Sagi, M., Ramat Negev Desert Agroresearch Ctr., D.N Halutza 85515, Israel
Dovrat, A., Hebrew University of Jerusalem, Dept. Fld. Crops, Vegetables Genet., Faculty of Agriculture, Rechovot, 76100, Israel
Kipnis, T., Agricultural Research Organization, Volcani Center, P.O. Box 6, Bet Dagan, Israel
Lips, H., Biostress Research Laboratory, Department of Life Science, Ben-Gurion University of the Negev, Sede Boqer 84990, Israel

The concentration of organic acids, organic nitrogen (N), nitrate (NO3), and total cations increased in annual ryegrass (Lolium multiflorum Lam.) with salinity and N concentration in the growth medium. Increasing salinity and N in the growth medium induced changes in the level of key enzymes of N assimilation and organic acids: nitrate reductase (NR, EC 1.6.6.1), phosphoenolpyruvate carboxylase (PEPc, EC 4.1.1.31), and glutamine synthetase (GS, EC 6.3.1.2). Plants grown in pots filled with sand were irrigated with nutrient solutions with an electroconductivity of 2 or 11.2 dS m-1 and N applied as ammonium nitrate (NH4NO3), sodium nitrate (NaNO3), or ammonium applied as ammonium nitrate (NH4NO3), sodium nitrate (NaNO3), or ammonium (NH4) as ammonium sulfate [(NH4)2SO4] at concentrations of 0.5, 4.5 or 9.0 mM. Nitrate reductase, PEPc, and GS increased with salinity and N level. Shoot NR was highest in the presence of NH4NO3 irrespective of salinity level, while root NR activity responded best to NO3. Enhancement of PEPc activity in both shoots and roots was highest with NH4NO3 and lowest with NH4. Nitrogen source had no significant effect on GS activity in shoots or roots of ryegrass. Shoot NR activity increased with NO3 concentration in the tissue, as calculated from regression coefficients. The PEPc activity correlated positively with total cations and NO3 concentrations in the plants, irrespective of the salinity level, suggesting that the increase in total cations and NO3 induced by salinity may have triggered the changes in enzyme activities. The concentration of organic acids in both shoots and roots correlated positively with PEPc activity irrespectively of the salinity level. The PEPc activity was higher in roots than in shoots, while organic acid concentration was higher in shoots. These results suggest that a significant part of the organic acids produced in the roots were used as carbon skeleton for transamination reactions. The increased activity of NR, PEPc, and GS in roots may constitute part of an adaptation strategy of the plant to increasing salinity in the medium. These enzymes have an important role in the metabolism of amino acids and the synthesis of organic N in annual ryegrass irrigated with saline water, and boosting them with suitable N fertilizers could increase the nutritional value and protein content of the crop.