The popularity of density-tolerant maize (Zea mays L.) hybrids greatly increased the yield of maize hybrids in China in recent years. Clarifying trends in yield and plant density responses of agronomic characters of maize hybrids released in different years can provide theoretical and practical guidance for breeding high-yield and density-tolerant maize hybrids. The results showed grain yield (GY) of maize hybrids released in the 1970s and 1980s decreased with increased plant density. But GY of maize hybrids released in the 1990s and 2000s increased with a plant density increase.The GY gains from the low to high plant density plots were 86, 135, and 198 kg ha -1 per year, respectively. In all plant density treatments, yield and biomass of newer hybrids (2000s) were significantly higher than those of older hybrids (1970s). The leaf area index (LAI) of maize hybrids released in different years (1970s ∼ 2000s) increased with plant density, whereas the extinction coefficient decreased. Compared with the old hybrids, new hybrids have larger population sink capacity and stronger source supply ability, which is more prominent under high density. Population sink capacity is the main factor limiting maize yield. The key to maize hybrid improvement is to increase the population sink capacity. Breeders should continue to select for hybrids under stressful and high plant density conditions to increase seed setting rate and decrease empty ear rate of maize hybrids.

To explore the response of summer maize leaf senescence, photosynthetic characteristics, and yield to the depth of one-time base application of controlled-release urea, which provides a theoretical basis for the light and simplified production of summer maize. Seven treatments were set up with Zhengdan 958 as the material under field conditions, including no nitrogen fertilizer (CK), surface spreading (DP0), furrow application depth of 5 cm (DP5), 10 cm (DP10), 15 cm (DP15), 20 cm (DP20), 25 cm (DP25). The results showed that under the same nitrogen application rate, there are significant differences in the effects of summer maize leaf senescence and photosynthetic characteristics with the increase of fertilization depth, and DP10 and DP15 have the best effects. The LAI of DP10 and DP15 increased by 5.1% and 5.5% compared to DP0 at tasseling stage, and chlorophyll content increased by 6.8% and 7.3% in 10 days after tasseling. Compared with DP0, superoxide dismutase (SOD) increased by 13.1% and 10.5%, the content of soluble protein increased significantly, while the content of malondialdehyde (MDA) decreased by 9.8% and 10.8%, respectively. In addition, Pn and Gs of the ear-leaf significantly increased by 13.9%, 16.5%, and 26.1% and 31.9% at tasseling stage, respectively, over DP0, while Ci decreased by 22.3% and 26.4%, respectively; meanwhile, the photochemical quenching (qP) and quantum yield (ΦPSII) of the reaction center of photosystem II (PSII) of the ear-leaf were significantly improved, the non-photochemical quenching (NPQ) was significantly reduced. The yield of DP10 and DP15 heightened significantly; two-year average value increased by 5.7% and 6.0% compared with DP0; the kernels per spike and 1000-kernels weight increased by 4.8%, 5.2%, and 4.1%, 5.2%, respectively. Comprehensive analysis of LAI, chlorophyll content, various protective enzyme activities and MDA, soluble protein content showed that 10–15 cm is the appropriate fertilization depth when the nitrogen application rate of controlled-release urea is 225 kg N per hectare. In consequence, optimizing fertilization depth of controlled-release urea as a simplified fertilization mode could improve the nitrogen utilization efficiency and obtain higher yield in summer maize, which provides technical support for large-scale application of controlled-release urea.