Cytokin/Auxin Control of Apical Dominance in Ipomoea nil

Abstract

Although the concept of apical dominance control by the ratio of cytokinin to auxin is not new, recent experimentation with transgenic plants has given this concept renewed attention. In the present study, it has been demonstrated that cytokinin treatments can partially reverse the inhibitory effect of auxin on lateral bud outgrowth in intact shoots of Ipomoea nil. Although less conclusive, this also appeared to occur in buds of isolated nodes. Auxin inhibited lateral bud outgrowth when applied either to the top of the stump of the decapitated shoot or directly to the bud itself. However, the fact that cytokinin promotive effects on bud outgrowth are known to occur when cytokinin is applied directly to the bud suggests different transport tissues and/or sites of action for the two hormones. Cytokinin antagonists were shown in some experiments to have a synergistic effect with benzyladenine on the promotion of bud outgrowth. If the ratio of cytokinin to auxin does control apical dominance, then the next critical question is how do these hormones interact in this correlative process? The hypothesis that shoot-derived auxin inhibits lateral bud outgrowth indirectly by depleting cytokinin content in the shoots via inhibition of its production in the roots was not supported in the present study which demonstrated that the repressibility of lateral bud outgrowth by auxin treatments at various positions on the shoot was not correlated with proximity to the roots but rather with proximity to the buds. Results also suggested that auxin in subtending mature leaves as well as that in the shoot apex and adjacent small leaves may contribute to the apical dominance of a shoot.