. and Williams, J.T. (Editors), 1984. Crop Genetic Resources: Conservation and Evaluation. Allen and Unwin, London. Jensen, N.F., 1988. Plant Breeding Methodology. Wiley, New York. Jinks, J.L., 1954. The analysis of continuous variation in a diallel cross of Nicotiana rustica varieties. Genetics, 39: 767-788. Jinks, J.L. and Pooni, H.S., 1976. Predicting the properties of recombinant inbred lines derived by single seed descent. Heredity, 36: 253-266. Kadkol, G.P., Macmillan, R.A., Burrow, R.P. and Halloran, G.M., 1984. Evaluation of Brassica genotypes for resistance to shatter. I. Development of a laboratory test. Euphytica, 33: 6373. Kadkol, G.P., Halloran, G.M. and Macmillan, R.H., 1986a. Inheritance of siliqua strength in Brassica campestris L. I. Studies of F2 and backcross populations. Can. J. Genet. Cytol., 28: 365-373. Kadkol, G.P., Halioran, G.M. and Macmillan, R.H., 1986b. Inheritance of siliqua strength in Brassica campestris L. II. Quantitative genetic analysis. Can. J. Genet. Cytol., 28: 563-567. Kirk, J.T.O. and Oram, R.N., 1978. Mustards as possible oil and protein crops for Australia. J. Aust. Inst. Agric. Sci., 44: 143-156. Lupton, F.G.H., 1961. Studies in the breeding of self-pollinating cereals. 3. Further studies in cross prediction. Euphytica, 10: 209-224. Mendham, N.J., Shipway, P.A. and Scott, R.K., 1981. The effects of delayed sowing and weather on growth, development and yield of winter oil-seed rape (Brassica napus). J. Agric. Sci., (Camb.), 96: 389-416. Mendham, N.J., Russell, J. and Buzza, G.C., 1984. The contribution of seed survival to yield in new Australian cultivars of oil-seed rape (Brassica napus). J. Agric. Sci., (Camb.), 103: 303-316. Meredith, W.R. and Wells, R., 1986. Normal vs. Okra leaf yield interactions in cotton. I. Performance of near-isogenic lines from bulk populations. Crop Sci., 26:219-222. Park, S.J., Walsh, E.J., Reinbergs, E., Song, L.S.P. and Kasha, K., 1976. Field performance of doubled haploid barley lines in comparison with lines developed by the pedigree and single seed descent methods. Can. J. Plant Sci., 56: 467-474. Ratinam, M. and Thurling, N., 1989. Early generation selection for grain yield in narrow-leaf lupin (Lupinus angustifolius L.) I. Studies with a simulated F2 population. Plant Breed., 102: 237-247. Reinbergs, E., Park, S.J. and Song, L.S.P., 1976. Early identification of superior barley crosses by the doubled haploid technique. Z. Pflanzenzucht., 76:215-224. Salmon, D.F. and Latter, E.N., 1978. Visual selection as a method for improving yield of Triticale. Crop Sci., 18: 427-430. Srinivasa Rao, M.S. and Mendham, N.J., 1985. The influence of the apetalous flower character on radiation distribution in the crop canopy and yield components of rapeseed. In: N. Thurling (Editor), Proc. 5th Australian Rapeseed Agronomists & Breeders Research Workshop, 1985, Perth. Univ. of Western Australia, Perth, pp. 108-112. Swanson, E.B., Coumans, M.B., San, C.W., Barsby, T.L. and Beversdorf, W.D., 1987. Efficient isolation of microspores and the production of microspore-derived embryos from Brassica napus. Plant Cell Rep., 6: 94-97. Syme, J.R., 1968. Ear emergence of Australian, Mexican and European wheats in relation to time of sowing and their response to vernalization and daylength. Aust. J. Exp. Agric. Anim. Husb., 8: 578-581.
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Thomas, R.L., Grafius, J.E. and Hahn, S.K., 1971. Transformation of sequential quantitive characters. Heredity, 26:189-193. Thurling, N. and Ratinam, M., 1989. Early generation selection for grain yield in narrow-leaf lupin. II. Variation in early segregating generations of a selected cross. Plant Breed., 102: 286-295. Thurling, N. and Vijendra Das, L.D., 1977. Variation in pre-anthesis development of spring rape (Brassica napus). Aust. J. Agric. Res., 28: 597-607. Thurling, N. and Vijendra Das, L.D., 1979. Genetic control of the pre-anthesis development of spring rape (Brassica napus). II. Identification of individual genes controlling developmental pattern. Aust. J. Agric. Res., 30:261-271. Townley-Smith, T.F. and Hurd, E.A., 1973. Use of moving means in wheat yiueld trials. Can. J. Plant Sci., 53: 447-450. Wehner, T.C. and Gritton, E.T., 1981. Horticultural evaluation of eight foliage types of peas near-isogenic for the genes af tl and st. J. Am. Soc. Hortic. Sci., 106: 272-278. Wells, R., Meredith, W.R. and Williford, J.R., 1986. Canopy photosynthesis and its relationship to plant productivity in near-isogenic cotton lines differing in leaf morphology. Plant Physiol., 82: 635-640. Whan, B.R., Rathjen, A.J. and Knight, R., 1981. The relation between wheat lines derived from the F2, F3, F4 and F5 generations for grain yield and harvest index. Euphytica, 30:419-430.
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Elsevier Science Publishers B.V., Amsterdam
Application of the ideotype concept in breeding for higher yield in the oilseed brassicas
N.
