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 (H