Effects of NaCl stress on nitrogen metabolism of cucumber seedlings

To investigate the effects of NaCl stress on plant growth, nitrogen assimilation, proline and soluble protein contents, and gene expression of enzymes of nitrogen metabolism, a hydroponic experiment using cucumber (Cucumis sativus L.) seedlings was performed. The seedlings were grown in nutrient solution supplemented with 0 or 84 mM NaCl for up to 9 days. Plant biomass, especially root biomass, was significantly decreased under NaCl stress. Salinity significantly increased ammonium content, but decreased nitrate and soluble protein contents in leaves and roots. Salt stress caused a significant increase in proline content, which peaked on day 3 of NaCl treatment. Moreover, salt stress significantly decreased activities of nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) in the roots and leaves on the 3rd, 6th, and 9th day of NaCl treatment. A semiquantitative RT-PCR approach showed that changes in NR, GS and GOGAT gene expression were consistent with the salt-induced changes in enzyme activities. These results suggest that salt stress-induced growth inhibition in cucumber seedlings may involve disruption of nitrogen absorption and decreased activities of enzymes associated with nitrogen assimilation.