Modulating cold tolerance in Mexican lime (Citrus aurantifolia Swingle): A combined approach using melatonin, acetic acid and mannitol

Mexican lime (Citrus aurantifolia Swingle), a widely grown commercial citrus cultivar, serves as a common rootstock for various citrus species. However, its susceptibility to cold temperatures poses a significant challenge to successful cultivation and productivity. This study was conducted to assess the combined effect of melatonin (ML) (500 μM), mannitol (Ma) (50 mM), and acetic acid (AA) (15 mM) on the chlorophyll fluorescence parameters and biochemical traits of Mexican lime plants exposed to low temperatures (0 and -6 °C). The results showed that both the ML+MA+AA and ML+AA treatments significantly enhanced the PIABS and PItotal parameters at 0 and -6 °C temperatures. Furthermore, the ML+AA treatment resulted in the highest level of chlorophyll a+b content (2.5 mg/gFW) in comparison to the control group and the other treatment. Notably, the ML+AA treatment effectively mitigated cold stress in Mexican lime plants by increasing total phenols, flavonoids, and carbohydrates, while reducing electrolyte leakage and malondialdehyde levels. In addition, the ML+AA treatment significantly decreased leaf abscission (82.3 %) at -6 °C in contrast to the control group, which experienced a 100 % rate of leaf abscission. Both ML+MA+AA and ML+AA treatments exhibited the highest antioxidant capacity (53.8 %). Moreover, the ML+MA+AA treatment significantly enhanced the activity of the SOD enzyme in contrast to the control. The PCA results indicated that both ML+MA+AA and ML+AA treatments clustered together and were distant from the control at both 0 and -6 °C, suggesting their effectiveness in mitigating cold stress. In conclusion, the ML+AA is recommended as an effective treatment to alleviate low-temperature damage in Mexican lime plants.