Thurling
Crop and Pasture Sciences, School of Agriculture, The University of Western Australia, Nedlands, W.A. 6009, Australia
ABSTRACT Thurling, N., 1991. Application of the ideotype concept in breeding for higher yield in the oilseed brassicas. Field Crops Res., 26:201-219. Approaches to yield-improvement breeding methodology which utilize information from biometrical genetic studies and from plant physiological studies aimed at developing ideotype models are reviewed here, with special emphasis on yield improvement in the oilseed brassicas. Major difficulties involved in yield-improvement programmes are the accurate selection of parents from which to generate superior breeding populations, and the selection of high-yielding genotypes from early-segregating generations. In both cases, the complex polygenic inheritance of yield and the magnitude of genotype × environment interactions affecting yield greatly reduce the chances of isolating superior genotypes. A wide array of biometrical procedures have been proposed in developing yield-improvement programmes. Some have been utilized successfully. However, their use in analyzing a character such as yield has been generally limited because of the need to conduct analyses with populations sown at much lower densities than commercial crops. Development of the ideotype concept has focussed the attention of plant physiologists on identification of simple morphological characters which have some influence on physiological processes determining the yield of the economic organs. Characters such as leaf inclination and leaf shape, for example, are often simply inherited and can greatly influence crop canopy structure and radiation interception. Such characters could be rapidly modified by selection to increase crop photosynthesis and yield. Ideotype definitions integrate information on these relationships and provide plant breeders with a clear blueprint of the characteristics of a high-yielding cultivar in a specified environment. Research with oilseed Brassica species in Western Australia and Tasmania has identified a number of highly heritable characters which provide a basic framework for the definition of a yield ideotype for spring rape (B. napus). These include an optimal time of flowering, apetalous flowers and long upright pods to improve light penetration of the dense pod canopy, and pod shattering resistance to allow for direct harvesting of the crop. A major advantage in breeding for this ideotype is that genes for certain characters can be easily introgressed from the related species B. campestris and B. jut,cea. It is also possible, using microspore cultures, to generate large numbers ofB. napus pure tioes directly from plants selected in F2 populations on the basis of ideotype characteristics. This wouM allow for large-scale yield testing of selections within two to three years of making the original crosses.
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INTRODUCTION
Yield improvement has always been a major objective of modern plant breeding. In Australia, and other countries with high-input crop production systems, yield improvement must be maintained to ensure that returns to farmers will be greater than the costs of production. However, maintenance of a steady rate of yield improvement could become more difficult with a depletion of genetic variability to breeding programs (Frey, 1971 ). Recognition of this decline in genetic diversity within domesticated plant species has prompted a major world-wide effort to conserve the genetic resources of these species surviving as land races and/or wild relatives in centres of origin (Holden and Williams, 1984). Given this enhancement of the gene pools of domesticated species, there would still be a need to further improve the efficiency with which the greater genetic variability could be utilized by selection to increase yield. Approaches to improving the methodology of breeding for higher yield have been the subject of considerable research (Jensen, 1988). For some time it was thought that biometrical genetics would offer an effective means of yieldimprovement breeding strategies by providing a better understanding of its complex polygenic inheritance. Unfortunately, early hopes of a major contribution of biometrical genetics to the improvement of yield-breeding methodology in self-pollinating species have not been realized. More recently, with a rapid expansion in our understanding of physiological determinants of yield, the ideotype concept has evolved to provide breeders with specific models upon which to base their yield-improvement strategies (Donald, 1968). In this case, the desired high-yield phenotype may be defined in terms of several relatively simple plant characters. Breeders should then have a greater chance of significantly improving yield by selecting more efficiently for higher heritability characters they intend combining in a superior cultivar. The main purpose of this paper is to show how the definition of a yield ideotype might be best utilized to improve the efficiency of breeding for higher oilseed Brassica yields. Some emphasis will be given to the application of information from biometrical genetic analyses to improving yield-breeding methodology. The problems encountered with the effective application of such analyses will serve to highlight the greater benefits expected from appropriate manipulations of more simply inherited yield determinants. BREEDING FOR HIGHER YIELD IN SELF-POLLINATING PLANT SPECIES
Bulk selection and pedigree selection have been the methods most commonly used in breeding for higher yield in self-pollinating plant species with populations derived from hybridization between selected parents. Although it is time-consuming, the pedigree method is normally preferred because se-
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lection can be commenced as early as the F 2 generation to isolate superior plants on the basis of progeny performance. This type of selection is repeated in the succeeding generations to isolate relatively few superior and genetically uniform lines for advanced testing over a range of environments. Selection for yield itself is normally delayed until the F4 generation, when sufficient seeds can be harvested from remnant plants of selected F3 lines to establish large replicated plots in the following generation. The first step in implementing pedigree selection for higher yield is selection of the parents which will be crossed to generate the breeding population. If yield itself is the only criterion on which parent selection is based, simply selecting the two highest-yielding lines from a germplasm collection is unlikely to isolate the best possible cross, i.e. the one generating a population in which transgressive segregation for high yield is greatest. Generally, because of the complex inheritance of yield, identification of superior crosses cannot be based solely on the yields of potential parents. Some evaluation of crossbred populations, such as bulk F2 or F3 populations, has been advocated as being a more reliable basis for cross-prediction (Harrington, 1940). However, the results from different experiments revealed inconsistencies, casting some doubt on the effectiveness of this approach to parent selection. Irrespective of the deficiencies in various methods used in selecting parents, the ability to accurately identify a clearly superior cross would obviously improve the efficiency of pedigree selection by allowing the breeder to concentrate on a much larger elite population. The selection of desirable phenotypes from a crossbred population can first be conducted in the F2 generation which, according to Jensen (1988) is a pivotal generation "when the population numbers are smallest, when selection or rejection has the greatest impact, and when the most information is present." Direct selection for yield in early generations of a pedigree selection scheme has invariably been ineffective when selection was based on either grain weight measurements (Whan et al., 1981 ) or visual criteria (TownleySmith and Hurd, 1973). However, it is interesting to note that experienced breeders were better able to visually identify high-yielding lines of Triticale than inexperienced breeders (Salmon and Larter, 1978 ). The general lack of success in identifying high-yielding genotypes in the F2 or F3 generations is not unexpected in that selections made in these generations are based on a plant population totally different to the commercial crop. Selection among widely-spaced F2 plants, for example, makes no allowance for the highly competitive environment of a crop population. In fact, selection for high yield at low density tends to isolate genotypes poorly adapted to crop densities (Hamblin, 1975). The difficulties of selecting accurately for yield in early generations of a pedigree selection program has prompted a search for highly-heritable yieldrelated characters to serve as alternative selection criteria. Time of flowering,
