Enhancing yield of modern maize (Zea mays L.) hybrids through optimization of population photosynthetic capacity and light-nitrogen use efficiency under high planting density

In maize production, the development of density-tolerant and lodging-resistant varieties has made dense planting an effective strategy for achieving high and stable yields, with superior hybrids serving as a prerequisite for successful high-density cultivation. However, the photosynthetic mechanisms underlying improved density tolerance in maize hybrids released across different eras in China remain unclear. This study investigates 40 years of breeding progress toward enhanced photosynthetic traits under varying planting densities and elucidates the physiological and ecological bases of improved density tolerance in maize hybrids. A three-year field experiment was conducted from 2019 to 2021 to compare eight major Chinese hybrids from four decadal cohorts under three planting densities: 45,000 (D1), 67,500 (D2), and 90,000 (D3) plants ha⁻¹. At high density (D3), modern hybrids exhibited a more optimal canopy architecture and superior leaf photosynthetic performance compared to older hybrids, despite a slight reduction in specific leaf nitrogen. Notably, modern hybrids (2000s) were able to maintain higher net photosynthetic rates and photosynthetic nitrogen use efficiency (PNUE) at D3, resulting in the highest grain yield (GY), which was 118.47% greater than that of older hybrids (1970s). Leaf area duration after anthesis, total chlorophyll content, key photosynthetic enzyme activities, and maximum quantum efficiency of PSII photochemistry were all positively correlated with GY. Among these, PNUE showed the strongest correlation with grain yield and thus represents a key indicator for optimizing maize hybrids. Based on these findings, breeders should continue selecting hybrids under high-density and suboptimal conditions, focusing on optimizing population architecture and enhancing photosynthetic capacity while fine-tuning leaf nitrogen status to develop high-yielding, density-tolerant hybrids capable of sustaining long-term increases in maize grain yield.