Salinity-induced effects on nitrogen assimilation related to growth in cowpea plants

This study was carried out to establish relationships between nitrate and ammonia assimilation and growth of cowpea (Vigna unguiculata (L.) Walp) plants when exposed to NaCl-salinity. Cowpea plants were initially pre-acclimated under a mild NaCl-stress (50 mol m(-3)) during 8 days in order to induce salt acclimation. Subsequently, pre-acclimated and non-acclimated plants received a NaCl-osmotic shock (100 mol m(-3)) in the nutrient solution for 4 days. Although pre-acclimated plants exhibited lower NO3- uptake rate, in situ nitrate reduction, nitrate reductase (NR) activity, and shoot growth, they did not show alterations in these parameters, expressed as % of control, during the time-course experiment. These data suggest a salt adaptative response to a mild NaCl-salt stress. In contrast, addition of high level of NaCl (100 mol m(-3)) induced a steady decrease in nitrate uptake and assimilation parallel to a reduction in the shoot growth, particularly when the salt-shock was applied on the non-acclimated plants. On the other hand, the leaf glutamine synthetase activity, free amino acids and proline concentration showed a slight increase in all of salt treatments. The leaf soluble protein was slightly reduced by salt treatments during the short-term exposure. Moreover, the plants pre-acclimated with NaCl exhibited lower concentrations of protein and amino acids when compared with non-acclimated plants. Our study indicates that the regulation processes of N acquisition and growth of cowpea plants do operate at the whole plant level resulting in N homeostasis, especially in plants exposed to a mild-salt stress. Furthermore, the nitrate uptake and leaf-NR activity are more limiting for shoot growth than the primary ammonia assimilation from the glutamine synthetase. (C) 2001 Published by Elsevier Science B.V.