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for example, is particularly relevant to improving yield in short-season environments such as occur in the West Australian wheatbelt. Studies with lupins (Lupinus angustifolius) in this environment (Ratinam and Thurling, 1989) showed that flowering times of ten cultivars in a low-density mixture were negatively correlated with their yields in large, crop density monoculture plots ( r = - 0 . 8 0 ) . An F4 population derived from the earliest-flowering plants in an F2 of a late × early lupin cross had a 40% greater yield than one derived from a random selection of F2 plants (Thurling and Ratinam, 1989 ). Grainyield components (ears m - 2, spikelet number, grains per ear and grain weight) have also been extensively studied in respect of their value as alternative criteria of selection for yield in early generations. As they are simple to measure and usually have higher heritabilities than yield, these characters seemed to offer considerable scope as selection criteria. However, their use in breeding programs has been limited because strong negative correlations between some components means that selection for one component will be compensated for by a decrease in the mean value of another, and yield will not be affected (Adams, 1967). The difficulty of improving the efficiency of selection for yield, either directly or indirectly, in Fz o r F 3 populations has also prompted the development of rapid generation advance techniques such as single-seed descent (SSD; Goulden, 1939) which involve a rapid turnover of generations (until F5 o r F 6 ) during which time selection is largely or wholly ignored. A major aim of this procedure is to produce uniform lines as quickly as possible and concentrate selection for yield among large plots of these lines. This type of selection would be far more efficient than selecting for yield between plants and single rows in early generations, and could be commenced as soon as two years after producing F1 seeds. Despite the fact that no selection may be practiced during the development of SSD lines, they appear to include as many superior genotypes as are found amongst pedigree lines (Park et al., 1976 ). Thus, with the shorter time required to reach the stage of advanced testing, single-seed descent is now considered an effective alternative to pedigree selection in improving the yield of self-pollinating species. The various difficulties encountered in pedigree selection with procedures such as parent selection and early-generation selection for yield have provided a focus for much biometrical genetic research. In fact, the need to improve methods of breeding for higher yield in all types of domesticated species has stimulated the development of a wide range of analytical techniques for studying the inheritance of polygenic characters in these different plant species. Some basic aspects of the application of biometrical genetic principles to improving pedigree selection procedures are considered in the following section.
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A P P L I C A T I O N S O F B I O M E T R I C A L G E N E T I C S TO P L A N T B R E E D I N G P R O B L E M S
Selection of parents for yield improvement in selfpollinating species Diallel analysis was the first biometrical technique to be used extensively in improving the efficiency of plant-breeding programs. Its main attractions were that relevant information could be obtained without going beyond the F~ generation and information on both genetic relationships among a group of potential parents and their potential as parents of a yield-improvement program could be obtained from the one experiment. The analytical method developed by Griffing ( 1956 ) measures the genetic potential of parental line in terms of combining ability and has the advantage of making no assumptions regarding genetic relationships among parents. By contrast, the other major analysis developed by Jinks ( 1954 ) and Hayman ( 1954 ) is primarily aimed at investigating genetic relationships among the parents, and its accuracy depends on the experimental materials conforming to several different genetic assumptions. Generally, no real advantage in accuracy of parent selection has been gained from the use of the Jinks-Hayman analysis (Lupton, 1961 ), probably because the assumptions are rarely satisfied in actual experiments (Baker, 1978 ). Also, since most diallel experiments with cereal crops are conducted at plant densities much lower than that of the commercial crop, it is not surprising that this technique has been of limited value in predicting cross potential. Genetic information from segregating generations of crosses between selected parental combinations has also been utilized in predicting cross potential through estimation of the number of transgressive lines likely to be obtained through single-seed descent (Jinks and Pooni, 1976). This prediction is based on a ratio of the difference between parental means to the additive genetic variance obtained from measurements of early-generation populations - F2 and first backcrosses. The accuracy of predictions based on this method has been confirmed from measurements of plant heights and flowering times of advanced lines, but these characters are genetically less complex than yield. Ideally, parent selection should be based on comparisons among groups of advanced lines derived from different crosses, particularly where such lines can be generated within a short period of time. The latter requirement is now possible in plant species where relatively large numbers of haploid plants can be obtained directly from F~ hybrids through chromosome elimination techniques or anther culture. Doubling of the chromosome numbers of haploid plants by colchicine treatment will result in each chromosome having an identical partner, and the seed produced will then constitute a genuine pure line (dihaploid line). Lines derived from an F~ hybrid in this way will be
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representative of the genetic variation released at meiosis in the hybrid, and can be generated in a period as short as six months. A major advantage of this technique, apart from the rapid development of experimental materials, is that lines can be tested at crop densities to obtain relevant yield assessments. This is normally not possible with biometrical analyses based on early-generation populations such as F~'s, F2's and backcross populations. Studies with dihaploid lines of barley (Reinbergs et al., 1976) showed that only twenty of these would be required for predicting the relative yield potential of a cross, so many crosses may be evaluated at the same time.
