Effect of salt stress on antioxidant defence system in soybean root nodules

Abstract

The antioxidant defence systems of soybean (Glycine max (L.) Merr) nodules responded differently to 50 and 200 mM NaCl. At 50 mM NaCl, leghaemoglobin content and nitrogenase activity remained unaltered but there was an overall increase in the antioxidant enzymes (ascorbate peroxidase, catalase, glutathione reductase and superoxide dismutase) and in reduced glutathione. After returning the salinised nodules to a non-saline environment (recovery), the enzymatic activities returned to the initial values but reduced glutathione remained high with respect to the controls measured at the end of the experiment (final controls). Severe salt treatment reduced the leghaemoglobin content and nitrogenase activity by 31% and 50%, respectively. Ascorbate peroxidase, catalase and glutathione reductase activities decreased between 30 and 100% while superoxide dismutase and reduced glutathione increased over the controls by 19% and 30% respectively. After recovery, glutathione reductase increased over the final controls and reduced glutathione remained as under 50 mM NaCl. Malondialdehyde content and total protein remained unchanged in nodules treated with the two salt concentrations. These results suggest that under mild saline stress, the elevated levels of the antioxidant enzymes and reduced glutathione protect nodules against the activated oxygen species thus avoiding lipid and protein peroxidation, and leghaemoglobin breakdown. However, severe saline treatment produced an irreversible decay in the leghaemoglobin content and nitrogenase activity despite the high reduced glutathione level and glutathione reductase activity.