Solar UV-B effects on composition and UV screening efficiency of foliar phenolics in Arabidopsis thaliana are augmented by temperature.

The accumulation of foliar phenolics constitutes one strategy of plants against the potentially harmful effects of ultraviolet-B and A (UV-B, UV-A) radiation. These compounds protect photosensitive tissues by shielding and antioxidative function. It is unknown, however, whether seasonal acclimation to natural conditions may modify the UV-B effect on phenylpropanoid composition and localisation, and thus their screening efficiency. To address this debate, a field experiment with the wildtype of Arabidopsis thaliana accession Landsberg erecta (Ler) was implemented over a whole year with plants exposed to different UV-filter treatments. While seasonal increases of UV-B radiation had a slight negative effect on the amount of hydroxycinnamic acids (HCAs), low temperatures increased foliar HCAs. HCAs, however, did not contribute substantially to seasonal changes of in vivo UV absorbance. Kaempferol and quercetin derivatives increased significantly under ambient UV-B radiation, and low temperature interacted with this effect. A shift of epidermal UV-A shielding from kaempferol to quercetin derivatives was elucidated in UV-B presence. Despite this, a substantial 20-fold increase of quercetin derivatives, during periods with high irradiance and low temperature, did not affect UV absorbance leading to the conclusion that quercetin accumulation was not exclusively in epidermal vacuoles. Using confocal microscopy, the potential occurrence of quercetin in mesophyll cells was demonstrated in plants grown with experimental UV-B radiation at low temperature for the first time in A. thaliana. The presented study discusses the idea that cross-talk of UV-B radiation and temperature might adjust the physiological function of quercetin from an (epidermal) screening to an antioxidant substance.