The Efficacy of Salicylic Acid Levels on Photosynthetic Activity, Growth, and Essential Oil Content and Composition of Pennyroyal Plants Under Salt Stress

This research was conducted to find out whether the foliar application of salicylic acid (SA) at 0.5, 1, and 1.5 mM in comparison with water spray (control) could mitigate the adverse effects of salinity (0, 25, 50, and 75 mM NaCl as non-saline and low, moderate, and high salinities, respectively) on pennyroyal (Mentha pulegium L.) plants. Salt stress increased Na+ content, while decreased the ratio of K+/ Na+, calcium and magnesium contents in both shoots and roots. Decreasing rubisco activity, photochemical efficiency of photosystem II and stomatal conductance led to a reduction in photosynthesis rate by 25.26% in SA untreated plants under high salinity. Application of SA especially at 1 mM concentration alleviated the harmful impacts of salinity via reducing the Na+ translocation from roots to the shoots and improving nutrients uptake and photosynthetic activity. Leaf area and root and shoot biomasses were decreased with increasing salt stress, but these growth parameters were improved by foliar spray of SA. The essential oil content of plants was significantly increased under low and moderate salinities by 18.57% and 35.71%, respectively. Foliar spray of 1 mM SA also enhanced essential oil content of plants in non-saline and all saline conditions by up to 36.84%. Salinity changed the relative proportions of essential oil constituents and induced production of new constituents such as α-thujene, myrcene, isophorone, and germacrene D. The antioxidant activity of essential oil in salt-stressed and SA-treated plants was higher than untreated plants. This research proved that SA-induced salt tolerance in pennyroyal plants was related to increasing nutrient uptake, photosynthetic activity, plant growth, and essential oil production.