Histochemical evidence for the relationship between peel damage and the accumulation of phenolic compounds in gamma-irradiated citrus fruit

Abstract

The first symptoms of radiation damage to citrus fruit were noted 24 to 48 hr after irradiation with 240 krad of gamma rays. At about the same time, radiation-damaged cells were observed in peel cross sections in the flavedo, the outer colored peel layers. The damaged cells were deformed and their protoplasts stained much darker with haematoxylin-safranin than protoplasts of intact cells. The cytoplasm in damaged cells either thickened at one side of the cell, sometimes filling up most of the cell space, or contracted into a small mass. Using several histochemical reagents, a considerable accumulation of phenolic compounds was found to occur in the damaged cells shortly after irradiation. No accumulation of phenolic compounds was observed in intact cells of irradiated fruit. It is suggested that the phenolic compounds which accumulate in flavedo cells following irradiation cause cell death and consequent peel necrosis (pitting).