Exogenous application of melatonin improves the growth and physiological properties of blueberry seedlings under salt stress

Melatonin is a secondary messenger in plants that is involved in the regulation of abiotic stress responses, but it is unclear whether it enhances the adaptive capacity of blueberries to cope with salinity stress. The effects of exogenous application of melatonin (100, 200, and 300 mu mol center dot L-1) on the growth, photosynthetic characteristics, leaf anatomy, and physiological and biochemical properties of blueberry plants under mixed salt stress (90 mmol center dot L-1: 30 mmol center dot L-1 Na2CO3 + 60 mmol center dot L-1 NaCl) were investigated. The results showed that blueberries under 90 mmol center dot L-1 mixed salt stress had significant salt damage symptoms, with reduced blueberry biomass, reduced chlorophyll content, and photosynthesis inhibition. In contrast, melatonin-treated blueberries showed significantly reduced salt damage symptoms compared with stressed blueberry plants, with the optimum effect at 200 mu mol center dot L-1, along with increased chlorophyll content, improved photosynthetic capacity, and increased leaf thickness under stress. Melatonin treatment resulted in reduced malondialdehyde (MDA) content and relative electrolyte leakage (EL), increased K+ content, and decreased Na+ content in blueberry roots, stems and leaves, thereby reducing the toxic effects of mixed salts on blueberries. The results indicate that melatonin can mitigate the effects of mixed salt stress on blueberries and that foliar spraying of melatonin could alleviate mixed salt-induced damage to blueberries, with the optimum effect at 200 mu mol center dot L-1 treatment. This study provided a valuable basis for exploring the mechanism of exogenous melatonin action in blueberries in saline environments.