The effects of UV-B radiation on photosynthesis in relation to Photosystem II photochemistry, thermal dissipation and antioxidant defenses in winter wheat (Triticum aestivum L.) seedlings at different growth temperatures.

To study the UV-B effect on photosynthesis in winter wheat at different day/night temperatures, biologically effective UV-B radiation at 4.2 (L) and 10.3 (H) kJ m d was provided on the seedlings at 25/20°C or 10/5°C. UV-B radiation inhibited net photosynthesis rate (P) by enhanced intensity and decreased temperature without change of intercellular CO concentrations (C). Decreased maximal quantum yield of Photosystem II (F/F) and increased minimum fluorescence (F) were observed in H at both temperatures and L at 10/5°C. H increased total pool size (VAZ) of xanthophyll cycle pigments, but decreased the de-epoxidation state (DEPS) of these pigments at both temperatures, while L only decreased DEPS at 10/5°C. The activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) and the redox states of ascorbate and glutathione (AsA/DAsA and GSH/GSSG) were enhanced at 25/20°C, while there were increased SOD and CAT, unaltered APX activities and AsA/DHA, as well as decreased GR activity and GSH/GSSG in L and H at 10/5°C. UV-B radiation resulted in higher HO and thiobarbituric acid reactive substance (TBARS) concentrations at 10/5°C than 25/20°C. It appears that low temperature alone did not influence photosynthesis but aggravated UV-B induced photoinhibition, which was associated with PSII photochemistry rather than stomatal limitation. Xanthophyll cycle pigments failed to provide photoprotection through thermal dissipation. The antioxidant system was up-regulated in L and H at 25/20°C, but was impaired at 10/5°C. Low temperature intensified UV-B induced photoinhibition and damage by weakening the antioxidant system.