Effect of exogenous melatonin on antioxidant defense system and osmo-regulatory solutes of drought-stressed <i>Morinda citrifolia</i>

Abstract

Morinda citrifolia is a small tropical tree that contains active natural metabolites in its leaves, stem, roots, and fruits. Despite these properties, drought stress has always been one of the limiting factors affecting its growth and productivity. This study investigated the role of melatonin in the regeneration of M. citrifolia in vitro under simulated drought stress. Nodal cuttings of six-month-old M. citrifolia were inoculated into Murashige and Skoog (MS) media supplemented with 2, 4-dichlorophenoxy-acetic acid (0.5 mg/L), indole acetic acid (0.5 mg/L) and varying concentrations of melatonin (0 μM, 50 μM and 100 μM) and polyethylene glycol (PEG) 6000 (0%, 20% and 40%). M. citrifolia experienced a significant increase in plant growth, stabilized chlorophyll contents, superoxide dismutase (SOD), and glutathione peroxidase (GPx) activities under drought stress possibly because it is a drought-tolerant plant. However, melatonin was involved in the accumulation of proline and ascorbic acid at 20% PEG. Osmoregulation of solutes stimulated and stabilized the production of catalase, GPx, and SOD activities. Upregulation of glutathione S-transferase augmented the biosynthesis of glutathione during drought stress. Also, a high accumulation of carotenoid function as photo-protectants and shields chlorophyll contents from drought-induced reactive oxygen species. Consequently, 40% of hydrogen peroxide was detoxified and plant growth was boosted. Therefore, melatonin acts as a stimulant of carotenoid, compatible solutes, enzymatic and non-enzymatic anti-oxidant defensive system, protects plants against oxidative injury, and boosted the growth of Morinda. citrifolia in vitro under drought stress.