Plant growth, metabolism and adaptation in relation to stress conditions: Further studies supporting nullification of harmful effects of salinity in lettuce plants by urea treatment

Foliar application of urea to lettuce plants induced pronounced changes in the total amount and in the relative composition of the nitrogen pool. As compared with untreated lettuce plants, urease (UR), nitrate reductase (NR), asparaginase (AS) and glutamine synthetase (GS) activities were, in general, increased with an increase in the concentration of urea. On the other hand, salinization of lettuce plants with NaCl induced a significant decrease in the activities of UR, NR, AS and GS, at vegetative and adult growth stages. In general, treatment of lettuce plants with increasing concentrations of urea fertilizer in combination with each of the levels of salinity resulted in significant increases in all enzyme activities. Treatment with increasing concentrations of urea fertilizer induced significant decreases both in glycine and proline contents below control levels. On the other hand, salinization of lettuce plants with NaCl induced significant pronounced increases in the contents of the two amino acids. Further increments in glycine and proline contents were observed in differently salinized lettuce plants foliary sprayed with increasing concentrations of urea at vegetative and adult growth stages. Salinity solely enhanced the occurrence of novel proteins that were detected neither in the water control nor in the urea-treated plants. Protein banding patterns of lettuce plants treated with urea either alone or in combination with NaCl showed different de novo protein bands with different molecular weights, induced by urea and/or NaCl at vegetative and adult growth stages.