Effects of water deficit on photosystem II photochemistry and photoprotection during acclimation of lyreleaf sage (Salvia lyrata L.) plants to high light.
Acclimation of photosynthetic light reactions to high light requires adjustments in photosystem II (PSII) photochemistry and may be affected by environmental stresses, such as water deficit. In this study, we examined the effects of this stress on PSII photochemistry and photoprotection, with an emphasis on the role of carotenoids and tocopherols, during acclimation of lyreleaf sage (Salvia lyrata L.) plants to high light. Violaxanthin was rapidly converted to zeaxanthin under high light, the de-epoxidation state of the xanthophyll cycle reaching maximum levels of 0.97 after 10 days of high light exposure. Under a higher photoprotective demand caused by water deficit, plants showed significant decreases in beta-carotene and enhanced oxidation of alpha-tocopherol to alpha-tocopherol quinone, which was followed by decreases in the F(v)/F(m) ratio. The levels of beta-carotene decreased more in water-stressed than irrigated plants during acclimation to high light, being particularly degraded (up to 73%) after 14 days of water deficit. Tocopherol levels increased significantly during acclimation to high light, particularly under water deficit, which caused 6.6- and 10-fold increases in alpha-tocopherol and alpha-tocopherol quinone, respectively. We conclude that when xanthophyll cycle-dependent excess energy dissipation could not afford further protection during high light acclimation and the photoprotective demand increased in lyreleaf sage plants by water deficit, enhanced oxidation of alpha-tocopherol and beta-carotene occurred. As stress persisted, enhanced formation of reactive oxygen species might ultimately damage the PSII, as indicated by the reductions in the F(v)/F(m) ratio.