Biochemical and physiological mechanism induced by melatonin in Mexican lime (Citrus aurantifolia Swingle) plants: cold and freezing stress

Mexican lime (Citrus aurantifolia Swingle) is one of the most important citrus rootstocks in the tropical and subtropical areas that is generally susceptible to cold and freezing. The aim of this study was to investigate the impact of melatonin (0 and 500 μM) on the cold tolerance of Mexican lime plants at 0 and − 6 °C. Findings indicated that melatonin notably enhanced the resistance of Mexican lime plants under both levels of stress. However, the strategy for cold tolerance was different between the two temperatures. Increased cold tolerance of melatonin-treated plants under 0 °C was related to the higher activity of antioxidant enzymes, proline and total soluble carbohydrate in addition to phenolic compounds. At a temperature of − 6 °C, non-enzymatic antioxidants such as carotenoids and phenolic compounds played a more significant role in enhancing cold resistance. Plants treated with melatonin at both stress levels exhibited lower electrolyte leakage, malondialdehyde, ABS/RC, DI0/RC and leaf abscission. Conversely, melatonin-treated plants exhibited elevated levels of pigments, Fv/Fm, PIABS, antioxidant capacity, total phenol content, and recovery. This study provides strong evidence that melatonin could improve plant resistance to cold stress in Mexican lime. This research presents compelling proof that melatonin has the potential to enhance the resistance of Mexican lime plants to cold stress.