Biometrical analyses of yieM component relationships in self-pollinating plant species As indicated earlier, grain-yield components were considered ideal candidates as alternative criteria of selection for high yield in early generations of pedigree selection programs. Despite their higher heritabilities and simplicity of measurement, they have not proved to be widely successful because of the common tendency for them to be negatively correlated. This so-called yieldcomponent compensation was seen as a manifestation of the sequential development of yield components, supported either by a limited constant input of metabolites or an oscillatory input which is limiting at critical stages of the developmental sequence (Adams, 1967). Such sequential development was thought to compromise information on the genetic control of yield components coming from conventional biometrical analyses. Accordingly, it was suggested that raw yield-component data should be transformed in such a way as to minimize the developmental influence and thereby enable the direct genetic control of components at the end of sequence to be more accurately defined (Thomas et al., 197 l; Driscoll and Abel, 1976). Following transformation of data on yield components from an experiment with oats (Grafius and Thomas, 1971 ), it was found that genotypes producing similar numbers of heads per unit area would be either high- or low-yielding, depending on whether they produced high or low numbers of seeds per head, respectively. Once these components have been determined, seed size will be largely determined by the amount of metabolites remaining to support this final stage of the developmental sequence. These results suggested that the best strategy for yield improvement would be to select primarily for a larger number of seeds per head. MORPHO-PHYSIOLOGICAL DETERMINANTS OF YIELD
The previous sections have considered some avenues for using biometrical genetics to improve the efficiency of breeding for higher yield in self-pollinating plant species. Overall, this approach has not been particularly successful,
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largely because of the complex inheritance of yield and the necessity to use low-density populations in obtaining estimates of genetic parameters. A more productive approach to improving the efficiency of yield improvement is likely to come from manipulation of relatively simple morphological characters found to have a significant influence on physiological processes determining seed yield. Enunciation of the ideotype concept (Donald, 1968 ) and pleas for greater collaboration between plant breeders and physiologists (Fischer, 1977 ) has encouraged a more rational approach to yield improvement based on combining specific physiological yield determinants in the superior cultivar. At the simplest level, this could involve crosses between a cultivar with a high biological yield and one with a high harvest index, to combine these characters and increase yield (Donald and Hamblin, 1976). Many of the characters which have been included in ideotype definitions for different plant species are, in fact, more simply inherited than biological yield or harvest index. Some may be determined by a single major gene, in which case introgression of the gene into a commercial cultivar through backcrossing affords a simple avenue to yield improvement. This procedure has often been used to develop isogenic or near-isogenic lines which provide the most effective basis for assessing the effects of a simply inherited character on seed yield. In this case, it is best to utilize a number of recipient parents, as the effects of a major gene can vary depending on genetic background. This was apparent in studies with soybean (Clawson et al., 1986 ) in which the incorporation ofa gene for dense leaf pubescence had a significant effect on growth rate in the cultivar Harosoy, but no effect in the cultivar Clark. More recently, leaf morphological variants have been studied more extensively with particular reference to their effects on crop canopy structure and the efficient interception of incident radiation. In peas, for example, three genes have been located which, singly or in combination, have profound effects on leaf morphology (Wehner and Gritton, 1981 ). Gene afreplaces leaflets with tendrils, tl replaces tendrils with leaflets and st reduces the size of the large stipules. Leafless peas ( a f a f s t st) have been the subject of considerable research in which modifications to existing crop management such as substantial increases in seeding rate appear necessary to achieve a leaf-area index of sufficient magnitude to ensure a high yield (Hedley and Ambrose, 1981 ). A series of alleles at a single gene locus are known to greatly modify the leaf shape of cotton and thereby influence light penetration of the canopy. Studies with isogenic lines (Wells et al., 1986) showed that alleles determining the okra and super okra leaf types were associated with reductions in leaf area index, but improved light penetration of the crop canopy. Generally, the okra leaf type is associated with about a 5% yield disadvantage relative to the normal leaf type. However, incorporation of the okra leaf gene into an appro-
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pilate genetic background can lead to a significant yield improvement over a corresponding normal leaf line (Meredith and Wells, 1986 ). DEVELOPMENT OF A YIELD IDEOTYPE FOR THE OILSEED BRASSICAS
Genetic diversity in the oilseed brassicas
Rapeseed is obtained from two closely related species ofBrassica, B. napus and B. campestris. Although grown primarily as a source of mustard, B. juncea is an important oilseed crop of the Indian sub-continent which also has a potential in lower-rainfall areas of southern Australia (Kirk and Oram, 1978 ). All three species are cytogenetically related, as shown in Fig. 1, and genes may be transferred among these and the other Brassica species included in this diagram. Improvement of the more important rapeseed species B. napus can therefore utilize an enormous reservoir of genetic diversity residing in its own primary gene pool and those of the related species. Two of the diploid Brassica species - B. campestris and B. oleracea - are also highly diverse and each comprises a range of distinct sub-species which have evolved in response to human domestication. Overall, the genetic variation within each of the six domesticated and semi-domesticated Brassica species may be utilized in the improvement of any of the amphidiploid species. However, the gene pools of B. campestris and B. juncea can be accessed most easily for the improvement ofB. napus. Phenological development
Time of flowering is a major criterion of selection for higher yield in grain crops grown throughout southern Australia. Although the length of the growing-season varies widely throughout this region, it is usually necessary to develop cultivars which will flower at a time allowing for the completion of seed development before the onset of drought stress at the end of the growingseason, as well as the accumulation of sufficient biomass by the crop before flowering to support this seed development. An optimal time of flowering has been identified for wheat crops grown in southeastern Australia such that higher-yieldingcrops are most likely to be obtained with cultivars which flower after early spring frosts, but are still early enough for seed development to be completed before the onset of drought stress (Syme, 1968 ). As rapeseed yields are not as susceptible to frost effects as cereals because of their indeterminate developmental pattern, an earlier time of flowering could be considered as optimal from the point of view of allowing for a longer period of reproductive development. Data collected from an experiment with oilseed brassicas in a relatively low-rainfall environment in Western Australia (own unpublished data, 1989 ).
