NaCl stress effects on enzymes involved in nitrogen assimilation pathway in tomato "Lycopersicon esculentum" seedlings

Tomato plants (Lycopersicon esculentum Mitt, cv. Chibli F1) grown for 10 days on control medium were exposed to differing concentrations of NaCl (0, 25, 50, and 100 mM). Increasing salinity led to a decrease of dry weight (DW) production and protein contents in the leaves and roots. Conversely, the root to shoot (R/S) DW ratio was increased by salinity. Na+ and Cl- accumulation were correlated with a decline of K+ and NO3- in the leaves and roots. Under salinity, the activities of nitrate reductase (NR, EC 1.6.6.1) and glutamine synthetase (GS, EC 6.3.1.2) were repressed in the leaves, while they were enhanced in the roots. Nitrite reductase (NiR, EC 1.7.7.1) activity was decreased in both the leaves and roots. Deaminating activity of glutamate dehydrogenase (GDH, EC 1.4.1.2) was inhibited, whereas the aminating function was significantly stimulated by salinity in the leaves and roots. At a high salt concentration, the nicotinamide adenine dinucleotide reduced (NADH)-GDH activity was stimulated concomitantly with the increasing NH4+ contents and proteolysis activity in the leaves and roots. With respect to salt stress, the distinct sensitivity of the enzymes involved in nitrogen assimilation is discussed. (c) 2005 Elsevier GmbH. All rights reserved.