Abstract
Melatonin (N-acetyl-5-methoxytryptamine) plays an essential role in abiotic stress in plants, but its mechanism in drought tolerance is unclear. To better understand the protective roles of melatonin against drought stress, we investigated the effect of foliar application with exogenous melatonin on plant growth, physiological responses, and antioxidant enzyme activities in rice seedlings under drought stress. Rice seedlings were grown for 21 days, and foliar sprayed with 0, 50, 100, 200, and 300 µM melatonin. The control plant was watered daily, while the others were subjected to drought stress by withholding water for seven days. The results showed that drought stress significantly reduced plant growth, relative water content, and chlorophyll content. Electrolyte leakage, malondialdehyde (MDA) content and hydrogen peroxide (H2O2) were also negatively affected by drought stress. Application of melatonin alleviated the effects of drought stress by increasing plant growth, improving relative water content and chlorophyll content, and decreasing electrolyte leakage, MDA, and H2O2. Foliar application with melatonin also increased antioxidant enzyme activities, including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (GPX). In addition, melatonin also enhanced proline and total soluble sugar accumulation during drought stress. It is, therefore, suggested that foliar application with 100 µM melatonin was the most effective for reducing the adverse effects of drought stress in rice plants.
Highlights
Rice (Oryza sativa L.) is a staple crop widely cultivated in Thailand and exported worldwide (Siriphollakul et al, 2017)
Treating the plant with 50 and 100 μM melatonin resulted in higher shoot fresh and dry weight, as well as higher root fresh and dry weight compared to the untreated group (Figure 1C, 1D, 1E, 1F)
Effects of exogenous melatonin on chlorophyll content in rice seedling leaves under drought stress Drought stress had a detrimental effect on the chlorophyll a, chlorophyll b, and total chlorophyll levels in rice seedling leaves
Summary
Rice (Oryza sativa L.) is a staple crop widely cultivated in Thailand and exported worldwide (Siriphollakul et al, 2017). Drought stress is still a significant limitation to rice production in Thailand, especially in the Northeastern region of Thailand, where this cultivar is popularly cultivated (Polthanee et al, 2014). Drought stress is one of the major abiotic stresses restricting crop growth and yield because it adversely affects many biochemical and physiological processes in plants. Drought stress induces stomatal closure, resulting in a lower transpiration rate and reduced photosynthesis (Farooq et al, 2009). Drought stress induces the generation of reactive oxygen species (ROS), leading to lipid peroxidation and causes membrane dysfunction (Li et al, 2011; Liang et al, 2019). In order to maintain the ROS balance and alleviate cellular oxidative injuries, plants have evolved
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