Exogenous salicylic acid and 24-epi-brassinolide improve antioxidant capacity and secondary metabolites of Brassica nigra

One of the important aspects of salt tolerance is the regulatory roles of some plant growth regulators. The objective of this work was to evaluate the capability of salicylic acid and 24-epi-brassinolide to induce salt tolerance in black mustard (brassica nigra L.). Treatments were four levels of salinity (0, 4, 8 and 12 dS m−1 NaCl as non-salinity and low, moderate and severe salinities) and plant growth regulators (control, 1 mM salicylic acid and 0.1 μM 24-epi-brassinolide). Root and shoot Na contents, peroxidase and superoxide dismutase activities, lipid peroxidation, proline and soluble sugars were enhanced, but root and shoot K, Ca and K/Na ratio, and leaf chlorophyll and carotenoid contents were reduced due to salt stress. Foliar sprays of salicylic acid and 24-epi-brassinolide increased K, Ca and K/Na ratio, chlorophylls a and b, catalase activity and soluble sugars, particularly under moderate and severe salinities. Phenolic compounds and flavonoids of black mustard were significantly increased with increasing salinity. The production of these secondary metabolites was also stimulated by growth regulators. Anthocyanin content was enhanced under low salinity, while it was reduced under severe salinity. Exogenous application of growth regulators elevated anthocyanin content under all salinity levels. In general, physiological performance, antioxidant capacity and secondary metabolites of black mustard were considerably enhanced by foliar spray of salicylic acid and 24-epi-brassinolide under salt stress.