Antioxidant responses of rice seedlings to salinity stress

The possible involvement of activated oxygen species in the mechanism of damage by NaCl stress was studied in leaves of four varieties of rice (Oryza sativa L.) exhibiting different sensitivities to NaCl. The 3-week-old rice seedlings were subjected to 0, 6 and 12 dS m−1 salinity levels for 1-week after which differences in antioxidant capacities and possible correlation, growth rate and Na+ uptake of the leaves were analyzed. High salinity treatment caused a decrease in growth rate in all the varieties tested except Pokkali. The salt-sensitive varieties, Hitomebore and IR28, exhibited a decrease in superoxide dismutase activity and an increase in peroxidase activity under high salinization. These varieties also exhibited increase in lipid peroxidation and electrolyte leakage as well as higher Na+ accumulation in the leaves under salt stress. The salt-tolerant variety Pokkali however, showed only slight increase and decrease in superoxide dismutase and peroxidase activity, respectively, and virtually unchanged lipid peroxidation, electrolyte leakage and Na+ accumulation upon salinization. On the other hand, the putative salt-tolerant Bankat variety, which showed a slight stimulation in growth rate similar to Pokkali at moderate salinity level, exhibited Na+ accumulation and symptoms of oxidative damage during salt stress similar to the salt-sensitive varieties rather than the salt-tolerant one. These results indicate that free radical-mediated damage of membrane may play an important role in the cellular toxicity of NaCl in rice seedlings and that salt-tolerant varieties exhibit protection mechanism against increased radical production by maintaining the specific activity of antioxidant enzymes.