Water Stress-induced Changes of Morphological, Physiological and Essential Oil Compounds in Thymus eriocalyx from Iran

Thymus eriocalyx is one of the most important medicinal plants in Iran, which has extensive pharmaceutical uses due to the biosynthesis of secondary metabolites such as thymol and carvacrol. Given the fact that drought stress is one of the factors limiting the growth and yield of plants in many regions of the world, the aim of this study was to investigate different drought stress treatments on morpho-physiological features and chemical compounds of Thymus eriocalyx essential oil. A factorial experiment was conducted with three replications in a completely randomized design with five levels of drought stress (20, 40, 60, 80, and 100 % of field capacity). The results showed significant decreases in both the fresh and dry weights, plant height, crop growth rate (CGR), relative growth rate (RGR), chlorophyll a content, total chlorophyll content, and soluble leaf protein content with increasing drought stress. The contents of proline and carbohydrate in the leaf solution were significantly affected by 20 and 40 % of field capacity compared to non-stress conditions. Essential oil analysis revealed that the major constituents of this plant were thymol, p-cymene, γ-terpinene, trans-caryophyllene, and carvacrol, which were affected by different drought stress treatments. Drought stress significantly raised the contents of thymol, carvacrol, and trans-caryophyllene, but had adverse effects on other chemical compounds of the essential oil. Overall, the results of this study indicate that drought stress, especially at high levels, has severe adverse effects on morpho-physiological features, but it can contribute to increases in some secondary metabolites of the essential oil such as thymol, carvacrol, and trans-caryophyllene. The present study, therefore, recommends that drought stress (irrigation at about 20 and 40 % of field capacity) can be used if there is a need to increase medicinal substances such as thymol and carvacrol.