The role of solar UV in long-term adaption of ripening apple fruits to high sunlight

The changes in chlorophyll, carotenoid and quercentin glycoside content were studied in two apple (Malus domestica Borkh.) varieties. Braeburn and Granny Smith, with different ability for acclimation to sunlight, during their von-tree ripening under full-spectrum solar radiation and without UV component. Exclusion of solar UV did not affect the development of variety-specific patterns of photo-synthetic pigments during fruit ripening and adaptation to thfrom solar UV-induciblee visible solar radiation. On the contrary, solar UV was crucial for the accumulation of quercentin glycosides, the main group of phenolics serving for UV acclimation in apple fruits. High content of quercentin glycosides (QG) was maintained in the sunlit skin of fruits grown with solar UV in the course of ripening. A part from solar UV-indicible QG in sunlit skin, there was a 'backgroud' level of these compounds in the shaded skin that was not affected by solar UV. The QG of shaded apple skin presumably originate from constitutive phenolic metabolism and may serve for protection against diffuse UV of sunlight scattered by canopy.