EFFECT OF NITROGEN NUTRITION SOURCE ON ANTIOXIDANT DEFENSE SYSTEM OF SOYBEAN PLANTS SUBJECTED TO SALT STRESS

Salt stress is major abiotic stress problem in arid and semi-arid regions and irrigation areas. Approximately 40% of the world's cultivated land, and nearly half of the irrigated land is affected by high salt contents. Salt stress affects plant physiology at both whole-plant and cellular levels through osmotic and ionic stress. A secondary effect of salt stress is the increase of reactive oxygen species (ROS), which include singlet oxygen, superoxyde anion radicals, hydroxyl radicals, and hydrogen peroxide. The effect of the source of nitrogen nutrition (nitrogen fixation or nitrate assimilation) on the response of antioxidant system of soybean plants subjected to salt stress was studied. Activity of some stress defense enzymes (catalase, peroxidase, and SOD), GSH and GSSG in nitrogen-fed and nitrogen fixing plants was measured. Catalase and peroxidase activity was higher in the leaves of nitrogen fixing plants compared to nitrate-fed. Therefore, symbioses showed different tolerance level under salinity condition which was essentially correlated with symbiotic performance at nonstressful conditions.