Hormonal regulation of fruit set and abscission in citrus: Classical concepts and new evidence

Abstract

Evidence has been presented in citrus suggesting that initiation of fruit growth is promoted by mediators of hormonal nature, such as gibberellins, that are able to respond to internal and external stimuli. In seeded citrus, pollination increases GA levels and reduces ovary abscission, whereas exogenous GAs mimic the effects of pollination on fruit set of non-pollinated fruits. In seedless cultivars, a large number of observations indicate that gibberellins are one of the limiting and crucial factors regulating parthenocarpic fruit initiation. While cytokinins have also been involved in the improvement of fruit set, the role of auxins appear to be related to the control of fruit size and retardation of pre-harvest abscission. After hormone stimulation, fruit development is mostly supported by the disponibility of nutrients. It is also known that unfavorable environmental conditions, as well as a shortage of carbohydrates, activate synthesis of regulatory compounds involved in the processes of abscission, such as abscisic acid and ethylene. Abscission of developing fruits appears to be under hormonal regulation according to the sequence: ABA --> ACC --> ethylene --> fruit abscission.