UV screening by phenolics in berries of grapevine (Vitis vinifera).

The role of phenolics in UV-screening was investigated in berries of a white grape cultivar (Vitis vinifera L. cv. Bacchus). Fluorescence microscopy revealed accumulation of phenolics in the skin of berries and, by high performance liquid chromatography and mass spectrometry, flavonols and hydroxycinnamic acids were identified as the main groups of UV-absorbing phenolics. Relationships between natural radiation and the synthesis of phenolics were studied in plants that were cultivated in the absence of UV radiation in a greenhouse before outdoor exposure to three different light regimes: the entire solar spectrum, the solar spectrum minus UV-B radiation and only visible radiation. During six days of exposure, flavonol synthesis was significantly stimulated by natural UV, in particular UV-B, but concentrations of hydroxycinnamic acids decreased under all conditions. Direct comparison of fluorimetrically-determined skin absorbance with absorbance of extracted flavonols or hydroxycinnamic acids suggested that acclimation of UV screening depends almost exclusively on flavonol synthesis. While increased flavonol levels resulted in efficient UV-A shielding, UV-B shielding was incomplete, probably due to decreased levels of the UV-B-absorbing hydroxycinnamic acids during exposure.