Morphophysiological determinants of yield in rapeseed (Brassica campestris and Brassica napus). II.* Yield components

Abstract

Analyses of yield component relationships in the oilseed rape species Brassica napus and Brassica 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. campestris 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. campestris. 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. campestris 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 development of more efficient procedures of selection for yield components. ____________________ *Part I, Aust. J. Agric. Res., 25: 697 (1974).