Modification of Branching Behavior in Apical-Dominant Apple Trees with Plant Growth Regulators and Their Residual Effects on Tree Growth After Transplanting

Abstract

Nursery trees of apple hybrids NY73334-35, NY75413-30 and NY75414-1 were treated once or twice with the plant growth regulator (PGR) Accel or Promalin, alone or in combination, to induce branching. Regardless of the PGR used, or the rate and frequency of application, treated trees produced more shoots per tree than untreated control trees for all three hybrid selections. Medium [750 mg·L ^-1N-(phenylmethyl)-1H-purine-6-amine (6 benzyladenine or BA)] or high rates (1000 mg·L ^-1BA) of both compounds were more efficient in shoot induction than low rates (500 mg·L ^-1BA). When comparable rates of both chemicals were applied, no differences in the number of lateral shoots, shoot length distribution, total tree extension growth and apical dominance among chemical treatments were noted. Regression analysis on rates of Accel and Promalin vs.total number of shoots demonstrated that Accel promoted a linear increase in this characteristic, whereas Promalin caused a parabolic effect on branching. The trees treated once or twice in the nursery at medium rates of either compound were transplanted into the orchard. One year after transplanting the trees exhibited a pronounced increase in the total number of shoots and their extension growth, and also considerable reduction in apical dominance. It is believed that those changes may facilitate canopy training, and thus accelerate the onset of bearing.