Photooxidative Sunburn of Apples: Characterization of a Third Type of Apple Sunburn

Abstract

ABSTRACT Photooxidative sunburn, a third type that is distinctly different from sunburn necrosis and sunburn browning, is described for apple (Malus domestica Borkh.). 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 temperatures (FST) below 31°C. Sunburn browning and sunburn necrosis require FST above 45°C. Ultraviolet-B radiation is involved in induction of sunburn browning, but blocking UV-A and UV-B radiation did not protect nonacclimated apples from photobleaching and necrosis. Infrared radiation (>700 nm) was ruled out as an induction factor. We suggest that visible radiation and the production of reactive oxygen species are the induction factors for photooxidative sunburn.