Larry E. Schrader

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

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.

Sunburn Browning Decreases At-Harvest Internal Fruit Quality of Apples (Malus domestica Borkh.)

Changes in internal fruit quality associated with increasing severity of sunburn browning of apples were investigated at harvest in five cultivars. Apples were sorted into five classes ranging from no sunburn (Sb-0) to severe sunburn browning (Sb-4). Fruit quality of flesh tissue was compared beneath the sunburned area, the shaded side, and the shoulder side of each apple. All two-way interactive effects (i.e., sunburn classes vs. sides of fruit, sunburn classes vs. cultivars, and cultivars vs. sides of fruit) were significant (P ≤ 0.05) for all fruit quality traits. For the treatment combination of sunburn class and side of fruit, flesh firmness, and soluble solids concentration increased on the sun side, but titratable acidity averaged over cultivars for the sun side generally declined as sunburn severity increased from Sb-1 to Sb-4. Starch cleared more quickly on the shaded and shoulder sides as compared to the sun-exposed side of all fruit with sunburn browning. Starch also generally cleared more rapidly in apples with more severe sunburn browning, suggesting that sunburned apples were more mature than nonsunburned apples harvested on the same day. For the treatment combination of side of fruit and cultivar, the sun side was higher than shade or shoulder sides in firmness and soluble solids concentration for all cultivars when averaged over sunburn classes. Starch index was lowest for the sun side. For the treatment combination of sunburn class and cultivar, firmness, soluble solids concentration, and starch index, when averaged over sides of fruit, generally increased in all cultivars as severity of sunburn increased; titratable acidity decreased.