Hormonal regulation of fruitlet abscission induced by carbohydrate shortage in citrus

The hormonal signals controlling fruitlet abscission induced by sugar short age in citrus were identified in Satsuma mandarin, Citrus unshiu (Mak.) Mar c, cv. Clausellina and cv. Okitsu. Sugar supply, hormonal responses and fru itlet abscission were manipulated through full, partial or selective leaf r emovals at anthesis and thereafter. In developing fruitlets, defoliations r educed soluble sugars (up to 98%), but did not induce nitrogen and water de ficiencies. Defoliation-induced abscission was preceded by rises (up to 20- fold) in the levels of abscisic acid (ABA) and 1-aminocyclopropane-1-carbox ylic acid (ACC) in fruitlets. Applications to defoliated plants showed that ABA increased ACC levels (2-fold) and accelerated fruitlet abscission, whe reas norflurazon and 2-aminoethoxyvinyl glycine reduced ACC (up to 65%) and fruitlet abscission (up to 40%). Only the fun defoliation treatment reduce d endogenous gibberellin Al (4-fold), whereas exogenous gibberellins had no effect on abscission. The data indicate that fruitlet abscission induced b y carbon shortage in citrus is regulated by ABA and ACC originating in the fruits, while gibberellins are apparently implicated in the maintenance of growth. In this system, ABA may act as a sensor of the intensity of the nut rient shortage that modulates the levels of ACC and ethylene, the activator of abscission. This proposal identifies ABA and ACC as components of the s elf-regulatory mechanism that adjusts fruit load to carbon supply, and offe rs a physiological basis for the photoassimilate competition-induced abscis sion occurring under natural conditions.