![]() Increases in natural UV radiation due to decreased stratospheric ozone concentrations have stimulated research on mechanisms and maximum capacities for protection against UV exposure ( Caldwell et al., 1998). ![]() To cope with UV radiation damage, plants have evolved a variety of mechanisms including: screening out UV radiation by accumulating UV-absorbing phenolic compounds in the leaf epidermis, repairing UV-induced DNA damage, and formation of antioxidants to scavenge peroxides and oxygen radicals ( Bornman and Teramura, 1993 Jordan 1996). In general, UV radiation damages lipids, nucleic acids, and proteins in leaves of higher plants, and specifically targets the photosystem II (PSII) reaction center, Rubisco, chloroplast ATPase, and violaxanthin deepoxidase ( Jordan, 1996 Vass, 1997). When harvesting light, photosynthetic organs are inevitably exposed to the UV region of natural radiation. Photosynthetic organisms form energy-rich compounds using the energy of the sun's visible radiation. We propose that protection of UV-B inactivation of PSII is observed because preceding damage is efficiently repaired while those factors determining UV-B inhibition of CO 2 assimilation recover more slowly. However, UV-B inhibition of CO 2 assimilation rates was not diminished by efficient UV-B screening. UV-B-dependent inhibition of maximum photochemical yield of photosystem II (PSII), measured as variable fluorescence of dark-adapted leaves, recovered in parallel to the buildup of epidermal screening for UV-B radiation, suggesting that PSII is protected against UV-B damage by epidermal screening. It is suggested that curvilinearity results from different absorption properties of the homogeneously dissolved phenolics in extracts and of the non-homogeneous distribution of phenolics in the epidermis. Transmittance in the UV of all extracted phenolics was lower than epidermal UV transmittance determined fluorimetrically, and the two parameters were curvilinearly related. In contrast, UV-B radiation specifically increased flavonoid concentrations resulting in more than a 10-fold increase. During exposure, synthesis of UV-screening phenolics in leaves was quantified using HPLC: All treatments increased concentrations of hydroxycinnamic acids but the rise was highest, reaching 230% of the initial value, when UV radiation was absent. Different light regimes were produced by filters transmitting natural radiation, or screening out the UV-B (280–315 nm), or screening out the UV-A (315–400 nm) and the UV-B spectral range. Grape ( Vitis vinifera cv Silvaner) vine plants were cultivated under shaded conditions in the absence of ultraviolet (UV) radiation in a greenhouse, and subsequently placed outdoors under three different light regimes for 7 d.
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