Effects of growth regulators on growth and fruiting of own-rooted ‘Golden Delicious’ apple trees

Abstract Established tissue-culture propagated own-rooted cultivar ‘Golden Delicious’ apple ( Malus domestica Borkh.) trees were treated with paclobutrazol at a dosage of 40 mg cm −2 of trunk cross-sectional area (average of 1.48 g per tree) via trunk drench at the end of the fifth growing season. They responded the second season after application. Treatment with the triazole GA biosynthesis inhibitor had more effect on vegetative growth than on flower cluster or fruit number. It reduced terminal shoot length and tended to reduce fruit yield and seasonal net photosynthesis on a leaf area basis. Weight per fruit was not changed but fruit length tended to be slightly reduced whereas russeting index was increased. Promalin (GA 4 + 7 + BA) applications to paclobutrazol-treated trees did not alter growth or fruiting. Despite the fact that paclobutrazol inhibited terminal shoot growth, treated trees went into biennial bearing as readily as untreated trees after a heavy fruit load season. Vegetative growth was vigorous the “off year” and terminal shoot growth was no longer inhibited by paclobutrazol. Applications of GA 4 + 7 to trees during the “off-year” did not affect vegetative growth that season or fruiting the following season. “On year” yield tended to be reduced from trees that had been treated with paclobutrazol 5 years earlier.