Exogenous putrescine changes redox regulations and essential oil constituents in field-grown Thymus vulgaris L. under well-watered and drought stress conditions

Drought stress affects a vast range of morphological, physiological and biochemical characteristics in plants; however, exogenous application of osmotically active materials such as polyamines has been regarded as a good alternative to counter the adverse effects of various environmental stresses on plant functions. The present study investigates the response of Thymus vulgaris L. plants to different concentrations of putrescine (0, 20 and 40 mg L−1) under well-watered and drought stress conditions in a factorial experiment based on randomized complete block design with three replications during 2-year field trial. Drought stress significantly impaired the plant growth and biomass accumulation, and altered leaf water status, photosynthetic pigment contents, hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels, free proline and soluble protein contents, ROS scavenging enzymes such as superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), and ascorbate peroxidase (APX), essential oil content and chemical profiles of the essential oil compared to the respective control values. However, exogenously applied putrescine (particularly at 20 mg L−1) improved leaf water content, accumulated dry matter, reduced cell injury indices and up-regulated antioxidant enzyme activities. Moreover, stressed plants treated with the highest concentration of putrescine increased their essential oil content by 23.07% compared to non-stressed plants without putrescine application. A total of 14 compounds predominantly consisted of monoterpenes such as Thymol, Carvacrol, γ-Terpinene and p-Cymene were identified in the essential oil of the plants in both years. Percentage of the major essential oil constituents substantially enhanced under drought stress, whereas exogenous application of putrescine further improved the content of these specific volatile compounds under well-watered and stressed plants compared to the respective control. A significant negative correlation was observed between essential oil percentage and H2O2 (r0.01 = −0.928) and MDA (r0.01 = −0.947) contents in plants under drought stress. The results of this study indicated that foliar application of putrescine in proper level may act as elicitor to trigger physiological processes and induce valuable metabolites biosynthesis, which may compensate the negative impacts of drought stress on plant biomass and essential oil quality and quantity.