Selenium-induced enhancement in carbohydrate metabolism in nodulated alfalfa (Medicago sativa L.) as related to the glutathione redox state

This study was undertaken to investigate the factors explaining the results of our previous study revealing that selenium (Se) may enhance the nodulation of alfalfa (Medicago sativa L.) via improved carbohydrate metabolism. Symbiotic and non-symbiotic alfalfa plants were cultivated without or with low (1 mu M) Se addition at low (2mM) and adequate (10mM) N supply levels. The plants were analyzed for root nodulation, photosynthesis and growth parameters. The antioxidative defense and glutathione (GSH) redox state were also investigated. In the inoculated low-nitrogen (N) plants, Se markedly increased the number and fresh weight (FW) of the nodules without any effect on nitrogenase activity. Particularly in the nodulated low-N plants, it also significantly increased the net assimilation rate and soluble sugars, and enhanced protein synthesis. Though Se generally diminished the activity of antioxidative enzymes, the concentrations of hydrogen peroxide (H2O2) and superoxide radical (O-2(center dot -)) remained constant or even significantly diminished. This gives evidence that Se enhanced the protection of plants against oxidative damage. Se tended to increase GSH particularly in the leaves, and significantly increased the reduced glutathione/oxidized glutathione (GSH/GSSG) particularly in the low-N plants. We suggest that Se exerts its positive effect on the carbohydrate metabolism via modulation in the glutathione redox state.