Salinity affects C/N ratio through differential responses of carbon and nitrogen metabolism in mangrove Avicennia marina leaves revealed by combined analysis of transcriptome and metabolome

Background and aims Salt is a critical environmental factor that profoundly impacts plant growth and development. However, the investigation into how plants regulate carbon and nitrogen metabolism in response to different levels of salinity has been limited. Methods In this study, we investigated the impact of fluctuating salinity on Avicennia marina, a typical pioneer mangrove plant. We measured physiological indicators like photosynthesis to understand these effects. To gain deeper insights, we conducted an integrated analysis of genes and metabolites associated with carbohydrate and nitrogen compound metabolic pathways using transcriptomics and metabolomics. Results Our findings showed that low salinity treatment increased the leaf C/N ratio compared to the control group without salt treatment. Low salinity improved chlorophyll content and up-regulated PsbB gene expression in A. marina leaves, enhancing light absorption capacity. It also reduced leaf specific area, improving photosystem II utilization efficiency and minimizing heat dissipation. Furthermore, low salinity up-regulated genes encoding large subunit of RubisCO (RBCL) involved in Calvin cycle within A. marina leaves, leading to improved photosynthetic capacity, resulting in an increased C/N ratio. Under high salinity, A. marina leaves enhanced glycolysis and the TCA cycle for energy production while activating N metabolic pathway to accumulate amino acids and lipids. The balance between carbon and nitrogen metabolism was restored by returning the C/N ratio to control levels under high salinity. Conclusions These findings highlight the distinct response strategies of leaf carbon and nitrogen metabolism in A. marina when adapting to fluctuating salt environments in intertidal zones.