Sunburn of Apple Fruit: Historical Background, Recent Advances and Future Perspectives

Sunburn is a physiological disorder of apples and other fruit species caused by excess solar radiation. Damage occurs in practically all growing regions of the world, causing severe crop loss every year. Direct factors required for induction of the three currently-known types of sunburn (i.e., sunburn necrosis, sunburn browning, and photooxidative sunburn) include excess radiant heating and/or exposure to excess sunlight. Several other factors (e.g., relative humidity, wind velocity, acclimation of fruit, and cultural management practices), which alone cannot induce sunburn damage, indirectly influence the induction of sunburn by interacting with the direct factors to influence the appearance and severity of the symptoms. Sunburn affects apple fruit at many levels; it causes structural and morphological changes, alters pigment composition, influences adaptive mechanisms, impairs photosynthesis, and consequently decreases fruit quality. Fruits employ multiple physiological and biochemical mechanisms as complex defense systems to minimize damage. Photoprotective pigments, antioxidant enzymes and metabolites, heat-shock proteins, and the xanthophyll cycle help mitigate damage, but are often inadequate under field conditions to fully protect from sunburn. Quality loss significantly affects postharvest behavior, marketing and consumer acceptance of fruit. Internal fruit quality (e.g., firmness, soluble solids concentration, and titratable acidity) is affected by sunburn, and changes in these traits continue during cold storage. Sunburn-related disorders (e.g., sunburn scald in ‘Granny Smith’ and ‘Fuji’ stain) can appear in cold storage. There are several methods with various modes of action (e.g., climate ameliorating techniques, and sunburn suppressants) available to growers to decrease sunburn under field conditions. At the end of this review, the potential impact of a changing climate on sunburn incidence is considered, as both UV-B radiation and temperature are projected to change. Finally, several topics that need further research are discussed.