Peppermint (Mentha piperita L.), with numerous medicinal properties is one of the most important plant from Lamiaceae family that its essential oils are widely used in pharmaceutical, food, cosmetic and sanitary industries. Abiotic stresses, especially drought stress are one of the most important problems in arid and semi-arid regions. Application of salicylic acid (SA) as a phytohormone has been increased due to resistance to stresses such as drought. In order to evaluate the effects of drought stress and salicylic acid on peppermint morphological characteristics, an experiment was conducted as factorial based on completely randomized design with four replications in medicinal plants greenhouse of Agricultural and Natural Resources College of Arak University in 2017. Studied factors included drought stress (35%, 65%, and 95%) and salicylic acid (0, 50 mg/l, 100 mg/l and 150 mg/l). Results showed that drought stress, salicylic acid and their interaction had a significant effect on some traits. Based on the results, salicylic acid improved most morphological traits including main stem height, leaf length and width, internode length, fresh and dry weight of shoot by 14, 57, 71, 40, 61, 52 percent in comparison to control, respectively. Drought stress also had an increasing effect on traits such as fresh and dry weight of root and root volume. The most suitable result was obtained in interaction of 100 mg/l SA with 65% stress with the highest percentage (56.6%) and yield (0.17) of essential oil. According to the results, it seems that using salicylic acid can increase the resistance of peppermint to drought stress by increasing the growth and physiological characteristics [1,2].
