Reduction of Early Fruit Abscission by Main-branch-girdling in Macadamia Is Related to the Favorable Status of Carbohydrates and Endogenous Hormones

Fruit abscission occurring severely in the early fruit development affects macadamia yield. Developing effective methods to improve fruit retention is a priority for macadamia cultivation and production. Girdling is an important horticultural practice that has been widely used to increase fruit yield. Previous studies have shown that girdling fails to increase macadamia yield despite enhancing the early fruit set, but few have examined the effect of girdling on its related physiological mechanism. The objective of this study was to investigate the effects of main-branch girdling (MBG) on early fruit retention and also on the levels of carbohydrates and endogenous hormones in the leaves, bearing shoots and fruit of macadamia. Herein, MBG was performed at fruit set using a single-blade knife on 9-year-old macadamia trees (Macadamia integrifolia). Results showed that MBG significantly reduced young fruit drop, concurrent with significant increases in the contents of starch in both the leaves and the bearing shoots and in glucose, fructose, and sucrose levels in the husk and seed. It was suggested that the availability of carbohydrate for fruit retention was improved by MBG. Additionally, MBG increased indole-3-acetic acid (IAA), gibberellin (GA3), and zeatin-riboside (ZR, a type of cytokinin) concentrations and decreased abscisic acid (ABA) contents in the husk and the seed, indicating that MBG reduced the early fruit drop by modifying the balance of endogenous hormones. Therefore, a positive interplay between carbohydrates and endogenous hormones induced by MBG was involved in the reduction of early fruit abscission in macadamia.