Methyl jasmonate enhances salt tolerance of almond rootstocks by regulating endogenous phytohormones, antioxidant activity and gas-exchange

The effects of methyl jasmonate (MeJA) foliar application (0, 0.025, 0.050 and 0.075 mM) on the growth and physiological responses of two almond rootstocks (GF677 and bitter almond) exposed to various concentrations of NaCl in irrigation water (0, 50, 100 and 150 mM) were evaluated. 60 days after salt stress exposure, the mitotic index of root apical meristem cells as well as shoot and root growth, activity of main antioxidant enzymes, gas exchange parameters and contents of cytokinins and ABA were determined. Salt stress decreased the plants' growth, particularly at higher levels. Application of MeJA in optimal concentrations of 0.025 to 0.05 mM alleviated the adverse effect of salt stress by increasing the photosynthetic rate, activity of antioxidant enzymes (APX, SOD and PDX), root and shoot dry mass, as well as cell membrane integrity. Furthermore, MeJA application brought about a two-fold increase in the concentration of leaf cytokinins. This reposition of cytokinins was due to restriction of both the activity of cytokinin oxidase and gene expression of this enzyme. The MeJA mitigating effect on the growth of salt-stressed plants could be a result of the inhibition of cytokinin decline under salt stress. The results revealed the effective impact of endogenous cytokinins in protective and growth improvement effects of MeJA on almond rootstocks under salt stress.