Drought-induced changes in flavonoids and other low molecular weight antioxidants in Cistus clusii grown under Mediterranean field conditions.

Mediterranean plants have evolved a complex antioxidant defense system to cope with summer drought. Flavonoids, and particularly flavanols and flavonols, are potent in vitro antioxidants, but their in vivo significance within the complex network of antioxidant defenses remains unclear, especially in plant responses to stress. To gain insight into the role of flavonoids in the antioxidant defense system of Cistus clusii Dunal, we evaluated drought-induced changes in flavonoids in leaves and compared the response of these compounds with that of other low molecular weight antioxidants (ascorbic acid, tocopherols and carotenoids). Among the antioxidant flavonoids analyzed, epigallocatechin gallate was present in the greatest concentrations (up to about 5 micromol dm(-2)). Other flavanols, such as epicatechin and epicatechin gallate, were found at concentrations below 0.25 and 0.03 micromol dm(-2), respectively. Neither of the antioxidant flavonols analyzed, quercetin and kaempferol, were detected in C. clusii leaves. Epigallocatechin gallate, ascorbic acid and alpha-tocopherol concentrations increased to a similar extent (up to 2.8-, 2.6- and 3.3-fold, respectively) in response to drought, but the kinetics of the drought-induced increases differed. Epigallocatechin gallate, epicatechin and epicatechin gallate concentrations increased progressively during drought, reaching maximum values after 30 days of stress. Ascorbic acid concentrations increased twofold after 15 days of drought, and maximum values were attained after 50 days of drought. In contrast, alpha-tocopherol concentrations remained constant during the first 30 days of drought, but increased sharply by 3.3-fold after 50 days of drought. The maximum efficiency of photosystem II photochemistry and the extent of lipid peroxidation remained constant throughout the drought period, whereas the redox state of ascorbic acid and alpha-tocopherol shifted toward their reduced forms in response to drought, indicating that the concerted action of low molecular weight antioxidants may help prevent oxidative damage in plants.