Salt Tolerance Induced by Exogenous Proline in Maize Is Related to Low Oxidative Damage and Favorable Ionic Homeostasis

We studied the ability of exogenous proline to counteract the effects of salt in maize plants (Zea mays). Ten-day-old plants were cultivated in Clark nutrient solution in either the absence or presence of salinity (NaCl at 80 mM) and sprayed with distilled water (control) or 30 mM proline solution. The most striking effects of exogenous proline were registered after 14 days of treatment. Proline was found to alleviate the salt damage to plant growth, a response closely related to reduced Na+ and Cl− accumulation together with increased K+ content. Also, exogenous proline efficiently activated the antioxidant systems by increasing both activity of catalase, ascorbate peroxidase, and superoxide dismutase and levels of reduced ascorbate and glutathione. Proline-sprayed stressed plants displayed a greater accumulation of this amino acid, which was correlated with reduced Δ1-pyrroline-5-carboxylate synthetase activity and increased proline dehydrogenase activity. As a result, proline-sprayed stressed plants showed decreases in H2O2 content and lipid peroxidation compared to only stressed plants. Our findings clearly evidence that proline treatment results in favorable changes in inorganic solute content and antioxidative systems, reducing salt-induced oxidative damage and improving salt acclimation in maize plants.