Brassinosteroid effects on some physical and biochemical properties and secondary metabolite accumulation in peppermint (Mentha piperita L.) under salt stress

• Peppermint plants were negatively affected by salt stress.
• Salt stress enhanced total phenolic contents but it decreased the growth parameters and total essential oil content in peppermint.
• Brassinosteroid contributed to the plant growth and development without any toxic effects in salt free plants.
• Brassinosteroid applications reduced the severity of damages caused by salt stress.
• When applied the suitable concentration, brassinosteroid had positively impact on secondary metabolite production in salt treated peppermint.

Brassinosteroids are plant steroidal phytohormones playing important roles in plant growth, secondary metabolite accumulation, stress responses and adaptation. This study was carried out to determine the effect of brassinosteroid applications on the growth, development, biochemical properties and secondary metabolite accumulation in peppermint (Mentha piperita L.) plants grown at different levels of salt. For this aim, three different NaCl (0, 100 and 150 mM) and four different brassinosteroid (0, 0.5, 1.5 and 2.5 mg l−1) concentrations were applied to peppermint plants. As a result of the study it was demonstrated that all of the investigated parameters were affected by the applications of salt and brassinosteroid. Fresh and dry weights of aerial parts and dry leaf weight decreased in line with the elevating level of salinity. Insomuch as lots of plants died at 150 mM NaCl. An increase in salinity led to reduction in essential oil content while lipid peroxidation, proline, antioxidant enzyme activities and total phenolic contents increased depending on rising of the salt concentrations. Brassinosteroid applications prevented the death of the plant even at 150 mM NaCl and obstructed adverse effects resulting from salt by improving the antioxidant enzymes activities and decreasing the membrane permeability and lipid peroxidation. Also brassinosteroid significantly increased the essential oil content in salt conditions. Brassinosteroid contributed to the plant growth and development without showing toxic effects and 0.5 mg l−1 of brassinosteroid significantly increased the essential oil content in nonstressed plants. It was determined that 0.5 mg l−1 of brassinosteroid for 100 mM NaCl and 2.5 mg l−1 of brassinosteroid for 150 mM NaCl were the most suitable concentrations in terms of injury degree, dry leaf weight, total phenolic contents and essential oil content in peppermint plants.