T HE EFFECTS of treatment with 100mM NaCl on photosynthetic activity, protein and proline contents , activities of key nitrogen metabolism enzymes, nitrate reductase (NR) and nitrite reductase (NiR), ammonia assimilating enzymes: Glutamine synthetase (GS), glutamate synthase (GOGAT) and glutamate dehydrogenase (NADH-GDH) activities and finally ammonia and nitrate contents were investigated in 40-day old leaves of tomato plants. There was a decline in total protein (TP) and insoluble protein (InsP) fractions accompanied with a significant increase in proline and soluble protein contents in response to NaCl-stress. Under salinized conditions, there was a significant inhibition in all tested leaf gas exchange parameters, stomatal conductance (g s), internal CO 2 concentration (Ci) and hence, CO 2 assimilation (A). Also, a significant inhibition of NR & NiR enzymes and a strongly decrease in nitrate content were observed. In contrast, ammonia assimilating enzymes (GS, GOGAT, NADH-GDH and NAD-GDH) activities were obviously increased in NaCl-salinized tomato leaves, this accompanied with a significant increase in ammonia content. Soaking tomato seeds in 10µM sodium nitroprusside (SNP) for 8h were elevated-to some extent-all the studied parameters and there was an improvement in TP, proline, all gas exchange parameters, NR & NiR activities and nitrate content. While ammonia assimilating enzymes (GS, GOGAT, NADH-GDH and NAD-GDH) activities and ammonia content were significantly decreased compared to NaCl-salinized tomato leaves.

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