POSTHARVEST CITRUS FRUIT ABSCISIC ACID AND ITS POSSIBLE PHYSIOLOGICAL SIGNIFICANCE

Citrus is an important commercial crop and the maintenance of postharvest quality of the fruits is a major concern of the citrus industry. Information on the production and utilization of the crop far exceeds the information on the influence of endogenous hormones affecting the postharvest quality of citrus fruits. The present report of postharvest citrus fruit abscisic acid (ABA) is part of an on-going broader study of environmental and chemical stresses on citrus fruits grown in different geographical regions and their differential phytotoxic responses to quarantine treatments (heat, cold, chemical fumigants). The hormone ABA was chosen for study because (a) it is well recognized that various forms of stresses (e.g. heat, cold, drought) induced changes in plant tissue ABA and (b) it is involved in the regulation of many physiological processes in plants. We analyzed both free and conjugated ABA by an indirect enzyme-linked immunosorbent assay and found their respective contents in the peel of citrus species to differ significantly. The ratios of conjugated to free ABA vary from 1.29 to 9.32. The distribution of free and conjugated ABA in lemon showed that the stylar tissues contained the predominant quantity of ABA and lesser quantities in the stamens and ovaries. The full physiological significance of ABA accumulation in various citrus fruit tissues is presently uncertain, but we suggest that the ABA changes reflects its probable involvement in stylar senescence, fruitlet abscission, fruit assimilate accumulation, peel coloration and maturation.