Changes in endogenous hormones in apple during bud burst induced by defoliation

Under the tropical conditions of East Java, terminal buds of apple burst at any time of the year in response to removal of the subtending leaves. Following two such defoliations, two weeks apart on separate trees, there was a decrease in abscisic acid (ABA), a three-fold increase in gibberellin-like substances (GAs) and only a slight increase in cytokinin-like substances (CKs) in the apex tissue of closed buds. These changes preceded bud opening and the associated increases in fresh and dry weight, and may be causally related to bud burst. In open buds (i.e. young expanding leaves) the concentration of CKs was greater, and the concentrations of ABA and GAs less, than the concentrations in closed buds. As the leaves expanded, ABA increased and GAs and CKs decreased in concentration. The decrease in concentration of GAs and CKs, however, was due to the rise in dry weight of the expanding tissue; the amounts of all three hormones (per apex) increased. During bud burst there was a concurrent decrease in the CKs of subtending stems, suggesting a transfer into the expanding bud tissues. Removal of the old leaves by defoliation may remove the source of ABA and allow the amount of GAs in the apex to rise, bud burst following. Stem CKs may be utilized in the expansion of the new leaves in the bursting buds.