Exploring the impact of exogenous melatonin on agro-morphological characteristics, carvacrol, and rosmarinic acid production in Satureja rechingeri Jamzad under drought stress

Satureja rechingeri Jamzad (known as “Jatra” in Persian), which belongs to the Lamiaceae family, is a rich source of essential oil particularly carvacrol, and rosmarinic acid. Drought stress has a detrimental impact on the physiological and biochemical parameters of plants, leading to a decline in plant productivity. Melatonin (MT), a new plant growth regulator found abundantly in plants, has been found to enhance the plant's internal resistance to various environmental stresses. The present study aimed to examine the impact of exogenously applied MT on the agro-morphological, physio-biochemical, and phytochemical traits of S. rechingeri plants cultivated under different levels of drought stress. The results indicated that plants treated with 200 µM MT obtained the highest plant height, length and width of leaf, fresh, dry and drug weight under different drought stress levels. The highest values of relative water content (RWC) (93.5%) and chlorophyll content (15.4 mg/g FW) were recorded by MT 200 µM and 100 µM, respectively, in 100% FC. Drought stress treatments (40, 60, and 80% FC) without foliar spray of MT significantly enhanced the H2O2 content, electrolyte leakage, and malondialdehyde content in leaves, whereas MT treatment under drought stress significantly decreased the above parameters. The lowest H2O2 content (11.5 nmol/g), electrolyte leakage (3.08%), and malondialdehyde content (0.78 µM/g) were obtained by 200 µM MT at 100% FC. In contrast, drought stress treatment increased the total phenol content (TPC), rosmarinic acid (RA), essential oils (EOs) content and yield, and carvacrol. The maximum values of TPC (28.1 mg GAE/g DW), EOs content (3.63%) and yield (0.96%), and carvacrol (95.66%) were achieved by 200 µM MT at 40% FC. The highest RA content (7.43 mg GAE/g DW) was recorded in 100 µM MT at 40% FC. Thus, foliar spray MT has the potential to enhance plant growth through the mitigation of reactive oxygen species (ROS)-induced oxidative harm, as well as the augmentation of photosynthesis pigments, secondary metabolites such as phenolics, EOs levels, overall antioxidant scavenging capacity, and the preservation of RWC during periods of drought stress.