Exogenous application of glutamic acid promotes cucumber (Cucumis sativus L.) growth under salt stress conditions

Salinity is expected to be the major destructive abiotic stress that causes ionic and oxidative damage leading to growth reduction and ultimately plant death. Glutamic acid (GA) is an alpha-amino acid that is used by almost all living beings in the biosynthsis of proteins. Therefore, in the present study, we tried to investigate the effect of foliar application of glutamic acid (GA) on cucumber (Cucumis sativus L.) under altered salinity levels. Cucumber seedlings were grown in plastic pots under greenhouse conditions by applying four levels of salinity (0, 3 dS/m, 6 dS/m and 12 dS/m) and two levels of foliar applied GA (0, 10 mM). Salinity was induced by mixing the salt and soil before seed sowing; however, exogenous GA was applied when the vine length was reached up to maximum height. Morphological characters showed disruptive response under saline conditions especially in indigenous cultivar (local cucumber represented as V1). Enhanced activities of superoxide dismutase (0.29 u g(-1) FW), guaiacol peroxidase (3.51 u g(-1) FW) and ascorbate peroxidase (0.39 mu mol AsA.mg(-1) Chl min-(1)) were observed in salt-stressed cucumber leaves. Both varieties showed unusual behavior for malondialdehyde in decreasing manner with increasing salinity levels (2.0333 mu mol g-1 FW at 12dS/m in local cultivar; while, 1.98 mu mol g(-1) FW at 12dS/m in hybrid cultivar SSC-228). However, exogenously applied GA played a beneficial role in promoting all morphological parameters under stress with increasing scavenging abilities against reactive oxygen species. Foliar application of GA improved plant defense mechanism with minimum destruction. Remarked calculations showed that under salt stress, GA improved plant stress tolerance against salinity by maximizing the growth rate.