Objective Most studies about wheat nitrogen use efficiencies focus on simple comparisons between with and without nitrogen fertilization treatments, which may lead to incomplete conclusions. We did the research through a gradient of nitrogen application rates, and focused on examining the difference in dry matter accumulation and transformation during maturing stage of wheat cultivars.

Methods The study was conducted from 2022 to 2024 in Houfuxi Village, Guocun Town, Suiyang District, Shangqiu City, Henan Province. The tested wheat cultivars included nitrogen-efficient wheat (Bainong 4199 and Xinong 979, HNC), and nitrogen-inefficient wheat (Fengdecunmai 21 and Zhengpinmai 8, LNC). Four nitrogen application treatments were set up, 0 kg/hm² (N0), 90 kg/hm² (N90), 180 kg/hm² (N180), and 240 kg/hm² (N240). Systematic measurements were conducted on indicators including dry matter accumulation and transport, nitrogen uptake and partitioning, and yield components. The variable relationships were further analyzed in conjunction with structural equation modelling.

Results N180 was the optimal nitrogen application rate for all the four cultivars. At this level, dry matter accumulation of both nitrogen-efficient and inefficient wheat varieties was significantly higher than other treatments, the number of grains per spike, the spike number per plant, and yield also reached their peaks; the nitrogen partial productivity (NPPE) under N180 was increased by an average of 10.56% compared with that under N240. With the increase of nitrogen application, dry matter and nitrogen accumulation at the maturity stage rose in both varieties. The dry matter accumulation in stems and leaves at maturity (DSLM) of nitrogen efficient varieties was significantly 16.17% and 11.49% higher than inefficient under N0 and N90. Nitrogen-efficient wheat had lower pre-anthesis dry matter translocation amount (DMRA), translocation rate (DMRR) and contribution to grains (DMRCR) than inefficient wheat, but higher pre-anthesis nitrogen translocation amount (NRA), translocation rate (NRR), contribution to grains (NRCT), post-anthesis nitrogen accumulation (NAG) and its contribution to grains (NACT), as well as post-anthesis dry matter accumulation (DMPA) and its contribution to grains (DMACR). Under different nitrogen rates, DMPA and DMACR of nitrogen-efficient wheat first increased then decreased, peaking at N180. Compared with nitrogen-inefficient varieties, the DMACR of nitrogen-efficient ones increased significantly by 18.57% at N180. The nitrogen uptake efficiency (NUPE) of nitrogen-efficient wheat varieties is significantly higher than that of nitrogen-inefficient wheat varieties. The structural equation model indicated that the main indicators affecting wheat nitrogen use efficiency were NRA, DSLM, and grain yield. DSLM exerted a negative effect on nitrogen use efficiency through grain yield and nitrogen accumulation in stems and leaves at maturity.

Conclusion Under nitrogen application, especially at the optimal nitrogen rate, nitrogen-efficient wheat exhibits lower pre-anthesis dry matter translocation than nitrogen-inefficient varieties. Moreover, the high accumulation of stem-leaf dry matter at maturity, post-anthesis dry matter, and nitrogen in nitrogen-efficient wheat restricts nitrogen uptake and utilization efficiency as well as nitrogen partial productivity. Therefore, breeding nitrogen-efficient wheat varieties with lower stem-leaf dry matter and nitrogen accumulation at maturity is a viable approach to improve nitrogen use efficiency.