Abscission Response and Color Changes of ‘Valencia’ Oranges1

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Abstract
Mature Florida ‘Valencia’ oranges [Citrus sinensis (L.) Osbeck] go through a period of reduced response to abscission chemicals. This nonresponsive period is characterized by both a reduction in the amount of ethylene induced by abscission chemicals and by decreased sensitivity of the abscission process to exogenous ethylene. During the nonresponsive period, application of the abscission chemical 5-chloro-3-methyl-4-nitro-1H-pyrazole (Release) induced less ethylene formation and less reduction in fruit removal force than at times either prior to, or following, the nonresponsive period. Experiments with radioactive Release showed that uptake was not a factor in the reduced effectiveness of this material during the nonresponsive period. Abscission of explants induced by exogenous ethylene was slower during the nonresponsive period. Mature ‘Valencias’ also go through a period of regreening. Regreening and the nonresponsive period may begin at about the same time, but regreening continues after the nonresponsive period is over. In addition, color changes induced by exogenous ethylene remain similar throughout the regreening and nonresponsive periods, but abscission of explants induced by exogenous ethylene is reduced only during the nonresponsive period. These differences in time-course and ethylene sensitivity between regreening and the nonresponsive period allow differentiation of these two processes.

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