The influence of reduced irrigation on herbage, essential oil yield and quality of Thymus vulgaris and Thymus daenensis

Drought stress, as a main abiotic stress, has a significant influence on growth and metabolic activities of plant species. In this study, the influence of reduced irrigation on dry herbage, essential oil yield, and chemical components of two thyme species were investigated. This study was done in an experimental greenhouse, I.A.U., Shahrekord Branch, Iran at 2012 in a CRD with three replications. Three irrigation levels were I1 (irrigated in field capacity or control), I2 (slight drought stress or irrigation in 75% field capacity), and I3 (mild drought stress or irrigation in 50% field capacity). Two thyme species included Thymus daenensis and T. vulgaris. The essential oils obtained by hydro-distillation and were analyzed by Gas Chromatography-Mass Spectrometry.  Results indicated that irrigation levels had a significant effect on many morphological and physiological characteristics, including dry matter, plant height, and leaf area index (LAI). In addition, levels of irrigation affected on oil yield and some major constituents in the essential oils. Carvacrol, γ-terpinene, and p-cymene contents in the essential oils were significantly increased under stressed conditions, whereas thymol amount was significantly reduced under stressed conditions.  According to the results of this study, drought stress reduces the essential oil yields and dry herbage in both species of thyme.