(Aims) To clarify the mechanisms though which dense planting could alleviate the negative effect of the reducing N rate on yield, (Methods) an experiment with four nitrogen levels—0 (N0), 120 (N1), 180 (N2) and 240 (N3) kg N ha⁻¹—and three plant densities—180 (D1), 240 (D2) and 300 (D3) × 10⁴ basic seedlings ha⁻¹—was conducted. (Results) Increasing plant density decreased the root length, root volume, root surface area and root tips of individual plant while it enhanced the aforementioned root traits in population. The chlorophyll content, photosynthetic rate, stomatal conductance and transpiration rate of the individual plants were decreased with the increase in plant density and enhanced with the increase in N level. The increasing density and N application rate enhanced the leaf area index, photosynthetic high-efficiency leaf area and canopy photosynthetically active radiation of population. N accumulation per plant was decreased with increasing density and was enhanced with an increasing N application level. Within the same N level, the N accumulation in the population, N production efficiency and N recovery efficiency were consistently D3 > D2 > D1. A high N application rate with high density was not conducive to improving the NR (nitrate reductase), GS (glutamine synthetase) and GOGAT (glutamate synthase) activities. The yield could be maintained as stable or improved if decreasing by 60 kg N ha⁻¹ with increasing 60 × 10⁴ basic seedlings ha⁻¹ within the range of N application in this experiment. (Conclusions) These results indicated that the yield of wheat could be improved with less N application by adjusting the compensatory effects from the plant density in populations.

To fully explore the effects of N on enhancing photosynthesis, grain quality, and yield of wheat (Ningmai 13), experiments with four nitrogen levels 0 (N0), 120 (N1), 180 (N2), and 240 (N3) kg N ha⁻¹ and four ratios of basal to topdressing R0 (0:0), R1 (7:3), R2 (6:4), and R3 (5:5) were conducted. The basal N was applied to soil before sowing and the topdressing N was applied at jointing stage. The effect of N topdressing ratio on improving leaf area of photosynthetic efficiency was insignificant under the same N level. The effect of N fertilization level on increasing chlorophyll content was more significant than that of N topdressing ratio. Within the same N level, the canopy photosynthetically active radiation in R2 was higher than that in R1 and R3, and increasing N by 60 kg ha⁻¹ significantly enhanced canopy photosynthetically active radiation. The effect of N topdressing ratio on photosynthetic rate, stomatal conductance, and transpiration rate were consistently R2 > R3 > R1; compared to N1, N3 could significantly increase photosynthetic rate. Increasing 120 kg N ha⁻¹ significantly enhanced grain protein content, wet gluten, and sedimentation value, while the effect of N topdressing ratio was insignificant. Increasing N dose from 120 kg ha⁻¹ to 180 kg ha⁻¹ significantly enhanced yield, and the yields and harvest indexes in 2019, 2020, and 2021 were consistently R2 > R3 > R1. The findings suggested that the effect of increasing N dose (60 kg ha⁻¹) was more considerable than that of N topdressing ratio, N3R2 (within the range of N application in this experiment) was more conducive to improving canopy photosynthesis, yield, and harvest index, and R3 was more conducive to increasing grain protein content, wet gluten, and sedimentation value.

Optimizing the N application amount and topdressing ratio can increase crop yield and decrease N loss, but its internal mechanisms have not been well studied, especially from the aspects of population dynamics and structure, ear fruiting traits. Here, field experiments, with three N rates 120 (N1), 180 (N2), and 240 (N3) kg N ha^-1 and three N topdressing ratios T1 (7:3), T2 (6:4), and T3 (5:5) were conducted. At the same N level, results showed that the N accumulation amounts in the leaf, grain, and plant in T2 were higher than in T3 and T1, and increasing 60 kg N ha^-1 (N3 compared to N2, N2 compared to N1) significantly enhanced N accumulation amounts. The effect of the N topdressing ratio on partial factor productivity of applied N was consistently T2 > T3 > T1, but T1 was more conducive to improving N utilization efficiency for grain and biomass production. After the jointing stage, compared to T1 and T3, T2 was more conducive to increasing the population growth rate of plant height, leaf area index, leaf area growth rate, dry matter weight, dry matter accumulation rate, light interception rate, and spikelets of population, and the above-mentioned indexes of population could be significantly enhanced by increasing 120 kg N ha^-1. T2 increased the fruiting spikelets per ear, grains per ear, grain weight per ear, fruiting rate per ear, grain filling rate per ear, and yield but decreased the sterile spikelets at the top and bottom and imperfect grains per ear. Increasing N from 120 kg ha^-1 to 180 kg ha^-1 or from 180 kg ha^-1 to 240 kg ha^-1 significantly enhanced yield. The N accumulation amount in the grain, leaf, plant, leaf area growth rate, dry matter accumulation rate, light interception rate, population spikelets, fruiting spikelets per ear, grain filling rate, and yield were significantly positively correlated with each other. This study demonstrates a suitable N application rate with a N topdressing ratio 6:4 would more effectively improve N efficiency, population dynamics, structure, ear fruiting traits, and yield, but the effect of the N topdressing ratio is not as significant as that of increasing 60 kg N ha^-1.

To fully explore the effects of N on enhancing photosynthesis, grain quality and yield of wheat, experiments with four nitrogen levels 0 (N0), 120 (N1), 180 (N2) and 240 (N3) kg N ha^− 1 and four ratios of basal to topdressing R0 (0:0), R1 (7:3), R2 (6:4) and R3 (5:5) were conducted. The effect of N topdressing ratio on improving leaf area of photosynthetic efficiency was insignificant under the same N level. The effect of N fertilization level on increasing chlorophyll content was more significant than that of N topdressing ratio. Within the same N level, the canopy photosynthetically active radiation in R2 was higher than that in R1 and R3, and increasing 60 kg N ha^− 1 significantly enhanced canopy photosynthetically active radiation. The effect of N topdressing ratio on photosynthetic rate, stomatal conductance and transpiration rate were consistently R2 > R3 > R1, compared to N1, N3 could significantly increase photosynthetic rate. Increasing 120 kg N ha^− 1 significantly enhanced grain protein content, wet gluten and sedimentation value, while the effect of N topdressing ratio was insignificant. Increasing N application amount from 120 kg ha^− 1 to 180 kg ha^− 1 significantly enhanced yield, and the yields and harvest indexes in 2019, 2020 and 2021 were consistently R2 > R3 > R1. The findings suggested that the effect of increasing N application amount (60 kg ha^− 1) was more obviously than that of N topdressing ratio, and R2 was more conducive to improving canopy photosynthesis, grain quality, yield and harvest index.