Influence of drought stress and plant growth promoting rhizobacteria (PGPR) inoculation on morphological characteristics, essential oil yield and composition of Thymus daenensis Clack

To investigate the effects of plant growth promoting rhizobacteria (PGPR) and drought stress on essential oil of Thymus daenensis Clack, a greenhouse experiment was conducted in Shahrekord, Iran in 2017. The experiment was arranged as a factorial in a completely randomized design with three replications. The first factor included four levels of irrigation: well-watered (No stress), irrigation after depletion of 20-25% of field capacity (FC) (Low stress), irrigation after depletion of 35-40% of FC (Mild stress) and irrigation after depletion of 55-60% of FC (Severe stress). The second factor included two levels of PGPR treatments: no inoculation (Control) and inoculation with PGPR. The results showed that the morphological parameters were significantly increased in PGPR treatments, while water stress decreased all parameters. The essential oil amount increased in low stress and decreased in severe stress. PGPR inoculation increased the amount of essential oil, although this increase was not statistically significant. PGPR incubation and drought stress had a significant effect on 13 and 14 components, respectively. The interaction of PGPR and drought stress had a significant effect on the oil components. Thymol and carvacrol, two important components of T. daenensis essential oil, decreased with increasing severity of drought stress, but PGPR inoculation increased them, especially at drought stress treatments.