DROUGHT INDUCES OXIDATIVE STRESS IN PEA-PLANTS

Pea (Pisum sativum L. cv. Frilene) plants subjected to drought (leaf water potential of approximate to -1.3 MPa) showed major reductions in photosynthesis (78%), transpiration (83%), and glycolate oxidase (EC 1.1.3.1) activity (44%), and minor reductions (approximate to 18%) in the contents of chlorophyll a, carotenoids, and soluble protein. Water stress also led to pronounced decreases (72-85%) in the activities of catalase (EC 1.11.1.6), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2), but resulted in the increase (32-42%) of nonspecific peroxidase (EC 1.11.1.7) and superoxide dismutase (EC 1.15.1.1). Ascorbate peroxidase (EC 1.11.1.11) and monodehydroascorbate reductase (EC 1.6.5.4) activities decreased only by 15% and the two enzymes acted in a cyclic manner to remove H2O2, which did not accumulate in stressed leaves. Drought had no effect on the levels of ascorbate and oxidized glutathione in leaves, but caused a 25% decrease in the content of reduced glutathione and a 67% increase in that of vitamin E. In leaves, average concentrations of catalytic Fe, i.e. Fe capable of catalyzing free-radical generation by redox cycling, were estimated as 0.7 to 7 mu M (well-watered plants, depending on age) and 16 mu M (water-stressed plants); those of catalytic Cu were approximate to 4.5 mu M and 18 mu M, respectively. Oxidation of lipids and proteins from leaves was enhanced two- to threefold under stress conditions and both processes were highly correlated. Fenton systems composed of the purported concentrations of ascorbate, H2O2, and catalytic metal ions in leaves produced hydroxyl radicals, peroxidized membrane lipids, and oxidized leaf proteins. It is proposed that augmented levels and decompartmentation of catalytic metals occurring during water stress are responsible for the oxidative damage observed in vivo.