Effects of Girdling and Defoliation on Hormonal Changes during Flower Induction in Longan (Dimocarpus longan Lour.)

The discovery of potassium chlorate (KClO3) as flower inducing agent in longan (Dimocarpus longan Lour.) has overcome the problem of irregular bearing and enables farmers to produce this fruit crop in off-season. However, the underlying physiological mechanism of how this chemical triggers flower induction (FI) remains unclear. A possible explanation might be the timely hormonal changes in longan leaves during the transitional growth stage from vegetative to floral buds following KClO3 application. The experiment consisted of a control treatment and four treatments with KClO3 application; (1) no shoot girdling and no defoliation, (2) shoot girdling and removal of all leaves and petioles above the girdle, (3) shoot girdling without defoliation above the girdle and (4) apical defoliation without girdling. All shoot treatments were applied to each of twenty mature trees. KClO3 was applied as soil drench at mature leaf stage in September 2007. The girdle was 2 mm wide and applied at about 10 cm below the shoot tip. Leaf samples for analysis of diffusible and extractable plant hormones were collected 8 times at 1- to 3-day intervals until 28 days after treatment (DAT). The concentration of endogenous hormones such as indole-3-acetic acid (IAA), gibberellins (GA(1, 3, 20) measured as GA(3) equivalents), and the cytokinins isopentenyladenin/isopentenyladenosin (iP/[9R] iP) and trans-zeatin/t-zeatin riboside (Z/[9R] Z) were determined in leaves. The results show that IAA concentrations in leaves of all treatments were slightly reduced between 4 to 13 days following KClO3 application but values remained rather constant thereafter. In general, leaves above the girdle had significantly higher concentrations of iP/[9R] iP but less Z/[9R] Z whereas the export of diffusible iP/[9R] iP from upper leaves of non-girdled shoots increased from 10 to 13 days after application.