Nitrogen deficiency impacts growth and modulates carbon metabolism in maize.

Nitrogen (N) deficiency in maize regulates carbon (C) metabolism by enhancing sugar and starch metabolism and related gene expression in both shoots and roots, while increasing root competition for assimilates causing carbohydrate accumulation in leaves and sheaths due reduced translocation to sink tissues. Soluble sugars are vital for plant development, with nitrogen (N) availability playing a key role in their distribution across plant organs, ultimately shaping growth patterns. However, the regulatory mechanisms governing carbon (C) assimilate allocation and utilization under different N forms remain unclear. This study examined C fixation, utilization, and spatial distribution in hydroponically grown maize seedlings subjected to four N treatments: 1 mM NO⁻ (low N, LN), 2 mM NO⁻ (medium N), 10 mM NO⁻ (high N), and 1 mM NH⁺ (low ammonium, LA). LN treatment significantly increased soluble sugar and starch contents while promoting greater root biomass at the expense of shoot biomass, leading to a higher root-to-shoot assimilate allocation. The activities of sugar and starch metabolism enzymes were more tightly regulated in both shoots and roots under LN, indicating enhanced C utilization and increased competition for assimilates, particularly in the root. Key genes involved in above-ground sugar and starch metabolism, ZmSPS1, ZmSuSy1, ZmCINV1, ZmVINV1, ZmCWINV1, ZmSTP2, ZmSUC2, ZmSWEET14, ZmSS1, ZmAMY1, ZmBAM1, and ZmAGPase1, were upregulated under LN, correlating with enhanced enzyme activity and resulting increased sugar and starch accumulation. Starch and sucrose accumulated more in LN-treated leaves than in other N treatments, with starch primarily stored in leaf tips and sucrose concentrated in the leaf sheath. This pattern suggests that excess C accumulation results from inefficient C utilization in sink tissues rather than impaired C assimilation. These findings provide new insights into how LN modulates C partitioning between leaves and roots for stress adaptation, highlighting the importance of improving C utilization in sink tissues to mitigate N deficiency and enhance plant growth.