Exogenous proline reduces NaCl-induced damage by mediating ionic and osmotic adjustment and enhancing antioxidant defense in Eurya emarginata

Proline accumulation at the cost of growth inhibition is commonly observed in plants subjected to salt stress, because an increased amount of energy is stored as nitrogen. The hypothesis tested in this study was that exogenous proline addition will alleviate deleterious effects of high salinity on plant growth. Eurya emarginata plants were treated with 10 mM proline and 200 mM NaC1 for 35 days. Growth-related parameters were determined in leaves, including fresh weight, tissue water content, concentrations of malondialdehyde (MDA), proline, Nat, and Kt, and activities of superoxide dismutase, catalase, peroxidase, glutathione peroxidase (GPX), H + -ATPase, pyrroline-5-carboxylate synthetase (P5CS) and proline dehydrogenase (PDH). Exogenous proline increased fresh weight, endogenous proline and Kt concentrations, and reduced Nat and MDA concentrations in under salt treatment. These effects were not observed in non-salt-treated soil. Increased Kt and decreased MDA concentrations were associated with increased Ht-ATPase activity and increased activities of antioxidant enzymes except GPX, respectively. Although P5CS activity was sharply reduced and PDH activity remained unchanged, exogenous proline compensated for the decrease in endogenous synthetic proline, indicating more energy stored as nitrogen would be used for growth. Correlations between enzyme activities and MDA or ion concentrations were observed, indicating that changes at the enzyme level may underlie the patterns of salt stress, and accordingly affect accumulation of ions and peroxide. Thus, exogenous proline significantly affected the salinity tolerance of Eurya emarginata, through diverse protective effects on water relations, ionic and osmotic adjustment, and antioxidant defense.