Signi®cance of skin ¯avonoids for UV-B-protection in apple fruits

An attempt has been made to assess the UV-B-protective capacity of phenolic compounds accumulated in super®cial structures of plants using apple fruit as a model. Two apple (Malus domestica Borkh.) cultivars (Braeburn and Granny Smith) differing in response to high ¯uxes of solar radiation were selected and exposed to increasing doses of UV-B radiation. The extent of UV-B-induced damage to photosystem II of apple skin correlated with its quercetin glycoside (but not anthocyanin) content. Granny Smith apples did not demonstrate a pronounced response to high sunlight in terms of the accumulation of phenolic substances in the skin and exhibited similar patterns of F o, F m, and F v/F m changes in the course of UV-B irradiation both on sun-exposed and shaded surfaces of a fruit. Unlike Granny Smith, Braeburn fruits were characterized by a signi®cant accumulation of quercetin glycosides in sun-exposed skin, however, shaded skin contained these compounds in similar amounts to those in Granny Smith. Accordingly, photosystem II in sunexposed skin of Braeburn apples was resistant to high doses of UV-B radiation (up to 97 kJ m ±2), whereas the susceptibility of the photosynthetic apparatus in shaded skin of Braeburn to UV-Binduced damage was much higher and similar to that of both sun-exposed and shaded skin of Granny Smith fruits. Anthocyanins, at least in the amounts found in Braeburn, did not show an additional effect in UV-B protection. Key words: Apple, phenolic compounds, photosystem II, protection, UV-B radiation