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
209
B.nigra 2n = 16 8 BB
B.carinata 2n = 34 1 8 BB + 9 CC
I
J
B.oleracea 2n = 18 9 CC
~ ~/ ~
B.napus 2n = 38 10 AA + g CC
I
ssp.gemmifera Brussels sprout)
Issp.oleifer a
ssp.acephala kale)
~swede)
i(ra pe )
issp.rapifer a
~sp.gongyloides kohl rabi) ~sp.capitata cabbage) ;sp.italica broccoli) ~sp.botrytis cauliflower)
B.juncea 2n = 36 10 AA + 8 BB
B.campestris 2n = 20 10 AA
I OILSEEDS ~sp.oleifera iturnip rape)
;sp.dichotoma itoria) ~spArilocularis sarson) .EAF VEGETABLES ~sp.pekinensis Chinese cabbage
~sp.chinensis Chinese mustar¢ :lOOT VEGETABLE ;ap.rapifera turnip)
Fig. 1. The Brassica gene pools.
reveals the importance of early flowering as a determinant of high yield (Table 1 ). Genes for early flowering had been introgressed into a leading B. napus cultivar (Wesbrook) from an early-flowering B. napus line (RU2) and an early-flowering B. campestris population (Chinoli C42). Introgression was achieved through three successive generations of backcrossing to Wesbrook and three subsequent generations of selfing, without any selection. The earliest-flowering B3 $3 lines from each cross were selected for field testing. Incorporation of the early-flowering trait into the Wesbrook background had a markedly beneficial effect on yield, although only the Wesbrook X RU2 line had a significantly higher yield than Wesbrook. The higher yield of this line was primarily associated with a much greater biological yield, and particularly with a greater increase in dry-matter between anthesis and maturity. Earlier flowering provided for a longer period of development before matu-
210
N. THURLING
TABLE 1 Agronomic performance of oilseed Brassica populations and their hybrid derivatives at a low-rainfall site in the Western Australian wheatbelt Time to 50% flowering (days)
Seed yield (g m - 2)
Harvest index
B. napus
84
225
(RU2) Best inbred backcross line
80
Dry-weight ( g m -2 ) Total (maturity)
Increment (anthesis to maturity )
0.26
858
646
251
0.26
965
721
94
151
0.23
656
404
78
173
0.27
650
390
75
157
0.29
560
368
4
41
0.04
184
197
(a3 S3) B. napus
(Wesbrook) Best inbred backcross line (B3 $3) B. campestris
(Chinoli C42 ) LSD (0.05)
rity, but it appeared that RU2 also contributed genes for a higher growth rate during the reproductive period. Variation in flowering time among spring cultivars of B. napus in the field is essentially continuous, but distinct differences in response to photoperiod and vernalization in controlled environments are detected among the same cultivars (Thurling and Vijendra Das, 1977). Differences in vernalization response were particularly marked, and it was possible to distinguish cultivars with little or no vernalization requirement (e.g., Target) from those which had a relatively high vernalization requirement (e.g., Isuzu and Bronowski ). Cultivars having a relatively high vernalization requirement also differed in their responses to increasing duration of the seedling vernalization treatment. Genetic analyses of populations derived from crosses among the cultivars Target, Isuzu and Bronowski showed that duplicate recessive genes determined the vernalization requirement of Bronowski, but two independent recessive genes (one having a markedly greater effect than the other) determined the vernalization requirement of Isuzu (Thurling and Vijendra Das, 1979). Target carried the dominant alleles of all four genes controlling vernalization requirement. Further analyses with inbred-backcross lines derived from the crosses Target)< Isuzu and Target XBronowski confirmed earlier analyses by identifying lines carrying specific major genes for a vernalization requirement, as well as major genes donated by Bronowski and Isuzu which caused flowering to occur earlier than the recurrent parent Target (Fig. 2 ).
211
APPLICATION O F THE IDEOTYPE C O N C E P T OILSEED BRASSICAS
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Such genes are apparently not expressed in the parents in the presence of genes for a vernalization requirement, but obviously have a marked effect in a different genetic background. These studies highlight the importance of appropriate genetic analyses in detecting useful major genes which could be easily manipulated in breeding B. napus cultivars closely adapted to a particular environment. Canopy structure
Early post-anthesis development of a B. napus crop is characterized by the rapid formation of a dense canopy of bright yellow flowers which reflects a significant portion of the incident radiation. At a later stage, the dense canopy of pods will tend to shade earlier-formed pods, with the result that seed development in the latter is adversely affected and only a small number of seeds are produced in these pods. Although the production of a large number of pods is generally favourable to high seed yield, pod production can often be excessive and seed yield is limited by the production of few seeds in heavily shaded lower pods (Mendham et al., 1981 ). The importance to seed yield in B. napus of the production of adequate numbers of seeds per pod can be seen from the results of experiments (Table 2) conducted in Tasmania (Mendham et al., 1984). A much greater seed survival per pod was the main factor contributing to the marked yield advantage of the two Victorian lines which had Japanese cultivars among their parents. Seed survival in B. napus could be significantly improved by increasing light penetration of the canopy. One way this might be achieved is through introgression of two major genes for the apetalous character into a commercial cultivar (Buzza, 1983 ). These genes inhibit petal development, and would therefore favour the penetration of the canopy by a greater proportion of the incident radiation. Comparisonsbetween normal and apetalous lines (Srinivasa Rao and Mendham, 1985 ) showed light penetration to the base of the inflorescence in the latter at peak flowering to be 30% greater than in the normal line (Fig. 3 ). The same study also showed that the apetalous line had TABLE 2 B. napus cultivar means for yield and yield components in an autumn-sown experiment in Tasmania
(adapted from Medham et al., 1984) Cultivar/ breeding line
Midas Marnoo RT2 Wesreo S.E.
