Relative importance of Na+ and Cl– in NaCl-induced antioxidant systems in roots of rice seedlings

The present study examined the response of antioxidant systems to NaCl stress and the relative importance of Na⁺ and Cl^– in NaCl-induced antioxidant systems in roots of rice seedlings. NaCl treatment caused an increase in the activities of ascorbate peroxidase (APX) and glutathione reductase (GR) in roots of rice seedlings, but had no effect on the activities of superoxide dismutase (SOD) and catalase (CAT). There were detectable differences in APX and GR isoenzymes between control and NaCl-treated roots. Levels of activity for SOD and CAT isoenzymes did not change in NaCl-stressed roots compared with the control roots. NaCl treatment produced an increase in H₂O₂, ascorbate (AsA), dehydro-ascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) levels. Treatment with 50 m M Na-gluconate (whose anion is not permeable to membrane) led to a similar Na⁺ level in roots to that with 100 m M NaCl. It was found that treatment with 50 m M Na-gluconate affected H₂O₂, AsA, and DHA levels, APX and GR activities, OsAPX and OsGR mRNA induction in the same way as 100 m M NaCl. These observed changes seem to be mediated by Na⁺ toxicity and not by Cl^– toxicity. On the other hand, it was found that NaCl, but not Na-gluconate and NaNO₃, caused an increase in GSH and GSSG levels, indicating that Cl^–, rather than Na⁺, is responsible for the NaCl-increased GSH and GSSG levels in roots of rice seedlings.