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

An attempt has been made to assess the UV-B-pro-tective capacity of phenolic compounds accumulated in super®cial structures of plants using apple fruit as a model. Two apple (Malus domestica Borkh.) culti-vars (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 quer-cetin glycoside (but not anthocyanin) content. Granny Smith apples did not demonstrate a pro-nounced response to high sunlight in terms of the accumulation of phenolic substances in the skin and exhibited similar patterns of Fo, Fm, and Fv/Fm 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 sun-exposed 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-B-induced 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