Low ammonium and high nitrate input improves nitrogen use efficiency and growth in sugarcane through coordinated...

Providing plants with adequate nitrogen (N) through an appropriate ammonium-to-nitrate-ratio (NH₄⁺/NO₃^--ratio) can enhance nitrogen use efficiency (NUE) and promote growth. However, how different NH₄⁺/NO₃^--ratios affect N and carbon (C) metabolic processes and growth in sugarcane, a fast-growing high-biomass crop, remain unclear. Here, a hydroponic experiment was conducted using sugarcane variety 'Yunzhe05-51', applying seven NH₄⁺/NO₃^--ratios under a constant total N supply level of 10 mmol·L^-1. Plant growth, activities of N-related enzymes, and dual transcriptomic and untargeted metabolomic responses were measured to investigate the regulation of N and C metabolism. The results indicated that an NH₄⁺/NO₃^--ratio of 20/80 was optimal for sugarcane growth. Enhanced activities of leaf NO₃^- reductase and NH₄⁺ anabolic enzymes, together with more balanced C and N metabolic processes, explained the potential regulatory mechanisms that promoted sugarcane growth under this ratio. NO₃^--only treatment downregulated the expression and abundance of several genes and metabolites associated with photosynthesis, while promoting NO₃^- transport and reduction. When NH₄⁺ supply exceeded 4 mmol·L^-1, sugarcane showed signs of toxicity. Although activities of NH₄⁺ assimilation enzymes were relatively lower under exclusive NH₄⁺ supply, high NH₄⁺ concentrations resulted in higher L-glutamate levels, indicating growth inhibition under such conditions related to disruptions in protein posttranslational modifications. Furthermore, several metabolites and metabolic pathways associated with stress tolerance and detoxification were activated under high NH₄⁺ conditions. Collectively, this study elucidates the optimal NH₄⁺/NO₃^--ratio for sugarcane growth under adequate N availability and provides insights into the underlying metabolic regulatory mechanisms.