Articles
CHANGES IN ENDOGENOUS HORMONES IN APPLE DURING BUD BURST INDUCED BY DEFOLIATION
This is made possible by hand defoliation soon after fruit harvest, a procedure which apparently prevents dormancy development in terminal buds.
By this time flowers have initiated.
Within four weeks of defoliation more-or-less synchronous bud burst follows.
Temperatures are favourable for a second growth cycle at the time when rest would occur in the Temperate Zone.
Timing of defoliation determines the time of bud burst, and thus flowering and harvest, so that continuous production throughout the year is possible by sequential defoliations.
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 increased in fresh and dry weight, and may be casually related to bud burst.
During bud opening there was an increase in CKs in the young expanding leaves and 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.