Role of melatonin in promoting growth attributes, thymol, rosmarinic acid and biochemical properties in Thymus vulgaris L. under water deficiency

Thymus vulgaris L. is a rich source of thymol and rosmarinic acid. Nowadays, water deficiency is one of the critical environmental factors affecting plants performance and metabolites. Melatonin, as a biostimulant, increases the tolerance of plant to environment stressors, especially drought. In order to exploit dry and low-yielding lands, this study was conducted on enhancing T. vulgaris' growth, phytochemical and biochemical traits by means of melatonin foliar spray (0, 50, 100, and 150 µM) under various water deficiencies (100%, 70%, and 40% FC) which were carried out as a factorial completely randomized design. Foliar application of melatonin caused to improve water deficit tolerance, successfully revived the suppressed morphological traits and significantly increased biochemical capacity. The findings of present experiment demonstrated that as water deficiency increased, growing parameters reduced although, the negative effect of water deficit on vegetative parameters such as plant height/diameter, leaf length/width, and shoot fresh/dry weight were ameliorated by melatonin specially at concentration of 100 µM. The maximum essential oil content and yield was recorded respectively at 40% FC and 100% FC in combination with 100 µM melatonin. Based on GC and HPLC analyses, the amount of thymol, carvacrol and rosmarinic acid in the treatment of 40% FC and 150 μM melatonin increased by 42.25%, 81.36%, and 85.34%, respectively, compared to the control. Increase in drought severity led to increase in total phenol content, total flavonoid content, and also antioxidant capacity which were multiplied by 100 µM melatonin application. The least malondialdehyde (MDA) content, and hydrogen peroxide (H₂O₂) generation as well as proline accumulation was related to the plants treated with 150 µM melatonin under normal conditions while plants grown at 40% FC without melatonin had the most concentrations of the mentioned osmolytes. Regardless to melatonin application, in T. vulgaris under 40% FC conditions the activity of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were considerably more than those in control plants (100% FC) by 4.1, 1.8, and 0.64 times respectively. However, increased levels of melatonin had a promoting effect on POD and CAT activity, although SOD was negatively affected by melatonin concentrations. In conclusion melatonin probably with protection against oxidative damage and scavenging of free radicals through the stimulation of the enzymatic system, enrichment of plasma with phytochemicals, and regulation of osmotic status of the cells through accumulation of various solutes leads to an increase in the T. vulgaris tolerance against drought stress and ultimately increases the growth and phytochemical compounds.