## 4 Discussion
### Uncertainties in the SIF-Based GPP Model  
Previous studies have demonstrated that the relationship between remotely sensed SIF and GPP can be characterized by a linear and ecosystem-specific function [23; 24; 25]. There is also evidence indicating that the SIF and GPP relationship can be affected by various factors such as the rate of NPQ [61; 87], light conditions [46; 87], nutrient availability [61], water availability [87], temperature [23; 46; 68], and canopy structure [34]. Recent studies have suggested that there is a curvilinear relationship between instantaneous SIF and GPP due to the rate of photosynthesis saturated under high illumination, and this relationship tends to become linear when both SIF and GPP are temporally aggregated [28; 34]. However, the saturation of GPP with APAR were not found in our experiment (Figure 4) as C4 species exhibit higher photosynthesis capacities at higher temperatures than C3 species. Therefore, the relationship between SIF and GPP could be adequately characterized by a linear function in our research. However, the SIF-GPP relationship may not be constant in spatial and temporal modeling, which can be attributed to the ecosystem-specific structural and physiological characteristics as well as the environmental situations associated with each ecosystem [34]. Therefore, for accurate quantification and characterization of the SIF-GPP relationship, further experiments that overlap the entire growth periods in various ecosystems are needed, especially for C3 species. Remote measurements of SIF are likely representative of the upper layer of the canopy, which is mainly characterized by sunlit leaves [34; 59]. Although the upper layer substantially contributes to the total GPP, there are still many layers of leaves that contribute to the canopy photosynthesis at different rates depending on their sunlit or shaded exposure [59]. Thus, the heterogeneity of the vertical canopy may lead to a mismatch between the SIF-based estimates and the EC-based GPP  
Figure 8: \(D\)Q\({}_{8}\) values of the measurements: (**a**) 10 July; (**b**) 17 July; (**c**) 18 July; (**d**) 21 August; and (**e**) 22 August. T1 to T4 represent the measurements of the four canopy targets. Additionally, under low light unstressed conditions, there exists a competitive relationship between leaf fluorescence and photosynthesis and a positive correlation between them under stressed and high light conditions [87; 88]. The relationships between SIF and GPP for sunlit and shaded leaves are different as sunlit leaves receive more light and shaded leaves only receive diffuse light [88]. Therefore, additional studies are needed to address the contributions of the shaded leaf groups and the structural sensitivities of canopy-leaving SIF signals. Light energy absorbed by leaf chlorophyll molecules has three different pathways: photochemistry, NPQ, and SIF. Accurately estimating any one of the de-excitation pathways require measurement of the other two. Some researches indicated that most absorbed energy is used in photosynthesis and both fluorescence and NPQ are low under low light conditions. Under high light and stressed conditions, NPQ becomes dominant and the yields of fluorescence and photosynthesis decrease [87]. As a result, the relationship between SIF and GPP is influenced by NPQ. The mechanism of NPQ and its regulation should be considered in the further studies. Plant photosynthesis starts with absorption of light, mainly by chlorophyll molecules. Accordingly, some mechanisms were used to modulate photosynthetic light. Former studies have indicated that chloroplasts can change their orientation within the cell in response to the intensity and direction of the incident light [89]. When light is limiting to photosynthesis, chloroplasts move to positions that yielding maximum light absorption, whereas in saturating light they hide themselves to make the light absorption minimized.