SUNFLOWER SEEDLINGS SUBJECTED TO INCREASING STRESS BY WATER-DEFICIT -CHANGES IN O-2(CENTER-DOT-) PRODUCTION RELATED TO THE COMPOSITION OF THYLAKOID MEMBRANES

Free radicals, such as the superoxide anion, can damage the photosynth etic apparatus. At the same time they are increasingly formed as by-pr oducts of electron transport during stress conditions. For this reason , we studied the kinetics of formation and decay of superoxide anions in relation to changes, induced by increasing water deficit, in the co mposition of thylakoids. Seedlings of sunflower (Helianthus annuus L. cv. Licia Stella) were subjected to either a mild, a moderate, or a se vere water deficit. Upon illumination of thylakoid membranes containin g Tiron, we found first order kinetics of formation and decay of the T iron semiquinone radical. At each stress intensity a higher rate of su peroxide formation than in the control was observed. The different dec ay rates suggested a changed chemical environment around the radical. In comparison with the control, the levels of carotenoids and hydropho bic proteins in the thylakoids decreased, and the lipid/protein ratio increased after a mild water deficit. After moderate drought, the incr eased levels of hydrophilic proteins and carotenoids seem unsufficient to limit the improved capacity to leak electrons from the thylakoids. After a severe water stress, a decrease in unsaturation and in linole nic acid as well as a loss of lipolytic antioxidants took place, which may have rendered the thylakoids more sensitive to attack by activate d forms of oxygen. The results suggest that during water deficit, the interactions among membrane components change in relation to a changed chemical composition. Such changes are probably among the causes for changes in the conformation of proteins and in the position of various molecular species in the lipid bilayer, so that they become more expo sed to molecular oxygen.