Response of Canopy Photosynthesis, Grain Quality and Harvest Index of Wheat to Different Nitrogen Application Methods

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.