## 4 Discussion
### Discussion on the Synergistic Mechanism of O3 and CO2  
The effect of CO2 fertilization was mainly caused by the higher affinity of Rubisco for CO2. Due to the lack of a CO2 concentration mechanism in C3 plant cells, more photorespiration products are invested in the production of Rubisco [68,69]; as [CO2] rises, control of Asat by Rubisco (Vcmax) decreases and control by the capacity for RubP  
Figure 6: After adding LAI to the SURD analysis, the obtained analysis results were obtained. (**a**) Incorporate LAI as an independent variable into the SURD analysis of GPP in the E2 scenario. (**b**) Incorporate LAI as an independent variable into the SURD analysis conducted in the E3 scenario. (**c**) After conducting a SURD analysis of LAI as the dependent variable along with meteorological factors CO2, and O3, the U value of CO2 on LAI was obtained. (**d**) The U value of GPP in the E2 scenario with CO2. The U value here represents the “Unique” aspect in SURD, indicating the degree of influence of a single variable on the dependent variable. The sum of all U values equals 1. -regeneration (Jmax) increases [70]. Rubisco is usually fully active and carbamylated at current [CO\({}_{2}\)] under steady-state high-light conditions [71; 72]. As [CO\({}_{2}\)] increases, carbon fixation increases; there is an increasing demand for ATP (required for RubP regeneration), and the control of photosynthesis shifts from being limited by Rubisco to being limited by the capacity for RubP regeneration [73]. Succinctly, when the supply of photosynthate from chloroplasts exceeds the capacity for export and utilization by sink tissue, the imbalance in supply and demand is sensed in mesophyll cells by a mechanism that possibly involves hexokinase acting as a flux sensor [69; 74]. It further leads to the decrease in stomatal conductance and the saturation of photosynthesis. However, the CO\({}_{2}\) concentration in most areas is not enough to reach the maximum limit of Rubisco, so the increase in CO\({}_{2}\) decreases the stomatal conductance while still enhancing the photosynthesis of vegetation. Generally, the increase in O\({}_{3}\) decreased the content of the Rubisco enzyme and nitrogen per unit area. This led to the decreases in Vcmax and Jmax and affected the change in stomatal conductance of vegetation [75]. A decrease in the stomatal conductance of plants leads to a decrease in ozone absorption, preventing plants from being further affected by ozone. Recently, some studies have shown that mesophyll conductance may be more significant than stomatal conductance. [PERSON] (2022) [76] et al., based on fumigation experiments, found that ozone significantly reduced the mesophyll conductance of four woody plants but did not significantly reduce the stomatal conductance of all species. Therefore, the decrease in mesophyll conductance of ozone-controlled plants may be the main reason for the decrease in photosynthesis. However, the reciprocal mesophyll conductance (gm) of CO\({}_{2}\) resistance during its propagation from cellular interstitium to photosynthetic carboxylation site is similar to stomatal conductance [77; 78]. We believe that ozone reduces mesophyll conductance while reducing stomatal conductance, thus reducing the propagation resistance of CO\({}_{2}\) in cells. Though this explanation lacks more solid physiological evidence, it gives us a very important implication in that ignoring the Vcmax and Jmax estimated by gm will accumulate the negative effect of O\({}_{3}\) on gm in its effect on photosynthetic biochemical capacity. In addition, gm is a key parameter of photosynthesis models, and its inclusion in vegetation models can significantly improve the simulation accuracy of carbon and water fluxes [79]. A number of studies have indicated that using well-validated ecophysiological mechanism models to assess surface-atmosphere feedback is a better and more relevant approach [36; 48; 80], although this is challenging and requires further development.