Pods m - 2 ( × 103)
7.42 7.16 6.50 5.65 1.13
Seeds (n p o d - t )
Mean weight (rag)
Yield (t ha -~ )
3.2 10.4 11.3 6.5 0.9
4.73 3.07 3.73 4.30 0.23
0.72 2.08 2.16 0.72 0.25
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
213
120
Apetalous
lOO 8o 60 ~
CE
40
20
• 0
•
I 20
•
•
i 40
•
•
i
•
60
,
i 80
•
•
i
•
100
•
i 120
Crop heightabove ground(cm.) Fig. 3. Difference between a standard cultivar and an apetalous line in light penetration of the canopy (Srinivasa Rao and Mendham, 1985 ). TABLE3
Yield and yield component means for B.napuscv. Marnoo and an apetalous line (Srinivasa Rao and Medham, 1985) Line/cultivar
Plant part
Pods m - 2 Prod.
Seeds Unprod.
( n p o d - J)
Yield ( g i n -2)
Apetalous
Mainstem Branches Total
1391 3633 5024
250 1526 1776
22.3 23.5 23.1
118 355 473
Marnoo
Mainstem Branches Total
2420 5076 7496
150 812 961
19.0 17.1 17.6
150 288 438
a slightly greater yield primarily because of the greater number of seeds produced by each pod (Table 3 ). An increase in seed number per pod or seed weight per pod might also be achieved through selection for longer pods, which could provide a better environment for a higher proportion of ovules surviving as mature seeds. Considerable variation in pod length has been detected in the cultivar China A, and long and short pod lines selected from this cultivar have been compared over a range of managerial treatments (Chay and Thuding, 1989a). Data from a June sowing in an experiment in which long- and short-pod lines were compared over a range of sowing dates and seeding rates are given in Table 4. The difference in pod length between lines remained large over a range of plant densities, with the long-pod line consistently producing at least five more seeds per pod than the short-pod line. This greater seed number per pod of the long pod line was associated with a higher seed yield only at plant densities lower
214
N. THURLING
TABLE4 Means ~rseedyieldanditscomponents,~rlong-andsho~-podlinesofB. nap~ ~ownatdifferent plantdensities(Chayand Thuding, 1989a) Plant density ( n m -2) 25 50 100
Line
LP SP LP SP LP SP
SE
Pod length (mm)
Pods per plant
87 53 89 53 87 54 1.2
211 250 153 173 69 121 12.3
Seeds ( n pod- l )
Wt ( × 103 ) (g)
Yield (kg m -2)
18.4 12.8 17.3 12.1 17.1 12.5 0.43
4.55 4.95 4.97 5.09 5.17 5.07 0.11
0.44 0.39 0.66 0.54 0.61 0.76 0.05
TABLE 5 Variation in seed yield and yield component means between and within groups of full-sib families separated on the basis of pod length Pod-length group (mm) > 64 (15) a 65-74 (33) 75-84 (49) > 84 (15)
Pods (n plant- ~)
Mean Range Mean Range Mean Range Mean Range
127 47-162 125 58-164 123 80-176 124 92-149
Seeds ( n p o d -t )
W t ( × 103) (g)
Wt (mg pod-1 )
Yield(g)
18.5 15.0-25.6 24.3 15.7-30.0 23.1 15.0-36.4 27.7 21.1-46.2
4.28 3.61-5.64 4.41 3.53-5.69 4.61 3.50-5.73 5.05 3.48-5.74
7.3 3.8-10.6 11.0 7.2-20.7 12.6 6.7-20.7 13.8 10.1-21.9
11.3 8.0-17.6 13.8 4.2-27.9 13.6 6.9-26.7 17.0 9.9-26.2
aThe number of families in each pod length group are in parenthesis (Chay and Thurling, 1989b ).
than that normally occurring in a commercial crop. Over all densities, any yield advantage accruing from an increase in the number of seeds per pod tended to be compensated for by a lower number of pods per plant. The invariance of pod length over environments suggested that this character had a high heritability, which was confirmed in a later genetic study (Chay and Thurling, 1989b) which showed that the long-pod character was determined by two dominant genes acting in a complementary manner. Families derived from controlled crosses among plants from the F2 of a cross between a short-pod and a long-pod line were also grown in a field experiment for measurements of yield and yield components. The negative association between pod number per plant and seed number per pod was apparently bro-
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
215
ken through this intercrossing, and it was evident that seed yield could be improved through selection of genotypes producing large numbers of long pods (Table 5 ). Reduced pod-shattering
The tendency for B. napus pods to shatter spontaneously when mature, and particularly when harvested by direct heading, requires farmers to windrow their crops before ripening. This is an additional cost which has deterred Australian farmers from including rapeseed production in their cropping operations. One obvious approach to overcoming this problem would be the introgression of genes from related species such as B. campestris and B. juncea, which are far less prone to pod-shattering. The development of accurate laboratory testing procedures (Kadkol et al., 1984) provides an efficient means of selection for reduced pod-shattering. Genetic studies with populations derived from crosses between B. campestris subspecies (Kadkol et al., 1986a,b) indicated that relatively few genes interacting epistatically determined pod strength. These results are most encouraging from the point of view of incorporating genes for increased pod strength into B. napus through some backcrossing procedure. BREEDINGSTRATEGIESFOR THE YIELDIMPROVEMENTOF B. NAPUS INVOLVINGSELECTIONFOR IDEOTYPECHARACTERS From the previous section it can be seen that a number of simply inherited characters could be useful as selection criteria in improving the seed yield of B. napus. Current Australian B. napus cultivars should have adequate field resistance to the disease blackleg (Leptosphaeria maculans), and must have Canola quality characteristics. The latter requires the erucic acid content of the oil to be less than 0.5% and the concentration of four specific glucosinolates to be less than 30/zmol g- 1 oil and moisture free meal. Thus, a blacklegresistant commercial cultivar producing Canola-quality seed should be pivotal to any breeding program. Selection amongst early-flowering inbred-backcross lines derived from the Wesbrook X RU2 cross (see Table 1 ) for blackleg resistance and seed quality would provide one of the parents required for the yield-improvement program. The inbred-backcross line derived from the cross of Wesbrook with B. campestris population Chinoli C42 (see Table 1 ) was found to produce strong shattering-tolerant pods, and would therefore be a suitable donor of tolerance to pod-shattering. Uniform apetalous and longpod inbred lines are also readily available at this time. A genetically variable population in which selection can be commenced will be generated from the following crosses:
