Melatonin-mediated alleviation of drought stress by modulation of physio-biochemical and metabolic status in Dracocephalum kotschyi Boiss. (Lamiaceae)

Dracocephalum kotschyi Boiss., a fragrant perennial plant belonging to the Lamiaceae family, contains many bioactive substances, including essential oil, terpenoids, phenolics, and flavonoids that possess a wide range of biological and pharmaceutical properties. Due to the importance of plant exploitation in dry and low-yield lands, the present study was conducted to explore the effect of the exogenous application of melatonin (0, 50, 100, 150, and 200 & mu;M) on physiological, biochemical, and phytochemical traits of the plant subjected to different levels of water deficit stress (80 %, 60 %, and 40 % of field capacity). The results showed that the application of 150 & mu;M melatonin caused the highest content in the relative growth rate (1.84 g/g d), chlorophyll content (14.55 mg/g FW), relative water content (93 %), and chlorophyll fluorescence (0.9) at different irrigation regimes. Reduced irrigation significantly reduced chlorophyll content and fluorescence, while exogenously applied melatonin enhanced their contents at all levels. Reduced irrigation caused a significant increase in the levels of malondialdehyde (7.47 & mu;mol/g FW) and hydrogen peroxide (51.86 nmol/g). Total phenol content, antioxidant activity, and antioxidant power were increased by applying of melatonin up to 200 & mu;M, while the irrigation rate decreased. The essential oil content was increased by reducing irrigation, so the highest content (3.0 % w/w) was observed at 40 % FC and 150 & mu;M melatonin. Geranyl acetate (44.6-60.4 %), & alpha;-pinene (9.0-19.1 %), and geranial (6.2-16.2 %) were the major essential oil components of the treated plants. Reduced irrigation and application of melatonin up to 150 & mu;M caused an increase in coumaric acid, rosmarinic acid, and rutin despite a decrease in the content of chicoric acid and apigenin. The highest oleanolic acid content (52.69 mg/g DW) was measured in the plants subjected to 40 % FC. The highest (132 mg/g DW) and lowest (43.58 mg/g DW) ursolic acid content was measured in 40 % FC with 200 & mu;M melatonin and 80 % FC without melatonin treatment, respectively.