The role of cytokinins in the regulation of apical dominance in Pisum sativum.
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.