Abscisic acid and apical dominance in Phaseolus coccineus L.

Abscisic acid (ABA) in lanolin, applied to the internode of decapitated runner bean plants enhances the outgrowth of lateral buds. The optimum concentration of the paste is 10(-5) M. The effect of ABA is counteracted by indoleacetic acid (IAA) but not by gibberellic acid (GA3). There is no effect when ABA is applied to the apical bud or lateral buds of intact plants. However, 13.2 ng given to the lateral buds of decapitated plants stimulate their growth, whereas higher concentrations are inhibitory. Consequently, ABA enhances growth of lateral buds directly, but only when apical dominance is already weakened. The growth of the decapitated 2(nd) internode was not affected by ABA. Radioactivity from [2-(14)C] ABA, applied to nonelongating 2(nd) internode stumps of decapitated runner bean plants moves to the lateral buds, whereas [1-(14)C]IAA-and [(3)H]GA1-translocation is much weaker. ABA transport is inhibited if IAA or [(3)H]GA1 is applied simultaneously. In elongating internodes [(14)C]ABA is almost completely immobile. [(14)C]IAA-and [(3)H]GA1-translocation is not affected by ABA. The amount of radioactivity from labelled ABA, translocated to the lateral buds, is highest during the early stages of bud outgrowth.