Effect of drought stress and salicylic acid on some of growth traits, photosynthetic pigments and yield essential oil of peppermint (Mentha piperita L.)

The current research was conducted with the aim to investigate the effect of salicylic acid on the growth traits, photosynthetic pigments and the amount of essential oil in Mentha piperita under drought stress. The factorial experiment was carried out in the form of randomized complete block design and with three repetitions. Factors included irrigation round at four levels (90 (control), 75, 45, and 20% field capacity) and salicylic acid at two levels (zero (control) and 60 ppm). The investigated traits included the plant height, number of lateral branches, length of lateral branches, number of nodes, fresh and dry weight of the plant, fresh and dry weight of the stem, chlorophyll a, b and total carotenoids, and essential oil percentage and yield. Results from this experiment showed that the effect of drought stress on all studied traits was significant at the statistical level of 1%. The mutual effect of drought stress on salicylic acid hormone for the length of lateral branches, fresh and dry weight of the plant,  chlorophyll  a, b and total and essential oil percentage and yield traits was significant. Stress caused a reduction in all traits except for fresh and dry weight of the stem; and the use of salicylic acid led to the reduced effects of stress such that the highest values of these traits were obtained from 60 ppm salicylic acid. Given the mutual effects, the highest values of essential oil percentage and yield obtained from 90% of field capacity and application of salicylic acid. Overall, the application of this hormone led to the improvement of qualitative and quantitative characteristics of Mentha piperita under drought stress.