Analyses of yield component relationships in the oilseed rape species Bvassica napus and Bvassica campestris revealed substantial component compensation in both species. There were, however, certain differences between these species in the morphological expression of the compensatory mechanism. In B, napus, the decline in yield with successive delays in sowing date was accompanied by a marked reduction in the number of pods per plant, but little change in the seed weight per pod. The yield of B. campestvis was higher in the second sowing than in either the earlier or later sowings; however, there was still a substantial decrease in the number of pods per plant. This decrease in the number of pods per plant was accompanied by an increase in the seed weight per pod which was substantially greater between the first and second sowings than between the second and third. From correlation analyses it was evident that variation in seed yield was related primarily to changes in the number of pods per plant in B. napus and to changes in seed weight per pod in B. canzpestuis.The results of this experiment supported the widely held view that yield component compensation in grain crops is an inevitable consequence of a limited input of metabolites to the developing inflorescence. A reduction in the metabolic input to the inflorescence should, therefore, be accompanied by appropriate adjustments in the yield component system. If it is assumed that the total dry weight of the plant at flowering reflects, to some degree at least, the potential metabolic input of the plant, then a response of this type was particularly apparent in changes in the relative magnitude of the different yield components between the first and second sowings. In B. napus, a sharp reduction in the total dry weight of the plant was accompanied by a decrease in the number of pods per plant without any significant change in the seed weight per pod. Although there was a slight rise in the total dry weight of B. campestvis over the same interval, there was still a substantial reduction in the number of pods per plant which, however, was associated with a proportionately greater increase in the seed weight per pod.Appropriate transformations of the basic data revealed that inter-trait correlations or stresses strongly masked the true influence of source variation on the component characters succeeding number of pod bearing branches in the developmental sequence. The main effect of removal of the stress influence was a substantial inflation in the contribution of the cultivar x environment interaction to the variation in the component characters. Yield component relationships in the rapeseed species were, in fact, characterized by a strong environmental control of the type and degree of stress as well as by significant genetic control only of characters early in the developmental sequence. It is considered that such a situation makes it possible for reliable yield predictions to be based on yield component data and is, therefore, encouraging as regards the devel...

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Experiments were undertaken to determine the inheritance of pod length in a cross with spring rapeseed, Brassica napus, and to assess the value of pod length as a criterion of selection for high seed yieid.Analyses of patterns of variation in F, and backcross populations derived from a eross between a short-pod line TB42 and long-pod line GA553 indicated that much of the variation in pod length could be attributed to two major genes interactmg in a complementary manner. Short-pods were produced when either one or hoth genes were homozygous for the recessive allele. Analyses of F3 progenies of seleeted Fi and inbred-backeross lines derived from the same cross supported the two-gene hypothesis but also indicated that the effects of the major genes on pod length were possibly modified by genes of minor effect.Field testing of families derived from random lntermating between F2 plants of the TB42 X CA553 cross showed that number of pods per plant varied independently of pod length, but seed weight per pod tended to increase with increasing pod length. As a result, families with the longest pods generally had significantly higher yields than those with short pods.It was concluded that yield improvement in B, napus could be achieved through introgression of long-pod genes into cultivars with an appropriate genetie background to ensure that selection for the long-pod character would be accompanied by an increase in seed weight per pod with little or no reduction in number of pods per plant.

Variation in sowing date was found to have a marked effect on the seed yield and different morphological and growth characteristics of spring cultivars of the oilseed rape species Brassica campestris and Brassica napus. The two species differed appreciably, however, in their responses to successive delays in sowing date. In B. napus, there was a significant and continuous decline in seed yield with later sowings. This decline in seed yield was associated primarily with a reduction in the total dry weight of the plant at the final harvest which, in turn, was most closely correlated with the duration of the vegetative phase of growth. The total dry weight of the plant and the seed yield were greatest in the first sowing where flowering was substantially later than in either of the later sowings. Since earlier-flowering cultivars of B. napus would be better adapted to Western Australian conditions, where there is a rapid decline in soil moisture reserves during the spring, significant improvements in seed yield could depend largely on increasing the rate of pre-anthesis growth. In this experiment, however, no significant variation in the relative rate of growth prior to flowering was observed because of the marked negative association between net assimilation rate and leaf area ratio. Further studies of the variation in growth amongst a far wider range of genotypes will, nevertheless, be necessary to properly assess the feasibility of increasing relative growth rates by selection. The seed yield of B. campestris was substantially greater in the second sowing than in either the first or third sowings. Although seed yield was significantly correlated with both total dry weight of the plant at final harvest and the harvest index, the latter appeared to be the most important determinant of seed yield in this species. By contrast with B. napus, most of the dry weight of the plant at the final harvest was accumulated during the post-anthesis phase of growth. Accordingly, the character leaf area duration between anthesis and final harvest was found to be the most significant determinant of the total dry weight of the plant in B. campestris. The leaf weight ratio of the plant at anthesis also contributed significantly to total dry weight at the final harvest; this indicates that the pattern of dry matter distribution in the plant during the vegetative phase could prove a useful criterion for selecting potentially high-yielding strains of this species.

Drought imposed at different times on four spring cultivars of each of two oilseed rape species (Brassica campestris and B. napus) markedly influenced seed yield and its components and other characteristics of the mature plant. In general, drought reduced those plant and yield components associated with plant size, whereas other characters were variable, depending on the species and treatments. Seed yield was lowest in both species when drought was applied either from stem elongation or from the time of anthesis. However, considering grain yields in terms of the amount of water added in each treatment, B. napus cultivars were most sensitive at anthesis, with the exception of Masoweicki which was most sensitive at pod-filling. In B. campestris, Span was most sensitive when drought stress was first imposed at stem elongation, whilst other cultivars were sensitive at later reproductive stages. Principal component analyses reduced the complex interrelationships between all variables excluding yield to two major factors in each species and treatment. In general, the first factor, which explained most of the variation in the parameters measured, was composed of variables associated with plant size, whereas the second was composed of variables which were independent of plant size. The relative importance of these two factors as yield determinants varied substantially with the different species and treatments. In B. napus the first factor was the major determinant of yield ill the control and in the drought treatment imposed from stem elongation. In B. campestris the second factor was a significant determinant of yield in all treatments and the major factor influencing yield in the control and stem elongation treatments. Increased root weight was associated with increased yield in the drought treatments in both species, but this was primarily due to its association with plant size. When the influence of plant size was accounted for, it was observed that a smaller root weight relative to the above-ground plant weight and a greater tap-root weight relative to lateral root weight was associated with higher yield in both species in all drought treatments. As well as these latter characters, seed weight per pod, and harvest index were the most important group of characters contributing to yield in both species in all treatments when variation in plant size was accounted for.