Drought stress is one of the most important environmental stresses limiting the growth and effectiveness of plants, however, exogenous application of salicylic acid (SA) as a signaling molecule may counteract the adverse effects of stress on plant productivity. The present study was designed to determine the effects of different concentrations of SA (0, 100, and 150 ppm) on morpho-physiological traits and essential oil constituents in different ecotypes of two thymus species (Thymus vulgaris and T. kotschyanus) under well-watered and water stress conditions. The results demonstrated that foliar application of SA results in better efficacy of thyme in water stress conditions by reducing the content of cellular injury indices such as hydrogen peroxide (H2O2), malondialdehyde (MDA), and improving growth and metabolite attributes including root and shoot dry weight, total chlorophyll, leaf carotenoids, proline, and total soluble carbohydrate contents. Treatment with SA under both water availability conditions led to the improvement of essential oil content in some ecotypes, especially in Zanjan ecotype of T. kotschyanus. Furthermore, gas chromatography–mass spectrometry analysis of essential oils exhibited that the thymol, carvacrol, linalool, p-cymene, and γ-terpinene were identified as the main components of essential oils under employed treatments. SA application at 100 ppm was increased both essential oils content and compositions as well as thymol content by 93.39% in T. kotschyanus (Zanjan ecotype) under both wellwatered and water stress conditions. Appropriate concentration of SA significantly reduced the adverse effects of water deficit on thyme species through improved morphological and physiological parameters, and increased the quantity and quality of essential oils.
