Nitrogen form modulates carbon and nitrogen metabolism and assimilates partitioning to shape maize ear development

Nitrogen (N) deficiency in maize modulates carbon (C) and N metabolism by enhancing biomass allocation towards vegetative organs, suppressing sucrolytic activity and gene expression, inhibiting ear sink strength and increasing vegetative competition for assimilates, and causing C accumulation in developing maize ears due to inefficient C utilization. Nitrogen (N) form influences carbon (C) and N metabolism, thereby shaping maize growth and development. However, the mechanisms modulating C assimilate allocation to ears under different N forms remain unclear. This study investigated C metabolism and spatial distribution in the mini maize line TX-40 J, supplied with four N treatments: 1 mM NO₃⁻ (low N, LN), 2 mM NO₃⁻ (medium N, MN), 10 mM NO₃⁻ (high N, HN), and 1 mM NH₄⁺ (low ammonium, LA). LN significantly reduced shoot and ear biomass, producing smaller cobs, while stimulating root proliferation and increasing shoot-to-ear (S/E), root-to-ear (R/E), and root-to-shoot (R/S) ratios. Maize developing ears under LN accumulated less carbohydrates (sucrose, glucose, fructose, starch) and exhibited reduced activities of key enzymes (SPS, SuSy, SS, CINV, VINV, AGPase). Diurnal and spatial analysis showed impaired assimilate translocation, with sugars and starch significantly retained in source tissues. Gene expression analyses showed that genes involved in sucrose metabolism and transporter genes (ZmSPS, ZmSuSy, ZmCINV, ZmSWEET, ZmSUT), and starch biosynthetic genes (ZmAGPase, ZmSS) were differentially regulated by N treatments in developing ear tissues. Compared with HN, MN and LA plants, LN-treated plants exhibited markedly lower expression of these genes, suggesting reduced carbohydrate synthesis and impaired sink allocation under N deficiency. Correlation analysis further linked increased vegetative-to-reproductive ratios with reduced nutrient deposition in ears. Collectively, LN triggered metabolic reprogramming that supported vegetative retention over reproductive investment, weakening sink strength and yield potential. These findings provide mechanistic insight into N form-dependent regulation and inform strategies to improve nutrient use efficiencies.