Effect of foliar‐applied gibberellins and soil‐applied paclobutrazol on reproductive and vegetative growth of ‘Braebum’ apple trees growing under a high‐density planting system

Abstract

Five‐year‐old ‘Braeburn'/MM.106 (Malus domestica Borkh.) trees growing under a high‐density (10 000 trees/ha) planting system were treated with paclobutrazol (PBZ) as a soil drench at 250 mg(a.i.) per tree in August 1993. Gibberellin (GA), GA, or GA4, was sprayed on vegetative buds at 0, 10, or 100 μg per bud at three times between 22 November and 22 December 1992 (before PBZ treatment), or in 1993 (after PBZ treatment). Data on yield and yield components and vegetative growth were collected for 2 years. Time of GA application did not affect vegetative and reproductive growth components. The total shoot number, total shoot length, and total bud number were consistently reduced with PBZ application for 2 years of application. GA caused a decrease in shoot number across 2 years whereas total shoot length and total bud number were increased with GA treatment across 2 years. Fruit yield was decreased with PBZ application by 42 and 23% across 2 years in Trial A and by 27 and 12% across 2 years in trial B. Total fruit number per tree were decreased with PBZ application and varied from year‐to‐year showing biennial bearing (BI) patterns in both trials. PBZ plus GA reversed the PBZ effects on fruit yield and total fruit number and reduced bienniality in trees for 2 years. GA application to the trees which had previously received PBZ was most effective in reducing BI values. Mean fruit weight was not affected with PBZ treatment. The deleterious effects of these two plant growth regulators were reversed by the combined use of PBZ and GA, which provided a balanced yield and vegetative growth and prevented the trees from going into biennial bearing. No consistent, significant effects of GA types or G A levels or their combination with PBZ on vegetative and reproductive growth were found. It is suggested that PBZ and GA can be used to manipulate apple tree growth in a high‐density apple production system and their use in a cultural management system for high‐density apple orchards should be explored.