## 4 Discussion
### Canopy Light Interception and Photosynthetic Characteristics under N Management Practices  
Canopy light interception plays a crucial role in improving wheat productivity [29; 30]. Our previous research showed that under traditional fertilization, in a high soil fertility field with a grain yield of 10,500 kg ha\({}^{-1}\), the canopy photosynthetic effective radiation interception rate was than in a high soil fertility field with a yield of 9000 kg ha\({}^{-1}\)[31]. In this study, in a super high soil fertility field with a grain yield of 10,500 kg ha\({}^{-1}\), the F treatment further increased the canopy photosynthetic effective radiation interception rate of after anthesis compared to the T (Figure 3). The fertigation matched the supply of water and N with the demand for wheat, reduced ineffective tillering, and promoted the production of large tillers [17]. The increase in the effective number of tillers reduced  
Figure 8: The N agronomic efficiency and water use efficiency under different treatments. N0, no N application; N1, N application rate of 150 kg ha\({}^{-1}\); N2, N application rate of 210 kg ha\({}^{-1}\); N3, N application rate of 270 kg ha\({}^{-1}\); T, traditional fertilization method; F, fertigation. Different letters indicate statistically significant differences (\(p<0.05\)).  
the loss of light leakage, provided a basis for better utilization of light energy during the filling stage.  
The vast majority of grain yield comes from post-anthesis photosynthetic products, especially the contribution of flag leaves to the accumulation of dry matter in grains. In this study, compared to traditional fertilization method, fertigation improved the Pn, Gs, and Trof flag leaves during the mid grain filling stage, which created conditions for the formation of photosynthetic product in the later stage (Figure 4). Under two N management practices, with the increase of N rate, the Pn of flag leaf first increased and then decreased from 0 to 14 d after anthesis. It could be seen that under N2, the Pn of flag leaf was significantly higher than other treatments, which was conducive to the continuous supply of carbon assimilates at the filling stage. With the development of reproductive organs, the Pn and Gs of flag leaves in each treatment showed a decreasing trend from 14 d to 21 d after anthesis. Meanwhile, the Ci of flag leaves in N0 and N1 treatments also showed a decreasing trend, while the N2 and N3 treatments showed the opposite trend. Research has shown that when the intercellular CO2 concentration decreases along with stomatal conductance, the reduction in photosynthetic rate is mainly caused by the decrease in stomatal conductance; On the contrary, if the limiting for photosynthetic rate is non stomatal factor, it is a decrease in the photosynthetic activity of plant mesophyll cells [32]. It could be seen that the N application rate affected the photosynthetic activity of mesophyll cells, as evidenced by the results of the activity of sucrose phosphate synthase (SPS) in flag leaves in the study (Figure 5). Photosynthetic carbon assimilates in leaves mainly existed and exported outward in the form of sucrose [33]. The synthesis of sucrose in mesophyll cells can reflect the supply capacity of photosynthetic assimilates [34]. In this study, under N2, fertigation treatment increased the SPS activity and sucrose content of flag leaves after anthesis, which was beneficial for the accumulation of photosynthetic products in the mid grain filling stage.