Ammonium Supply Enhances Growth and Optimizes Carbon–Nitrogen Coordination by Modulating Carbohydrate Metabolism in Maize

Nitrogen (N) is primarily taken up by most plant species in the form of nitrate (NO3-) and ammonium (NH4+) to support growth and metabolic functions. However, the regulatory mechanisms modulating carbon (C) assimilate allocation under varying N forms remain unclear. This study investigated C metabolism and its spatial distribution in maize seedlings subjected to five N treatments (T1-T5): T1, nitrogen-free (control N); T2, 1 mM NO3- (sole NO3-); T3, 1 mM NH4+ (sole NH4+); T4, 0.5 mM NH4NO3 (mixed N supply); and T5, substitution of 1 mM NH4+ with 1 mM NO3- (NH4+→NO3-) at 10 days after seedling transfer (DAT). NH4+ treatment triggered significant physiological and molecular adaptations, such as enhanced growth, improved photosynthetic performance, and increased sucrose and starch accumulation. These elevated carbohydrate levels were closely associated with increased activity of sucrose-metabolizing enzymes (SuSy, SPS, and INVs) and starch-metabolizing enzymes (AGPase, SS, AMY, and BAM), alongside the upregulation of key genes involved in sucrose metabolism (ZmSPS1, ZmSuSy1, and ZmINVs), sucrose transport (ZmSWEET14, ZmSUT2, and ZmSTP2), and starch metabolism (ZmSS1, ZmAGPase1, ZmAMY1, and ZmBAM1). Spatial and diurnal analyses revealed dynamic patterns of C partitioning across the leaves, roots, and leaf sheaths. These findings advance our understanding of how different N forms, particularly NH4+, regulate C metabolism and shoot-root allocation to facilitate carbon utilization in sink tissues to improve plant resilience to N fluctuations. Future research will focus on exploring these adaptive mechanisms across diverse maize genotypes under field conditions, with the goal of improving nitrogen use efficiency (NUE) and productivity in variable N environments.