Changes of ABA, IAA and GAs Levels in Reproductive Organs of Citrus
This review provides information about the levels of abscisic acid (ABA), indole-3-acetic acid (IAA) and gibberellins (GAs) in reproductive organs and the effects of uniconazole-P application on those hormones in citrus to analyze their possible relationships with fruit set and growth. Endogenous phytohormone levels in flowers and fruitlets were studied in ‘Hyuganatsu’ (Citrus tamurana [Hort.] Ex. Tanaka), which had no parthenocarpic ability, and Satsuma mandarin (Citrus unshiu Marc. cv. Ozaki). ABA concentration in the style of Satsuma increased 6-fold from the bud to the anthesis stage. Stamens in Hyuganatsu contained higher concentrations of IAA and GAs than in Satsuma. Immediately after pollination, Hyuganatsu showed peaks of ABA and IAA concentrations in fruitlets and parthenocarpic Satsuma also showed smaller peaks of ABA and IAA concentrations. Parthenocarpic fruitlets contained higher concentrations of GAs than pollinated fruitlets after fruit-set. ‘Sepals’ which included floral disks, sepals and receptacles in both setting types contained higher concentrations of ABA than in the nonsetting type, suggesting that ABA in ‘sepals’ plays a positive role. ‘Sepals’ in both setting types contained higher concentrations of IAA after fruit-set, suggesting that IAA from ‘sepals’ may hinder fruitlet abscission. Uniconazole-P, a GA biosynthesis inhibitor, markedly stimulated fruitlet abscission in Satsuma and increased 4-fold the concentration of endogenous ABA in fiuitlets.