216
N. THURLING
Early-flowering (Wesbrook× RU2 ) selection × Shattering-tolerant (Wesbrook × Chinoli C42 ) selection-. F 1 (A) selection Apetalous line × Long-pod line--, F 1 (B) F 1 ( A ) X F 1 ( B ) --. Genetically v a r i a b l e F 2 population There are four possible approaches to selecting higher-yielding lines from this breeding population. ( 1 ) Pedigree selection: not ideal because of deficiencies indicated in Section B. (2) Backcrossing: This would be an appropriate technique for the introgression of the relatively few genes determining apetaly, long pods and podshattering tolerance into an early-flowering selection from the cross Wesbrook × RU2 (recurrent parent ). (3) Sin#e-seed descent: Plants combining blackleg resistance, Canola seed quality, early flowering, apetaly, long pods, and shattering tolerance selected EARLY (WESBROOK x RU2) SELECTIONX SHATTERING TOLERANT(WESBROOKX CHINOLI C42) SELECTION JUNE (YEiR
i
APETALOUS LINE x LONG POD LINE
]
SEGREGATING POPULATION
I I REMOVEMAIN STEM INFLORESCENCEFOR MICROSPORECULTURE SELECT EARLYFLOWERING,APETALOUSPLANTS
JUNE (YEiR
YEARS 3 -
DIHAPLOID LINES
I I IDENTIFY BLACKLEGRESISTANTLINES WITH CANOLASEEDQUALITY I I DIHAPLOID REPLICATED
SEED INCREASEAND IDENTIFYLONG POD,SHA'I-rERINGTOLERANTLINES
YIELD TESTS
I
LINES
I
CULTIURE
MICROSPORE HIGHEST LINES YIELDING
YEARS 5-7
I
INTERCROSSING
I
FINAL TESTING
COMMERCIAL CULTIVAR
[
Fig. 4. Yield improvement in Brassica napus based on selection for ideotype characters among dihaploid lines.
APPLICATION OF THE IDEOTYPE CONCEPT OILSEED BRASSICAS
217
to initiate single-seed procedure which could generate F 6 lines for extensive yield testing within 12-18 months. This method has some appeal because of the short time in which a superior cultivar could be identified. (4) Doubled haploid method: Since relatively high numbers of haploid plants can be produced quickly using isolated microspore-culture techniques with B. napus (Swanson et al., 1987 ), this method is preferred as genetically uniform dihaploid lines can be generated directly from selected F2 plants in a single step (Fig. 4 ). Efficient selection for yield among dihaploid lines having the required ideotype characteristics could be conducted within two years of making the original crosses, and it would also be possible to undertake an efficient recurrent selection for higher yield using selected dihaploid lines (Griffing, 1975). REFERENCES Adams, M.W., 1967. Basis of yield compensation in crop plants with special reference to field bean, Phaseolus vulgaris. Crop Sci., 7: 505-510. Baker, R.J., 1978. Issues in diallel analysis. Crop Sci. 18, 533-536. Buzza, G.C., 1983. The inheritance of an apetalous character in Canola (Brassica napus). Crucif. Newsl., 8:11-12. Chay, P.M. and Thurling, N., 1989a. Variation in pod length in spring rapeseed (Brassica napus L. ) and its effect on seed yield and yield components. J. Agric. Sci (Camb.), 113:139-147. Chay, P.M. and Thuding, N., 1989b. Identification of genes controlling pod length in spring rapeseed, Brassica napus L., and their utilization for yield improvement. Plant Breed., 103: 54-62. Clawson, K.L., Specht, J.E. and Blad, B.L., 1986. Growth analysis of soybean isolines differing in pubescence density. Agron. J., 78:164-172. Donald, C.M., 1968. The breeding of crop ideotypes. Euphytica, 17: 385-403. Donald, C.M. and Hamblin, J., 1976. The biological yield and harvest index of cereals as agronomic and plant breeding criteria. Adv. Agron., 36: 97-143. Driscoll, G.H. and Abel, M.F., 1976. A correct logarithmic transformation for standardizing multiplicative trait variables. Crop Sci., 16: 302-303. Fischer, R.A., 1977. The physiology of yield improvement - past and future. In: R.W. Downes (Editor), Proc. 3rd Int. Congress, SABRAO, 1977, Canberra. CSIRO, Canberra 3(a), pp. 1-13. Frey, K.J., 1971. Improving crop yields through plant breeding. In" K.D. Eastin and R.D. Munson (Editors), Moving Off The Yield Plateau. ASA, Madison, Wisc., pp. 15-58. Goulden, C.H., 1939. Problems in plant selection. In: R.H. Burnett (Editor), Proc. 7th Int. Genetics Congress, Edinburgh. Cambridge University Press, Cambridge, pp. 132-133. Grafius, J.E. and Thomas, R.L., 1971. The case for indirect genetic control of sequential traits and the strategy of deployment of environmental resources by the plant. Heredity, 26: 422442. Griffing, B., 1956. Concept of general and specific combining ability in relation to diallel crossing systems. Aust. J. Biol. Sci., 9: 465-493. Griffing, B., 1975. Efficiency changes due to use of doubled-haploids in recurrent selection methods. Theor. Appl. Genet., 46: 367-386. H amblin, J.C., 1975. Effect of environment, seed size and competitive ability on yield and survival ofPhaseolus vulgaris L. genotypes in mixtures. Euphytica, 24: 435-445.
