Photosynthesis is a major trait of interest for the development of high-yield crop plants. However, little is known about the effects of high-density planting on photosynthetic responses at the whole-canopy level. Using the high-yielding maize (Zea mays L.) cultivars “LY66,” “MC670,” and “JK968,” we conducted a 2-yr field experiment to assess ear development in addition to leaf characteristics and photosynthetic parameters in each canopy layer at 4 planting densities. Increased planting density promoted high grain yield and population-scale biomass accumulation despite reduced per-plant productivity. MC670 had the strongest adaptability to high-density planting conditions. A physiological analysis showed that increased planting density primarily led to decreases in the single-leaf area above the ear for LY66 and MC670 and below the ear for JK968. Furthermore, high planting density decreased chlorophyll content and the photosynthetic rate due to decreased canopy transmission, leading to severe decreases in single-plant biomass accumulation in the lower canopy. Moreover, increased planting density improved presilking biomass transfer, especially in the lower canopy. The yield showed significant positive relationships with photosynthesis and biomass in the lower canopy, demonstrating the important contributions of these leaves to grain yield under dense planting conditions. Increased planting density led to retarded ear development as a consequence of reduced glucose and fructose contents in the ears, indicating reductions in sugar transport that were associated with limited sink organ development, reduced kernel number, and yield loss. Overall, these findings highlighted the photosynthetic capacities of the lower canopy as promising targets for improving maize yield under dense planting conditions.

Understanding the contribution of aboveground dry matter (DM) production and harvest index (HI) to maize grain yield is essential to further improve grain yield to ensure future food growing requirements. In this study, we collected 548 sets of data of DM, HI, and grain yield from a multisite (40 sites) and multiyear (2009–2016) experiment conducted in major maize‐growing regions across China and 615 sets of data from 40 published papers distributed from 1988 to 2018 to assess the performance of DM, HI, and their contributions to grain yield. It was found that the average contribution rate of DM was 73.71%, which was much higher than those of HI (26.28%) across China. For the five different modern hybrids, the contribution rates of DM to grain yield (ranged from 71.36% to 89.10%) were also significantly higher than those of HI (ranged from 10.79% to 28.64%) on a large spatial scale. The significant differences in grain yields between cultivars were mainly due to the differences in HI. Among the four maize‐growing regions, the contribution rates of DM in both NW (57.34%) and NM (64.46%) were higher than those of HI (42.66% and 35.54%, respectively). In contrast, the contribution rates of HI in HM (52.67%) and SW (50.92%) were slightly higher than those of DM (47.33% and 49.08%, respectively). The interregional differences in grain yield were mainly attributed to the differences in DM between regions. Additionally, combining the data of this study and data collected from previous studies, we found that the increase in maize grain yield under lower yield levels (15 Mg/ha) was mainly dependent on the increase in DM.

Improved canopy structure was instrumental in setting maize yield records, and yet it has rarely been examined in China. At Qitai Farm in Xinjiang, we conducted a 4‐year field experiment using China's six highest‐yielding maize hybrids sorted into three yield level groups that were grown at similar growth durations and at optimum densities. The average yield of high‐yield level (HL, 22.3 Mg ha⁻¹) was 7.2% and 24.6% higher than that of medium‐yield level (ML) and low‐yield level (LL), respectively. For each yield level, we measured morphological traits that influence canopy structure and yield. They included plant height, ear height, ear ratio, internode length, leaf numbers, leaf angle, LOV, leaf area, and spatial density of leaf area. Among the preceding morphological traits of the three yield levels, HL’s best optimized the canopy structure, as shown by improved light distribution (19.0% light transmission at the ear) and increased light interception per unit leaf area per day (LIPA, 51.7 MJ m⁻² day⁻¹) in the canopy. In comparison, light transmission was 12.2% and 15.9% at the ear and the total LIPAs were 37.2 and 29.0 MJ m⁻² day⁻¹ at silking for ML and LL, respectively. HL had significantly longer leaf area duration and a higher photosynthetic rate, especially at the grain filling stage, and its total accumulated biomass at maturity was significantly better (by 13.9%) than LL’s. HL’s harvest index (0.54) was significantly higher than that of ML (0.52) and LL (0.48). HL’s radiation and heat use efficiencies were 2.61% and 1.37 g °C⁻¹ day⁻¹ m⁻², both significantly greater than those of ML and LL. Therefore, optimum maize plant types can significantly improve canopy structure and increase resource use efficiency and grain yield.

Matching of maize growth with solar radiation is of great importance for achieving high yield. We conducted experiments using different maize cultivars and planting densities under different solar radiations during grain filling to quantitatively analyze the relationships among these factors. We found that a decrease in solar radiation after silking caused a drop in maize grain yield and biomass, with lower solar radiation intensities leading to worse grain yields and biomass. Cultivar ZD958 was more sensitive to solar radiation changes than cultivar XY335; slight decreases in solar radiation (i.e., 15% shading) caused significant declines in ZD958 grain yield. When total solar radiation during grain filling was less than 486.9 MJ m⁻² for XY335 and less than 510.9 MJ m⁻² for ZD958, the two cultivars demonstrated high yields at lower planting density of 7.5 × 10⁴ plants ha⁻¹; average yields were 13.36 and 11.09 Mg ha⁻¹, respectively. When radiation intensities were higher than 549.5 MJ m⁻² for XY335 and higher than 605.8 MJ m⁻² for ZD958, yields were higher at a higher planting density of 12 × 10⁴ plants ha⁻¹, with average yields of 20.58 Mg ha⁻¹ for XY335 and 19.65 Mg ha⁻¹ for ZD958.