Photooxidative sunburn of apples, which is distinctly different from sunburn necrosis and sunburn browning, is described. Shaded (nonacclimated) apples suddenly exposed to sunlight became photobleached and eventually necrotic. Electron micrographs and electrolyte leakage studies indicated that cell death occurred with photobleaching. Photobleaching occurred at fruit surface temp. (FST) of 45 degrees C. UV-B radiation was involved in the induction of sunburn browning, but blocking UV-A and UV-B radiation did not protect nonacclimated apples from photobleaching and necrosis. IR (>700 nm) was ruled out as an induction factor. Results suggest that visible radiation and the production of reactive oxygen species are induction factors for photooxidative sunburn.
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