Hormonal and Nutritional Changes in the Flavedo Regulating Rind Color Development in Sweet Orange [Citrus sinensis (L.) Osb.]

The objective of this research was to determine the changes in the levels of endogenous gibberellins GA1 and GA4, abscisic acid (ABA), and ethylene during fruit coloring of on-tree fruits of sweet orange. The time course of carbohydrates and nitrogen content in the flavedo prior to fruit color break and during peel ripening were also studied. To identify nutritional and hormonal changes in the fruit, 45 days before fruit color break the peduncles of 15–30 fruits per tree of ‘Washington’ navel, ‘Navelate,’ and ‘Valencia Delta Seedless’ sweet orange, located in single-fruited shoots, were girdled to intercept phloem transport. A set of 15–30 fruits per tree remained intact on the peduncle for control. Girdling significantly delayed fruit coloration for more than 2 months; the delay paralleled higher GA1 and GA4 concentrations in the flavedo and retarded the rise of ABA concentration prior to color break. Girdling also reduced carbohydrate concentrations and increased N concentrations in the flavedo compared to control fruits; no ethylene production was detected. Therefore, in sweet orange, fruit changes color by reducing active gibberellin concentrations in the flavedo, which are involved in regulating sugars and ABA accumulation and in reducing N fraction concentration as rind color develops. This was demonstrated in vivo without removing the fruit from the tree. Comparable results were obtained with experiments carried out over four consecutive years in two countries (Spain and Uruguay).