Peppermint (Mentha piperita L.) is an economically important medicinal and aromatic plant grown in different areas worldwide. Secondary metabolites were fundamentally produced by genetic processing; however, environmental factors affect their biosynthesis. Salinity is the most important abiotic stress which induces morphological, physiological, and biochemical changes in plants. To investigate the influence of salinity stress (0, 25, 50, 75, 100 and 125 mM NaCl) on chlorophyll content, stomatal conductance, relative water content (RWC), proline, Na+ and K+ content, antioxidant enzymes of catalase (CAT), peroxidase (POX) and polyphenol oxidase (PPO), protein, essential oil yield and dry weight of peppermint, a greenhouse study was conducted. The results indicated that salinity had a significant effect on foregoing parameters. Changes in chlorophyll content were peak and stomata conductivity was a single function. Based on estimations, the highest chlorophyll content was recorded for low salinity (60 mM NaCl). The plant proline content was higher in stress condition compared to control plants. The highest proline content observed in 125mM NaCl concentration was two times higher than that of control plants. There was an increase and then decrease in CAT and POX activities, respectively, in lower and sever levels of salinity. A single equation was the best-fit equation for changing PPO enzyme activity under stress conditions. The dry matter has been affected dramatically by salinity and decreased from 11.34g under the non-stress condition to 4.24 g under high stress condition. Essential oil percentage (in dry matter) increased in moderate salinity stress. We found that the amount of essential oil per plant was linearly decreased. So, the highest (9.78 g plant-1) amount of essential oil per plant belonged to control group and the lowest (4.6 g plant-1) was observed for full stress condition.
