Antioxidant defense in the leaves of C3 and C4 plants under salinity stress

The effect of salt stress (50, 100 and 150 m M of NaCl) on the activity of superoxide dismutase (SOD, EC. 1.15.1.1), ascorbate peroxidase (APX, EC. 1.11.1.11), glutathione reductase (GR, EC. 1.6.4.2) enzymes and also on the rate of lipid peroxidation in terms of thiobarbituric acid-reactive substances (TBARS) content and photosynthetic capacity in two wheat (C3 plants) and two maize (C4 plants) varieties was studied. In the non-salined control plants, the antioxidant enzymes activities were significantly higher for maize than for wheat. Adding salt to the nutrient solution increased the level of antioxidants in leaves of both maize and wheat. The first substantial response to salinity was found for SOD on the 2nd day, whereas changes occurred for APX on the 4th day and for GR on the 4th/5th day of salt treatment. Although SOD activity increased considerably more in wheat (C3), it never reached as high levels as in maize (C4) grown in the same treatment combinations. The total increase in APX activity was similar for wheat and maize, whereas GR activity was higher in leaves of maize. Lipid peroxidation analyses showed an increase in TBARS contents in both plants' species grown under salinity that corresponded to the damage that occurred in secondary oxidative stress. However, as a result of advanced antioxidant defense in maize, the TBARS quantities did not elevate to as high level as in wheat. Chlorophyll fluorescence measurements revealed a considerable decrease in the efficiency of PS II and electron-transport chain (ETC). Assimilation rate of CO₂ decreased in both plant groups; however, in C4 maize, we observed a much better capacity to preserve the photosynthetic apparatus against overproduction of ROS. Results suggest that efficient antioxidant defense plays an important role in maize, the C₄ plant, resistance to environmental stresses like salinity or drought.