Diurnal changes in photosystem II photochemistry, photoprotective compounds and stress-related phytohormones in the CAM plant, Aptenia cordifolia

Acclimation of photosynthetic light reactions to daily changes in solar radiation requires adjustments in photosystem II photochemistry and may be affected by environmental stresses, such as drought. In this study, we examined the effects of a short-term, severe water deficit on diurnal variations in photosystem II photochemistry, photoprotective compounds (tocopherols and carotenoids, including the xanthophyll cycle) and stress-related phytohormones (abscisic acid and salicylic acid) in the CAM plant, Aptenia cordifolia L. f. Schwantes. Violaxanthin was rapidly converted to zeaxanthin under high light, the de-epoxidation state of the xanthophyll cycle reaching maximum levels of 0.95 at midday in irrigated plants. Under a higher photoprotective demand caused by water deficit, plants showed significant increases in abscisic acid and γ-tocopherol levels, which were followed by decreases in β-carotene and the F_v/F_m ratio at later stages of stress. Decreases in this ratio below 0.70 correlated with sustained increases in the de-epoxidation state of the xanthophyll cycle, which kept above 0.90 at night after 15 days of water deficit. In contrast to abscisic acid, salicylic acid levels kept constant under water deficit and showed a sharp decrease during the day both under irrigated and water stress conditions. We conclude that the CAM plant, A. cordifolia showed several strategies of acclimation to short-term water deficit, including abscisic acid and γ-tocopherol accumulation, as well as sustained increases in the de-epoxidation state of the xanthophyll cycle, which was tightly coupled to daily variations in photosystem II photochemistry. The differential accumulation of tocopherol homologues under water deficit and the diurnal fluctuations of salicylic acid levels in this CAM plant will also be discussed.