Efficient nitrogen (N) utilization is crucial for maintaining grain yield under low N input. Less is known about the role of within-plant N allocation and reallocation on ear development and the factors determining N allocation during the critical period around silking. In this study, two maize hybrids, ZD958 (N-efficient) and LY99 (N-inefficient), were evaluated in a 2-year field experiment under two N rates (60 and 180 kg N ha^-1). N transport and allocation into the ear during critical period were investigated using ¹⁵N stable isotopic tracer. The number and area of vascular bundles in ear shank, above- and below-ear internode were measured. The two hybrids did not differ in grain yields under high N rate. However, the grain yield of ZD958 was 43.6% higher than that of LY99 under low N rate, deriving from 26.3% and 13.9% increment in grain number and grain weight, respectively. At early critical growth stage before silking, ZD958 increased allocation of soil-derived N to the ear by 225.2% compared with LY99 under low N rate. At late critical growth stage after silking, ZD958 increased allocation of soil-derived N and reallocation of vegetative-N to the ear by 45.5% and 116.6%, respectively, compared with LY99 under low N rate. As a result, ear growth rate and ear N content of ZD958 was 22.2% and 69.1% higher than that of LY99 at the end of critical period. During N allocation and N reallocation, the lower leaves were sacrificed and the N status of the ear leaf and upper leaves was mostly maintained to sustain photosynthesis. In the ear shank, flux rate and N concentration of the xylem sap in ZD958 were 53.1% and 32.5% greater at silking stage, and were 40.8% and 27.5% greater at 14-days after silking, respectively, compared with LY99 under low N rate. Correspondingly, the number and average area of big vascular bundles in ear shank of ZD958 were 56.2% and 31.0% greater compared with LY99. Parameters characterizing the number and area of big vascular bundles were positively correlated with N allocation and grain yield, while that of small vascular bundles were negative. It is concluded that efficient N allocation to the ear at critical period is essential for ear growth and the subsequent vegetative-N remobilization, so as to improve low-N tolerance in high-yielding maize hybrids. A superior vascular system around the ear, especially in the ear shank, can enhance N allocation into the ear and could be regarded as a physiological selection trait in maize breeding to improve nitrogen use efficiency.

有效的氮（N）利用对于在低氮输入下保持谷物产量至关重要。关于植物内氮分配和再分配对穗发育的作用以及决定吐丝关键时期氮分配的因素知之甚少。在这项研究中，在为期 2 年的田间试验中，在两种 N 施用量（60 和 180 kg N ha ^-1）下对两种玉米杂交种 ZD958（N 效率高）和 LY99（N 效率低）进行了评估。使用¹⁵N 稳定同位素示踪剂。测量耳柄、耳上、耳下节间的维管束数量和面积。两种杂交种在高施氮量下的籽粒产量没有差异。然而，在低施氮量下，ZD958的籽粒产量比LY99高43.6%，这主要是由于粒数和粒重分别增加了26.3%和13.9%。在抽丝前的早期关键生长期，ZD958在低施氮量下较LY99增加了225.2%的土壤源氮分配到穗部。在抽丝后的关键生长期后期，ZD958在低施氮量下较LY99分别增加了45.5%和116.6%的土壤源N分配和营养N重新分配到穗部。结果表明，ZD958的穗生长率和穗氮含量在关键期末比LY99高出22.2%和69.1%。在N分配和N再分配过程中，下部叶片被牺牲，而穗叶和上部叶片的N状态大部分被维持以维持光合作用。在穗柄中，与LY99相比，ZD958在抽丝期的通量率和N浓度在抽丝期分别增加了53.1%和32.5%，在抽丝后14天分别增加了40.8%和27.5%。速度。相应地，ZD958耳柄大维管束的数量和平均面积比LY99分别增加了56.2%和31.0%。表征大维管束数量和面积的参数与N分配和粮食产量呈正相关，而小维管束则为负相关。得出的结论是，在关键时期对穗部进行有效的氮分配对于穗部生长和随后的营养氮再利用至关重要，从而提高高产玉米杂交种的低氮耐受性。穗部周围，尤其是穗柄的优良维管系统，可以提高氮素向穗部的分配，可作为玉米育种的生理选择性状，提高氮素利用效率。