ANTIOXIDANT RESPONSE OF TWO SALT-STRESSED BARLEY VARIETIES IN THE PRESENCE OR ABSENCE OF EXOGENOUS PROLINE

The antioxidant response to salt stress as well as the effect of proline metabolism on the activity of antioxidant enzymes were investigated in two barley varieties (Hordeum vulgare L. var. Sahand and var. Makoui) after exposure to salt stress and exogenous proline. The MDA content of Makoui plants grown under different salt regimes remained nearly constant but it strongly increased in Sahand plants under the same conditions. There was a linear and significant increase in CAT, APX, DHAR, and GR activities in Makoui plants in response to increased salt concentration. The strong and positive correlation between antioxidant enzymes and salt concentrations may account for the MDA level in Makoui plants remaining constant in response to different salt regimes. The MDA level in Sahand plants grown at 150 and 200 mM NaCl decreased significantly in response to exogenous proline. This decrease was independent of SOD and APX activities. At the same time, there was a significant decrease in DHAR activity and an increase in GR activity under the same conditions. These results show that proline is able to quench oxygen radicals not only chemically but also by its effect on the activity of DHAR and GR. The PDH activity in Makoui variety decreased significantly in response to 150 mM NaCl and disappeared in plants grown at 200 mM NaCl. The PDH activity in Sahand plants was very high under all salt regimes. Sahand plants may have an active proline cycle, though highly active proline cycling in plants is usually detrimental during stressful