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Harrington, J.B., 1940. Yielding capacity of wheat crosses as indicated by bulk hybrid tests. Can. J. Res., C., 18: 578-584. Hayman, B.I., 1954. The theory and analysis of diallel crosses. Genetics, 39: 798-809. Hedley, G.L. and Ambrose, M.J., 1981. Designing 'leafless' plants for improving yields of the dried pea crop. Adv. Agron., 34: 225-277. Holden, J.H.W. and Williams, J.T. (Editors), 1984. Crop Genetic Resources: Conservation and Evaluation. Allen and Unwin, London. Jensen, N.F., 1988. Plant Breeding Methodology. Wiley, New York. Jinks, J.L., 1954. The analysis of continuous variation in a diallel cross of Nicotiana rustica varieties. Genetics, 39: 767-788. Jinks, J.L. and Pooni, H.S., 1976. Predicting the properties of recombinant inbred lines derived by single seed descent. Heredity, 36: 253-266. Kadkol, G.P., Macmillan, R.A., Burrow, R.P. and Halloran, G.M., 1984. Evaluation of Brassica genotypes for resistance to shatter. I. Development of a laboratory test. Euphytica, 33: 6373. Kadkol, G.P., Halloran, G.M. and Macmillan, R.H., 1986a. Inheritance of siliqua strength in Brassica campestris L. I. Studies of F2 and backcross populations. Can. J. Genet. Cytol., 28: 365-373. Kadkol, G.P., Halioran, G.M. and Macmillan, R.H., 1986b. Inheritance of siliqua strength in Brassica campestris L. II. Quantitative genetic analysis. Can. J. Genet. Cytol., 28: 563-567. Kirk, J.T.O. and Oram, R.N., 1978. Mustards as possible oil and protein crops for Australia. J. Aust. Inst. Agric. Sci., 44: 143-156. Lupton, F.G.H., 1961. Studies in the breeding of self-pollinating cereals. 3. Further studies in cross prediction. Euphytica, 10: 209-224. Mendham, N.J., Shipway, P.A. and Scott, R.K., 1981. The effects of delayed sowing and weather on growth, development and yield of winter oil-seed rape (Brassica napus). J. Agric. Sci., (Camb.), 96: 389-416. Mendham, N.J., Russell, J. and Buzza, G.C., 1984. The contribution of seed survival to yield in new Australian cultivars of oil-seed rape (Brassica napus). J. Agric. Sci., (Camb.), 103: 303-316. Meredith, W.R. and Wells, R., 1986. Normal vs. Okra leaf yield interactions in cotton. I. Performance of near-isogenic lines from bulk populations. Crop Sci., 26:219-222. Park, S.J., Walsh, E.J., Reinbergs, E., Song, L.S.P. and Kasha, K., 1976. Field performance of doubled haploid barley lines in comparison with lines developed by the pedigree and single seed descent methods. Can. J. Plant Sci., 56: 467-474. Ratinam, M. and Thurling, N., 1989. Early generation selection for grain yield in narrow-leaf lupin (Lupinus angustifolius L.) I. Studies with a simulated F2 population. Plant Breed., 102: 237-247. Reinbergs, E., Park, S.J. and Song, L.S.P., 1976. Early identification of superior barley crosses by the doubled haploid technique. Z. Pflanzenzucht., 76:215-224. Salmon, D.F. and Latter, E.N., 1978. Visual selection as a method for improving yield of Triticale. Crop Sci., 18: 427-430. Srinivasa Rao, M.S. and Mendham, N.J., 1985. The influence of the apetalous flower character on radiation distribution in the crop canopy and yield components of rapeseed. In: N. Thurling (Editor), Proc. 5th Australian Rapeseed Agronomists & Breeders Research Workshop, 1985, Perth. Univ. of Western Australia, Perth, pp. 108-112. Swanson, E.B., Coumans, M.B., San, C.W., Barsby, T.L. and Beversdorf, W.D., 1987. Efficient isolation of microspores and the production of microspore-derived embryos from Brassica napus. Plant Cell Rep., 6: 94-97. Syme, J.R., 1968. Ear emergence of Australian, Mexican and European wheats in relation to time of sowing and their response to vernalization and daylength. Aust. J. Exp. Agric. Anim. Husb., 8: 578-581.
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Thomas, R.L., Grafius, J.E. and Hahn, S.K., 1971. Transformation of sequential quantitive characters. Heredity, 26:189-193. Thurling, N. and Ratinam, M., 1989. Early generation selection for grain yield in narrow-leaf lupin. II. Variation in early segregating generations of a selected cross. Plant Breed., 102: 286-295. Thurling, N. and Vijendra Das, L.D., 1977. Variation in pre-anthesis development of spring rape (Brassica napus). Aust. J. Agric. Res., 28: 597-607. Thurling, N. and Vijendra Das, L.D., 1979. Genetic control of the pre-anthesis development of spring rape (Brassica napus). II. Identification of individual genes controlling developmental pattern. Aust. J. Agric. Res., 30:261-271. Townley-Smith, T.F. and Hurd, E.A., 1973. Use of moving means in wheat yiueld trials. Can. J. Plant Sci., 53: 447-450. Wehner, T.C. and Gritton, E.T., 1981. Horticultural evaluation of eight foliage types of peas near-isogenic for the genes af tl and st. J. Am. Soc. Hortic. Sci., 106: 272-278. Wells, R., Meredith, W.R. and Williford, J.R., 1986. Canopy photosynthesis and its relationship to plant productivity in near-isogenic cotton lines differing in leaf morphology. Plant Physiol., 82: 635-640. Whan, B.R., Rathjen, A.J. and Knight, R., 1981. The relation between wheat lines derived from the F2, F3, F4 and F5 generations for grain yield and harvest index. Euphytica, 30:419-430.