Effect of salicylic acid on proline metabolism in lentil grown under salinity stress

The objective of the present work was to determine the effect of salicylic acid (SA, 0.5 mM) on plant growth parameters, free osmolytes (proline and glycinebetaine) and enzyme activities involved in proline metabolism, namely, pyrroline-5-carboxylate reductase (P-5-CR), proline oxidase and gamma-glutamyl kinase enzyme activities in shoot and root tissues of lentil plants grown in saline and nonsaline control. SA markedly improved germination under salt stress. Exogenously applied SA resulted increased plant growth significantly both in saline and non-saline conditions. The magnitude of increase in free proline accumulation was significantly higher in 100 mM NaCl but highly significant with the addition of 0.5 mM SA to 100 mM NaCl in shoots. Proline was found by 5.0 folds increased in shoots and 1.1 folds only in root with 100 MM NaCl + 0.5 mM SA treatment on day 10 of plant growth over that of non-saline control. Shoot always maintained higher level of free proline contents than the root. Pyrroline-5-carboxylate reductase (EC: 1.5.1.2) and gamma-glutamyl kinase (EC: 2.7.2.11) activities increased in all three treatments than the non-saline control and the magnitude of increase was found more in 100 mM NaCl + 0.5 mM SA than the others treatments. In addition, the proline oxidase (EC: 1.5.99.8) activity was inhibited in all treatments. Nevertheless, the reduction in the activity was more in 100 mM NaCl + 0.5 mM SA than the others. The results suggested that during the exogenously applied SA to salt stress, proline metabolism was significantly altered and the extent of alteration varied between the SA and salt stress, leading to the maintenance of the turgor by accumulating higher levels of free proline in lentil, supporting its protection from salt stress. Further, 100 mM NaCl + 0.5 mM SA treatments to lentil was evident from the higher level of glycinebetaine (GB) compared with non-saline control or/others treatments. Therefore, the enhanced activities of P-5-CR and gamma-glutamyl kinase, and proline content in response to NaCl and/or SA treatment, whereas their interaction had an additive effect. Moreover, the toxic effects generated from 100 mM NaCl was completely overcome by the application of SA. SA treatments can ameliorate the negative effect of salinity on the growth of lentil and SA could be used as a potential growth regulator to improve plant salinity tolerance. It was, therefore, concluded that SA ameliorated the stress generated by NaCl through the alleviated proline metabolizing system. Overall, the adverse effects of salt stress could be alleviated by exogenous application of SA. (C) 2009 Elsevier Ireland Ltd. All rights reserved.