Regulation of proline metabolism and antioxidant system by tocopherol and cysteine mitigates salt stress in fenugreek

The effect of exogenously supplied α-tocopherol (200 µM Toc) and cysteine (500 µM Cys) was explored in reducing the NaCl triggered growth alteration in Trigonella foenum-graecum . Salt stress reduced the growth characteristics like height, fresh, and dry weight of the shoot and root; however, the Toc and Cys application alleviated the decline. The application of Toc and Cys mitigated the reduction of chlorophylls and carotenoids due to NaCl stress. The salt stress induced decline in the content of nitrogen and potassium was alleviated by the applied Toc and/or Cys. The content of sodium was less in Toc and Cys treated seedlings. NaCl stress significantly increased the oxidative stress attributes like hydrogen peroxide and lipid peroxidation; however, foliar application of Toc and Cys reduced these parameters and also reduced the activity of lipoxygenase (LOX) and NADPH oxidase, with the highest decline exhibited by plants receiving their combined treatment. Exogenous Toc and Cys differentially regulated the proline metabolism by increasing the activity of pyrroline-5-carboxylate synthetase (P5CS) and γ-glutamyl kinase (γ-GK), and decreasing the activity of proline oxidase (PROX). In addition, salt stress increased the activity of enzymes of the antioxidant system (SOD by 95.51%, APX by 126.21%, MDHAR by 85.68%, DHAR by 99.67%, and GR by 132.06%) and the enzymes of the glyoxylase cycle (glyoxylase I by 104.94% and glyoxylase II by 100.25%), and exogenous Toc and Cys caused further increase in their activities. The content of methionine, glutathione, ascorbic acid (AsA), Toc, and Cys increased due to exogenous Toc and Cys treatments, further increasing the functioning of the antioxidant system. Salt stress increased the endogenous concentrations of nitric oxide (NO) by 98.16% and hydrogen sulfide (HS) by 145.55% however, treatment of Toc and/or Cys to NaCl-treated plants imparted a decline in NO and HS. Hence, the exogenous Toc and/or Cys treatment can be exploited to benefit fenugreek growth and metabolism under salt stress.