Effects of foliar application of salicylic acid and proline on the growth traits and the essential oil content of Dracocephalum kotschyi Boiss. under water deficit conditions

In the drought conditions of the last few decades, the use of novel scientific methods to increase the growth and production of effective substances of the medicinal and aromatic plants has been essential. This study was done to investigate the effect of foliar application of salicylic acid and proline on the growth traits and essential oil content of Dracocephalum kotschyi Boiss. (Lamiaceae family) under water deficit conditions at the Research Farm of the Research Center of Agricultural and Natural Resources of Chaharmahal va Bakhtiari Province, Iran during the two crop years of 2017-2019. The experiment was performed as a factorial in a CRBD with three replications. Experimental factors included irrigation treatment at two levels of normal irrigation (non-stress, irrigation after draining 30% of F.C.) and deficit irrigation regime (irrigation after draining 50% of F.C.), and three levels of foliar application (control, salicylic acid 1 mM, and proline 1 mM). The essential oil from the aerial parts of the herb at early flowering stage was extracted by water distillation. According to the results, the highest content of chlorophyll a, b, and total (0.96, 0.45, and 1.39 mg/g, respectively) were observed in water deficit irrigation treatment. Also, application of water deficit stress increased the proline content by 70% compared to the control while it did not show a significant effect on the morphological traits of plant height, stem diameter, and plant dry weight. Salicylic acid as a defense factor reduced the negative effects of deficit irrigation conditions on the essential oil, proline, and chlorophylls a, b, and total contents. The highest and lowest contents of essential oils were obtained in foliar-application of proline (1.12 mL/100 g dry matter) and control (0.84 mL/100 g dry matter), respectively. In conclusion, the foliar-spraying of salicylic acid in conditions of deficit irrigation is recommended to reduce the negative effects of stress and increase the content of D. kotschyi essential oil.