Melatonin mitigates cold damages by modulating the photosynthetic-photorespiratory supercycle and ROS/RNS homeostasis in Kandelia obovata

Extreme winter temperatures limit the distribution and survival of mangroves in northern China. Melatonin (MT), a bioactive molecule involved in stress responses, is known to alleviate cold stress, but its physiological role in cold tolerance in mangroves remains unclear. In this study, we examined the effects of exogenous MT pretreatment on Kandelia obovata seedlings exposed to low-temperature stress. MT application significantly reduced cold-induced damage in the exposed K. obovata seedlings, including leaf browning, oxidative stress, and photoinhibition of photosystems I and II. It enhanced redox homeostasis by activating antioxidant defenses and reducing lipid peroxidation, thereby mitigating cellular damage. MT also stabilized the photosynthetic machinery, improved electron transport efficiency, and supported the photosynthetic-photorespiratory supercycle. These physiological improvements were associated with increased cold tolerance and better maintenance of photosynthetic function under stress. Our findings suggest that MT pretreatment offers a promising strategy for improving cold resilience in K. obovata by maintaining redox balance and protecting core photosynthetic processes in the presence of cold stress.