Apical dominance in Pisum sativum L

Abstract

Application of quantitative methods to the histological study of the vascular connections of the first bud of seedlings of Pisum sativum L. var, Meteor showed that the inhibition of growth of the first lateral bud, whether promoted by the presence of the apex or by application of IAA in lanolin to the cut surface of decapitated plants, was not due to any noticeable lack of vascular connections with the stele of the main stem. The results have also indicated that in inhibited buds the smaller development of the vascular connections is rather a consequence than a cause of the bud inhibition. Experiments in whichychanges in the lateral bud behaviour, in the patterns of P translocation and IAA movement were induced either by a constriction of the stem or by a ring of lanolin with NPA (naphthylphthalamic acid) around the stem or by application of CHL (chloramphenicol) to the stump yielded results which do not support the "nutrient diversion" theory. Although no direct and conclusive proof of the irrelevance of the"nutrient diversion" theory could be obtained several features of the results indicate that the lack of nutrients should not be the basic cause of the lateral bud inhibition. Similar conclusion was obtained from the study of the time-course changes of translocation by mapping the radioactivity along the stem and relating them to the behaviour of the bud. The direct application of a cytokinin (6-Benzylaminopurine) in lanolin paste has caused the release of the lateral bud from apical dominance both in intact and decapitated IAA treated plants, but the supply of 6-Benzylaminopurine via the basal cut end of stem cuttings was not able to relieve the inhibition promoted by applied IAA, although a stimulatory effect on the lateral bud growth have been obtained in plain lanolin treated stem cuttings. The results, as a whole, lend support to the idea that the incapacity of the lateral bud to compete with the apex of the main stem for some growth regulator(s), indispensable for bud growth and probably from the cytokinin-type, coming up from the root system is the primary cause of its growth inhibition. An explanation of the apical dominance phenomena is attempted with a theoretical model which is based both on the results presented here and on other evidence available from the literature.