Proline Metabolism and Cross-Tolerance to Salinity and Heat Stress in Germinating Wheat Seeds

Germination/growth of wheat (Triticum aestivum L., cv. Zimai 1) seeds and changes in the levels of proline and protein as well as in activities of key enzymes involved in proline metabolism in response to salinity-, heat-stresses and their cross-stress were studied. With decreasing water potential caused by increasing concentrations of NaCl, germination percentage, fresh weight of seedlings and protein amount markedly decreased, whereas proline amount slightly increased. The activities of pyrroline-5-carboxylate synthetase (P5CS), ornithine aminotransferase (OAT), and proline dehydrogenase (PDH) peaked at −0.2 MPa water potential. Germination percentage and amounts of proline and protein increased as germination temperature elevated to 25°C from 15°C, and decreased above 25°C; fresh weight of seedlings increased to 30°C from 15°C, and decreased above 30°C. However, the activities of P5CS, OAT and PDH gradually decreased with elevaing temperature. Seeds pretreated at 33°C or in −0.8 MPa NaCl solution for various time length increased tolerance to subsequent salt + water stress or heat stress, as measured by germination percentage and fresh weight of seedlings 5 days after beginning of experiment. The acquisition of cross-tolerance resulting in limitation of negative stress effects does not relate directly to proline level and activities of P5CS, OAT and PDH involved in proline metabolism. Proline amount as measured four days or later after stress imposition cannot be considered a symptom of salt-, water- and heat-stress injury or an indicator of the resistance.