Effect of excess manganese on the antioxidant system in Cucumis sativus L. under two light intensities

Effects of excess manganese (Mn) on plant growth, lipid peroxidation, contents of Mn, Mg and Fe, activities of antioxidant enzymes and contents of ascorbate and glutathione were studied in cucumber plants under two (optimum and suboptimum) light intensities. The plant growth was inhibited by excess Mn, and the inhibition induced by excess Mn was much bigger under optimum light intensity than under low (suboptimum) light intensity. Excess Mn significantly increased the Mn content and reduced the Mg and Fe content in the leaves, particularly under optimum light intensity. Excess Mn increased accumulation of thiobarbituric acid reactive substances (TBARS) and low light intensity decreased TBARS concentration. The activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR) and dehydroascorbate reductase (DHAR) were increased by excess Mn, especially more significantly under optimum light intensity. The contents of reduced ascorbate (ASC), dehydroascorbate (DHA), total ascorbate, reduced glutathione (GSH), oxidized glutathione (GSSG) in the leaves were higher under optimum light intensity than under low light intensity, and decreased by excess Mn, particularly more considerably under optimum light intensity. It may be concluded that enhanced oxidative stress and increased Mn content be responsible for increased growth inhibition by excess Mn, especially under optimum light intensity.