Plant-water responses of different medicinal plant thyme (Thymus spp.) species to drought stress condition

Thymus with more than 200 species is known for its medicinal and culinary uses. There is an increasing demand for thyme and its products, requiring cultivation under different conditions, including drought. To understand the response of thyme to water deficit and its variation across the genus, eleven populations from a range of species (Thymus daenensis, T. kotchyanous, T. vulgaris, T. serpyllum, T. capitata and T. zygis) were evaluated physiologically to identify the response of different populations to water deficit stress. The drought stress was imposed by withholding water 30 days after planting. Root/shoot ratio, survivability, water content and water potential were recorded. The results showed that populations had significantly different ratios of root/shoot under drought conditions. Leaf water potential in watered plants (-3.4 bar) decreased to -10.5 bar in droughted plants. On average, water content for watered plants were 85%± 3.7 and for droughted plants were 73.4%±15.5, after the droughted period before wilting. Analysis of the data found a significant negative association between water content and water potential. This correlation ranged from – 0.075 in T. kotchyanous (from Iran) to – 0.91 in T. serpyllum (from Spain). T. serpyllum (from Spain) survived longer than all other populations (for 22 days) while T. vulgaris (Spain) did not survive beyond 12 days. Among the screening indicators tested in this study, survivability was found to be a reliable tool to screen germpalsm for drought stress tolerance. Our experiments identified T. serpyllum as being more tolerant than other species, and Spanish T. vulgaris population was the most susceptible.