## 1 Introduction
### The Current State of Investigation on Photosynthesis and COS Assimilation
The gradient between the atmospheric CO\({}_{2}\) concentration (C\({}_{a,\text{CO}_{2}}\)) and the intercellular CO\({}_{2}\) concentration (C\({}_{i,\text{CO}_{2}}\)) is mainly regulated by the stomatal guard cells. Specifically, the mesophyll CO\({}_{2}\) concentration is controlled by the diffusive pathway. Then, the above processes regulate CO\({}_{2}\), COS and H\({}_{2}\)O fluxes [92; 93]. There are differences between the plant morphology (herbs, trees), tree foliage types (conifer, deciduous and evergreen) and the photosynthesis pathways (C3 and C4 plants). It has been estimated that 85% of photosynthetic CO\({}_{2}\) uptake is regulated by g\({}_{\text{s}}\) and mesophyll conductance (g\({}_{\text{m}}\)) [94]. Stomata play an important role in linking carbon and water processes [95]. On the other hand, because COS is a trace gas in the troposphere and the catalytic efficiency of CA enzyme is high, the intercellular COS concentration (C\({}_{i,\text{COS}}\)) is kept at a very low level [41; 51; 96]. As they share the same assimilation pathway, COS and CO\({}_{2}\) are also maximally resisted by the stomata and the mesophyll [37; 7; 39]. Furthermore, based on parallel observations of the atmospheric concentration of COS (C\({}_{a,\text{COS}}\)) and C\({}_{a,\text{CO}_{2}}\)[98; 56], recent studies found that COS can be used to obtain g\({}_{\text{s}}\) across different biomass and scales [60; 73; 99]. From the perspective of both diel and seasonal variation, COS can not only be slightly continuously taken up by the partially closing stomata at night [65], but when some branches wither, COS can be released from these dead or drought-damaged components [65; 100; 101]. In the process of plant photosynthesis, by eliminating the interference of non-foliage components [90], the conductance of the leaf boundary (g\({}_{\text{b}}\)) is caused by adhesion and COS. The COS assimilation follows \(\text{F}_{c,\text{COS}}=(1/\text{g}_{\text{b}}+1/\text{g}_{\text{s}}+1/\text {g}_{\text{m}})^{-1}\cdot(\text{C}_{a,\text{COS}}-\text{C}_{i,\text{COS}})\)[51; 56; 60; 73; 97]. The pathway conductance controls the COS assimilation rate at the leaf scale [60]. Meanwhile, the COS flux is found to be regulated by radiation and the vapor pressure deficit (VPD) [102; 103; 104; 105]. In the diffusional pathway, environmental conditions, such as radiation, the leaf VPD, relative humidity (rH), nitrogen deposition, salinity and aerosol concentrations, also affect stomata conductance [106; 107; 108]. In the process of COS, CO\({}_{2}\) and H\({}_{2}\)O exchange, the molecule of COS is larger than CO\({}_{2}\) and the adhesional coefficient of COS is different from CO\({}_{2}\)[51; 56]. Moreover, stomata strongly respond to COS, which is mediated by H\({}_{2}\)S [51]. Further, in hydrolysis, the mesophyll COS is catalyzed by the CA enzyme [37; 41], while in photoreaction, CO\({}_{2}\) is catalyzed by RuBisCO [51; 87]. This causes a different diffusional coefficient between COS and CO\({}_{2}\) in each pathway. For other processes, because COS cannot be released by plant respiration, the difference is also shown in the photosynthetic feedback pathway between COS and CO\({}_{2}\)[109]. The assimilation processes of both COS and CO\({}_{2}\) also vary between species. It is important to explore the differential regulation mechanism of environmental conditions in the whole diffusional pathway of COS and CO\({}_{2}\) assimilation (Figure 1). In observational and modeling studies, water use efficiency (WUE), as the ratio of carbon assimilation (i.e., GPP) to water losses, has been employed to represent the coupling of the carbon and water cycles. Controlled by leaf stomata, WUE is broadly correlated with C\({}_{a,\text{CO}_{2}}\) at the leaf boundary layer [110].