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

Abstract

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 μM) 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 μM 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 µmol/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 μM, 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 μM melatonin. Geranyl acetate (44.6–60.4 %), α-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 μM 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 μM melatonin and 80 % FC without melatonin treatment, respectively.