## 4 Discussion
### Uncertainties and Limitations  
In our study, we found that the GPP/SIF ratio differed for 14 vegetated PFTs using the TROPOMI SIF product and FLUXCOM GPP data. The GPP/SIF ratio for forest types was generally higher than that of grass and crop types, which is not consistent with previous research [58]. [PERSON] et al. [58] reported that there was a generally consistent slope of the SIF-GPP relationship among biomes using the OCO-2 SIF and flux tower GPP data. The reason why our results are not consistent with previous research may be as follows: (1) the different datasets used in research; (2) different biome types, as we used a vegetation map produced from CCI LC, LUH2 and a C3/C4 vegetation map, while [PERSON] et al. [58] used the MODIS Land Cover Type product (MCD12Q1) based on the University of Maryland (UMD) classification scheme; and (3) different linear fitting methods, as we used a linear regression with no intercept, while [PERSON] et al. [58] used a linear regression with an intercept. Theoretically, SIF is related to GPP through the coordinate origin. In addition, we found that the slope between GPP and SIF for C4 CRO was lower than that for C3 CRO, which is not consistent with the results of [PERSON] et al. [58]. In our study, the selected samples of C4 CRO were mainly distributed at tropical regions (Figure 3) and could experience temperature and water stress. SIF is mainly driven by APAR and is also influenced by environmental stress (temperature and water stress), which determines photosynthetic light use efficiency (LUE). Therefore, both SIF and GPP for selected C4 CRO samples had a lower value than that for selected C3 CRO (Figure 3). Some studies have reported that the slope between GPP and SIF for C4 CRO was generally higher than that for C3 CRO [7; 58]. However, the potential influences of environmental conditions and the distributions of different PFTs are ignored. In addition, less C4 CRO samples were used, which may also have resulted in some uncertainties in our study. The potential physiological changes caused by low-temperature conditions may result in the decrease in the GPP/SIF ratio in response to low temperatures. Group A of each PFT generally experienced low-temperature stress (below 10 \({}^{\circ}\)C), whereas group B of each PFT generally experienced relatively optimal temperature conditions (above 10 \({}^{\circ}\)C). In this study, we demonstrated that the seasonal patterns of the GPP/SIF ratio in group A of each PFT generally differed from those associated with group B, indicating that different temperatures affect the SIF-GPP relationship differently (Figure 12), especially under low-temperature conditions (Figure 13). The GPP/SIF ratio for group A generally exhibited a \"hump-shaped\" seasonal pattern, and that for group B showed a slightly \"bowl-shaped\" seasonal pattern. The above results are different from previous research that did not consider the effect of temperature conditions [34; 58]. There were multiple sources of uncertainty in the seasonal variations in the GPP/SIF ratio [34]. The complex interactions of environmental factors (e.g., light intensity, temperature, and available water) on the photosynthetic activities play a role in changing the GPP/SIF ratio. Increasing light intensity led to a decline in the GPP/SIF ratio [3; 21; 38]. As the photosynthetic efficiency presents a non-linear decrease and the chlorophyll fluorescence quantum efficiency is not sensitive to increasing light intensity, the GPP exhibits a saturation effect under high light levels, while the SIF tends to continuously increase [37; 83]. Decoupling of the GPP flux and SIF signals under water stress conditions may also alter the GPP/SIF ratio [41]. The rapid response of the stomatal conductance to water stress may explain the decline in the GPP/SIF ratio under water stress conditions [30; 42].