Interactive Effects of Hybrid and Planting Density on Summer Maize Growth and Grain Yield Under Subsurface Drip Irrigation in the North China Plain

Selecting suitable maize hybrids and appropriately increasing planting density is a crucial strategy for improving maize yield under subsurface drip irrigation. A two-year field experiment was conducted to assess the impacts of two maize hybrids (Zhengdan 958, ZD958; Jingke 968, JK968) and four planting densities of 60,000 (PD1), 75,000 (PD2), 90,000 (PD3), and 105,000 plants ha−1 (PD4), on maize growth indices, ear leaf photosynthetic parameters, nitrogen content, grain yield, and yield components. The results indicated that with increasing planting density, the plant height of ZD958 initially increased and then decreased, whereas that of JK968 continued to increase. The leaf area index of both hybrids consistently increased, while stem diameter, rind puncture strength, and stalk breaking strength gradually decreased. Dry matter accumulation initially increased and then decreased, peaking at PD3. Ear leaf nitrogen content, relative chlorophyll content, net photosynthetic rate, and stomatal conductance all decreased with increasing planting density, while intrinsic water use efficiency (iWUE) first increased and then declined, reaching its maximum at PD3. Notably, iWUE of JK968 was significantly higher than that of ZD958 during the dough stage (p < 0.01). Ear traits, including ear length, ear diameter, kernels per ear, and grain weight per ear, all decreased continuously with increasing planting density. Grain yield followed a unimodal curve, peaking at the PD3 treatment, with two-year average yields of 12.7 and 13.5 t ha−1 for ZD958 and JK968, respectively. JK968 exhibited significantly higher leaf area index, stem diameter, rind puncture strength, stalk breaking strength, ear length, kernels per ear, grain weight per ear, and grain yield compared to ZD958 (p < 0.01), demonstrating superior tolerance to high planting density and enhanced source–sink coordination. In conclusion, in the North China Plain, JK968 planted at a density of 90,000 plants ha−1 can synergistically optimize population structure, improve stalk mechanical strength, and enhance photosynthetic efficiency, under subsurface drip irrigation.