Summary: | Increasing the planting density of summer maize to improve the utilization efficiency of limited soil and water resources is an effective approach; however, how the leaf water-use efficiency (WUE<sub>L</sub>), yield, and RUE respond to planting density and genotypes remains unclear. A 2-year field experiment was performed in the North China Plain (NCP) to investigate the effects of planting density (high, 100,000 plants ha<sup>−1</sup>; medium, 78,000 plants ha<sup>−1</sup>; and low, 56,000 plants ha<sup>−1</sup>) and genotypes (Zhengdan 958 and Denghai 605) on the leaf area index (LAI), photosynthetic characteristics, dry-matter accumulation, WUE<sub>L</sub>, and RUE of maize. The objective was to explore the effect of density and genotype on the WUE<sub>L</sub> and RUE of maize. Increasing planting density boosted LAI, light interception, dry-matter accumulation, and spike number but reduced the chlorophyll content, net photosynthetic rate, transpiration rate, and 1000-kernel weight. Both high and low planting densities were averse to RUE and yield. Zhengdan 958 increased the WUE<sub>L</sub> by 19.45% compared with Denghai 605, but the RUE of Denghai 605 was 18.19% higher than Zhengdan 958, suggesting that Denghai 605 had a greater production potential as the planting density increased. Our findings recommend using 78,000 plants ha<sup>−1</sup> as the planting density with Denghai 605 to maintain summer maize yields in the NCP.
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