Enhanced α-tocopherol quinone levels and xanthophyll cycle de-epoxidation in rosemary plants exposed to water deficit during a Mediterranean winter

Photosynthesis operates in a constantly shifting balance between efficient capture of solar energy and its rapid dissipation when captured in excess. In an attempt to better understand the role of α-tocopherol in plant photoprotection, we examined the changes in α-tocopherol quinone (α-TQ), in parallel with those of other low-molecular-weight antioxidants, in rosemary plants exposed to water deficit during a Mediterranean winter. Relative leaf water content (RWC) decreased from about 85% to approximately 65% in drought, but plants did not show symptoms of oxidative damage, as indicated by constant image ratios and malondialdehyde (MDA) levels. α-TQ was present at concentrations of 20 mmol per 100 mol of chlorophyll, and represented less than 1% of total tocopherol content in non-stressed leaves. Although α-tocopherol levels were not significantly altered, α-TQ reached up to 36 mmol per 100 mol of chlorophyll under stress (under both high light and after exposure to increasing water deficit at lower light intensities). Furthermore, both α-TQ and xanthophyll cycle de-epoxidation were strongly negatively correlated with the relative efficiency of photosystem II photochemistry (φPSII) at midday. The biological significance of α-tocopherol and α-TQ in the network of photo- and antioxidative protection mechanisms evolved by plants to withstand stress is discussed.