Thymus is one of the most important medicinal genuses. This study investigated the effects of water stress on seed germination, growth, essential oil content and composition, water use efficiency, growth indices and some physiological characteristics of Thymus daenensis (Chadegan, Kohrooyeh and Margoon genotypes), T. kotschyanus , T. carmanicus and T. vulgaris under laboratory, greenhouse and field conditions. Water stress treatments were 0, -0.3, -0.6, -0.9 and -1.2 MPa of water potential levels in germination test, while in greenhouse and field condition were 20, 50 and 80% of soil water depletion, representing the non-stressed control (T 20 ), moderate drought stress (T 50 ) and severe water stress (T 80 ), respectively. Water stress reduced seed germination and speed, root and shoot lengths, plant height, leaf area, herbage production, chlorophyll content, irrigation and water use efficiency based on dry matter (IWUE dm ), crop growth rate, relative growth rate and net assimilation rate, while increased proline, K + , essential oil content and irrigation water use efficiency based on essential oil yield (IWUE eso ). The highest leaf area, dry matter and crop growth rate were obtained in T. daenensis genotypes. Thymol was the main essential oil component in T . vulgaris and T. daenensis , while carvacrol was the main component in T. carmanicus and T. kotschyanus . In all species, water stress increased thymol content, while decreased carvacrol. The results showed that there is a high potential for water saving through deficit irrigation practice and using an adapted plant species, while the highest herbage yield was produced under 20% soil water depletion irrigation regime. T. daenensis (Margoon genotype) could produce the highest herbage and essential oil yield under controlled and stressed conditions (3526 and 64 kg ha -1 in T20, and 2188 and 58 kg ha -1 in T 80 , respectively). It was concluded that irrigation of T. daenensis based on 50% water depletion should be an appropriate choice for first growing season and 80% water depletion for the second growing season in semi-arid climatic conditions. Keywords: Water stress, Thymus daenensis , Thymus carmanicus , Thymus kotschyanus , Thymus vulgaris , growth.
