Abstract:

Objective: With the mangrove plant Kandelia obovata as the test material, this study investigated the mechanism by which nano zinc oxide (ZnO NPs) enhances the cold resistance of K. obovata seedlings, providing crucial technical support for the safe overwintering of mangroves on the northern margin or in higher latitudes. Method: K. obovata seedlings were pre-treated with foliar spraying method with varying concentrations of ZnO NPs to determine the optimal application concentration. Seedlings were then sprayed with optimal concentration of ZnO NPs twice daily in the morning and evening for 3 days, followed by exposure to low-temperature stress at 8 ℃ (day)/–3 ℃ (night) for 3 days. Physiological indices such as photosynthetic rate, antioxidant enzyme activity, and melatonin (MEL) synthesis pathway were measured. Result: The leaf net photosynthetic rate (P_n) and melatonin (MEL) content of seedlings initially increased and then decreased with increasing ZnO NP concentration, with 200 mg·L^?1 being the optimal concentration under low-temperature stress. Low-temperature stress significantly decreased endogenous MEL and nitric oxide (NO) contents and increased contents of superoxide anion (${\mathrm{O}}_2^{\overline{\;\cdot\; }} $) and hydrogen peroxide (H₂O₂), thereby inhibiting photosynthesis. Spraying treatment with 200 mg·L^?1 ZnO NPs on leaves was able to effectively increase the activities of N-acetylserotonin methyltransferase (ASMT) and tryptophan decarboxylase (TDC) in the MEL synthesis pathway of K. obovata seedlings under low temperature stress, and promote the production of various intermediate products (tryptamine, 5-hydroxytryptamine, and 5-methoxytryptamine) in the synthesis process of melatonin from tryptophan. Additionally, ZnO NPs not only effectively increased antioxidant enzyme activities, reduced ${\mathrm{O}}_2^{\overline{\;\cdot\; }} $ and H₂O₂ contents, but also activated nitrate reductase (NR) activity by H₂O₂ in the leaves under low temperature stress, promoting NO₂^? production, and ultimately improving photosynthesis, which is closely related to the melatonin synthesis pathway. Conclusion: ZnO NPs enhances the cold resistance of K. obovata seedlings by regulating the MEL/ROS/ reactive nitrogen species (RNS) redox network under low-temperature stress. ZnO NPs may induce the tryptophan/tryptamine/5-hydroxytryptamine/5-methoxytryptamine/melatonin synthesis pathway in K. obovata seedlings under low-temperature stress, influencing the redox network formed by melatonin, ROS, and RNS.

Key words: ZnO NPs, low temperature stress, mangroves, melatonin synthesis pathway, ROS, RNS, photosynthesis