Reasonable canopy structure and leaf physiological characteristics are considered as important factors for improving canopy nitrogen (N) distribution by matching the available light resources and thus increasing the grain yield of maize (Zea mays L.). However, the determinants of different maize varieties in light–N matching and grain yields with specific canopy structures and leaf physiological characteristics, as well as the response to the N application rate, remain poorly understood. In this study, we analyzed the relationships between different canopy structures and the enzyme activity and light utilization of spring maize in the field. Two maize varieties (XY335 and ZD958) with different canopy structures were used as the experimental material in a 2-year field experiment from 2014 to 2015, grown under different N inputs of 0, 100, 200, and 300 kg N ha⁻¹ (N0, N1, N2, and N3) at a planting density of 90,000 plants ha⁻¹ in Jilin Province on the Northeast China Plain. The results show that XY335 combined with N3 had a greater leaf angle, upper internode length and number, and upper leaf area index of the upper layer compared with ZD958. Higher N assimilatory enzyme (glutamine synthase (GS), glutamate synthase (GOGAT), and nitrate reductase (NR)) activities in the upper and middle leaves were observed in XY335 compared to ZD958. Furthermore, the light interception and light utilization efficiency of the upper leaves of XY335 increased, especially at higher N application rates, which significantly affected the N translocation post-silking and its distribution in different populations. As a result, the photosynthetic N use efficiency (PNUE) values of the upper leaves (10.4%) and middle leaves (5.2%) of XY335 were higher than those of ZD958, coordinating the canopy light and N distributions and being positively correlated with the maize grain yield. This suggested that the superior canopy structure of the upper layer and N assimilatory enzymes of the upper and middle leaves of this maize variety significantly increased the light interception of the canopy, while the synchronization of light and the N of the upper and middle leaves increased the light and N utilization efficiency of maize, which ultimately increased the grain yield at a high plant density.

In order to evaluate the effects of climatic conditions on maize grain yield (GY), kernel weight (KW), and kernel filling and identify the optimal climatic factors for GY and KW, 2-year field experiments in three seasons, i.e., spring (S_PM), summer (S_UM), and autumn (A_UM), on maize were conducted in Central China. The results showed that S_UM had more growing degree days (GDDs) than S_PM and A_UM due to the higher mean temperature (MT), and also resulted in higher temperature stress (killing degree days (KDDs)) in maize growth duration. Meanwhile, after silking, S_PM and S_UM had more GDDs and KDDs than A_UM because of the higher MT, and the accumulated solar radiation (Ra) for S_UM was significantly higher than for S_PM and A_UM. The GY of S_PM was significantly higher than that of S_UM and A_UM, while S_UM’s GY was always the lowest, because the GDD_GD, MT_GD, and KDD_GD played significantly negative roles on GY. The final KW for S_UM was always the lowest, with GDD, MT, KDD, and Ra causing significantly negative effects, and M△T and precipitation having significant positive effects, resulting in a lower kernel filling rate during the linear kernel filling period (KFR_lkf) and a lower GDD at the maximum kernel filling rate (GDD_KFRmax). Maize KFR_lkf has significant negative linear dependences on GDD, MT, and Ra. In summary, because of the higher MT, KDD, and GDD during maize growth and kernel filling duration negatively affecting the maize kernel filling rate, the GY and KW for S_PM were the highest, and for S_UM, they were the lowest; therefore, farmers should plant S_PM first and then A_UM in Central China.

Spring wheat–summer maize (SWSM) annual crop systems were formed to satisfy the maize grain mechanized harvest thermal requirement in the thermal–resource–limited region of the North China Plain. However, the nitrogen (N) application rate effect on SWSM annual yield formation, N accumulation and utilization were barely evaluated. Two–year field experiments were conducted to evaluate the effects of the N application rate on the annual yield of SWSM, observe N accumulation and utilization, and identify the optimized N application. The experiments were conducted under 5 N levels of 0 (N0), 180 (N180), 240 (N240), 300 (N300), and 360 (N360) kg ha⁻¹. The results showed that spring wheat, summer maize and annual cereal yield under the N₂₄₀ and N₄₈₀ treatments obtained the highest grain yield (GY) of 5038, 1282 and 16,320 kg ha⁻¹, respectively, and the optimal N application rate was estimated using a linear–plateau model to be 231–307, 222–337 and 463–571 kg ha⁻¹ with maximum GY of 4654–5317, 11,727–12,003 and 16,349–16,658 kg ha⁻¹, respectively. With the increase in the N application rate, the dry matter accumulation (DM) were significantly increased by 16.9–173.5% for spring wheat and 11.1≈–76.8% for summer maize, respectively; and the annual cereal DM was 15.1–179.7% greater than that with N₀ treatment, respectively. Spring wheat, summer maize and the annual cereal total N accumulation (TN) under N₃₆₀ and N₇₂₀ treatments were significantly increased by 5.4–19.1%, 16.6–32.3% and 11.5–26.2%, respectively, compared to the other treatments; however, N use efficiency for biomass and grain production (NUEbms and NUEg) were decreased significantly by 10.9–13.6% and 8.9–20.7%, 6.8–13.8% and 12.2–15.6%, and 5.5–11.7% and 10.0–16.0%, respectively. Meanwhile, the N partial factor productivity (PFPN), N agronomy use efficiency (ANUE), N recovery efficiency (NRE) and N uptake efficiency (NEupk) under the N₂₄₀ treatment for spring wheat and summer maize obtained high levels of 20.99 and 47.01 kg⁻¹, 9.27 and 16.35 kg⁻¹, 32.53% and 32.44%, and 0.85 and 0.72 kg⁻¹, respectively. Correlation analysis showed that the N application rate, TN and NEupk played significantly positive roles on GY, spring wheat spilke grain number, summer maize ear grain number and 1000–grain weight, DM LAImax and SPADmax, while NUEbms, NUEg, PFPN and ANUE always played negative effects. These results demonstrate that spring wheat, summer maize and annual cereal obtained the highest GY being 4654–5317, 11,727–12,003 and 16,349–16,658 kg ha⁻¹ with the optimal N application rate 231–307, 222–337 and 463–571 kg ha⁻¹, respectively, which provide N application guidance to farmer for spring wheat–summer maize crop systems to achieve annual mechanical harvesting in the thermal–resource–limited region of the North China Plain.