Recently developed methods of growth and yield analysis were applied to the results of a field experiment to determine (i) the effect of seed inoculation on the growth and yield of two cultivairs of garden pea (Pisum sativum L.) - 'Dark Skin Perfection' (DSP) and 'Early Frosty' (EF), (ii) the physiological basis for yield variability and (iii) the dynamics of reproductive growth in the pea. Seed inoculation had no noticeable effect on yield. There were, however, significant cultivar differences--DSP was larger, matured later but had lower yields than EF. Growth analysis revealed that these differences were due to the extended vegetative growth phase, higher leaf area ratio and lower harvest index of DSP. Further analysis indicated that EF had a more efficient growth strategy, as the maximum rate of partitioning of dry matter into the reproductive structures coincided with high leaf activity. By contrast maximum sink activity in DSP occurred during leaf senescence. Stem length, average leaf area and inverse leaf weight ratio were the main components of biomass variability. Yield variation, however, was affected indirectly by stem length, average leaf area, reproductive effort and average seed weight, and directly by the number of nodes, pod set and inverse leaf weight ratio. These results imply that the supply of photosynthetic material is important for increased pea yields. Significant negative correlations between vegetative components and average seed weight suggest compensation and competition between these components. Thus, yield improvement may be attainable by (1) enhancing the component which is unaffected by this compensation or (2) reducing the competition by shifting the equilibrium. Two crucial periods when source supply may affect yield were detected during the reproductive phase. The first period (61 days after planting in DSP and 55 days after planting in EF) was more pronounced in DSP. In the second phase (day 75 for DSP and 65 for EF) seed growth became important and seemed to influence leaf activity. This finding suggests that the rate of canopy establishment is as important as the rate of pod filling for improving yields. A dynamic model simulating pod yield in relation to source supply is outlined. / Land and Food Systems, Faculty of / Graduate

Intact or decapitated Pisum sativum seedlings were used to investigate the correlative inhibition of lateral buds by the shoot apex - a developmental process known as apical dominance. Apical dominance is considered to be regulated by the relative ratios of growth regulators, especially auxin and cytokinin, and resource availability in the plant. This study considered the role of cytokinins in the regulation of correlative inhibition, and was closely linked to theoretical models of auxin and nutrient gradients in the whole plant. Firstly, the response of all lateral buds on the plant to decapitation of the shoot apex, and the subsequent growth of these buds, was documented. The pattern of lateral bud branching following decapitation, noted to change with increasing age of the plant, was found to be consistent with the view that apically synthesized auxin, moving basipetally, exerts an inhibitory influence on lateral bud growth. Removal of selected lateral buds on decapitated plants which left various combinations of buds to compete indicated that correlation between lateral buds on the same plant was likely an important factor controlling the patterns of lateral bud branching. Secondly, a quantitative study of the biological responses which result from the application of a cytokinin to a lateral bud were performed. The different abilities of ten cytokinins tested to release lateral buds from dominance paralleled their activity in a soybean callus bioassay, and were likely a result of the intrinsic activities of the cytokinins due to their structure and their subsequent rr.etabolism in the plant following application. A consistent feature of these experiments was the low biological activity of isopentenyladenine in relation to the high activity of zeatin. Further investigation of the role of isopentenyladenine and zeatin in apical dominance control indicated that lateral buds differed in their sensitivity to these two cytokinins. On decapitated plants, lateral buds were more able to utilize applied isopentenyladenine, and high zeatin concentrations were no longer supraoptimal for growth. Finally, the metabolism of [³H] isopentenyladenine in the intact plant or in isolated explants was investigated in an attempt to relate the biosynthesis, transport, interconversion and degradation of cytokinins to the developmental process of apical dominance. Comparison of the uptake and metabolism of [³H] isopentenyladenine by isolated organs of Pisum sativum indicated that the roots, leaves and stems, but not isolated lateral buds had the capacity to metabolise [³H] isopentenyladenine to zeatin metabolites. This metabolite activity was very notable in the stem, where it occurred as a gradient and was related to the age of the plant. Auxin was considered to be the factor controlling this distribution of cytokinin metabolic activity in the stem since parallel gradients had been noted in the patterns of lateral bud branching and the response of buds to cytokinin application. Indeed, it was shown that indole-3-acetic acid affected the metabolism of [³H] isopentenyladenine in isolated stem sections. These results are discussed in relation to the control of lateral bud growth via the auxin mediated distribution and metabolism of root produced cytokinins in the shoot system, necessary for the release of apical dominance. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1988.