Water Deficit Stress Mitigation by the Foliar Spraying of Salicylic Acid and Proline on the Volatile Oils and Growth Features of Hyssop (Hyssopus officinalis L.)

To achieve a product with a high quantity and quality of active substance in the main and valuable medicinal and aromatic plants like hyssop (Hyssopus officinalis L.) under semiarid conditions requires the use of novel agronomic technics. The effects of the foliar application of salicylic acid (SA) and proline were investigated on the hyssop growth and phytochemical traits at the field conditions during two experimental years. The experiment was performed as a factorial in a completely randomized block design (RCBD), in which irrigation treatment (optimum and reduced irrigation) and the foliar application (control, the foliar applications of SA and proline). The volatile oils from the aerial parts harvested at the flowering were analyzed by GC-FID and GC/MS. Results indicated that water deficit stress significantly affected some physiological and phytochemical characteristics of hyssop. In general, the amounts of proline and chlorophyll (a, b, and total), the volatile oil content, and the concentrations of the major constituents of the volatile oil improved under water deficit stress. The highest values of the volatile oil content and the concentrations of the major constituents of the volatile oil including beta-pinene (24.59%), trans-pinocamphene (14.20%), cis-pinocamphene (65.57%), and myrtenol (6.95%) were obtained from the plants treated with deficit irrigation x foliar spraying of SA. In conclusion, the foliar-spraying of SA could decrease the harmful impacts of water deficit on the concentrations of the main compounds in the volatile oil, and it is recommended for the stability of the active substance yield of H. officinalis in a semiarid climate.