## 4 Discussion and Conclusions  
[PERSON] et al. [25] showed that leaf-scale fluorescence signals (SIF and F\({}_{8}\)) did not exhibit a clear trend after stomatal closure, suggesting that changes in SIF emissions only responded to changes in the light reactions of photosynthesis. They further hypothesized that the linearity between SIF and GPP observed in many previous studies resulted from a shared driver: APAR. However, our results show that both ChIF and F\({}_{8}\) could track changes in the photosynthetic CO\({}_{2}\) rate due to variations in the CO\({}_{2}\) concentration and leaf temperature under constant light conditions. Further, we observed a significant reduction in both ChIF(F\({}_{8}\)) and \(A_{\text{G}}\) after inducing stomatal closure. Our findings, in contrast, suggest that changes in SIF can not only track the impacts of stresses on the light reactions of photosynthesis but also offer relevant insight into carbon assimilation. Under steady-state conditions, the efficiencies of the light and dark reactions would coordinate with each other to optimally match the surrounding environmental conditions and internal substrate availability [36]. Downregulation or upregulation of the photochemical quantum yield should accompany the inhibition or promotion of light reaction centers, resulting in variations in the fluorescence intensity and NPQ. However, changes in the SIF magnitude related to changes in the overall efficiency of the dark reactions should be limited. Our experiment showed that the variations in ChIF and F\({}_{8}\) were less than 22% in response to the experimental treatments. It should also be noted that the light intensities were kept constant in this study. Further research is needed to quantify the contribution of the dark reactions to SIF emissions under other light levels. [PERSON] et al. [25] used a two-spectrometer approach to retrieve SIF at O\({}_{2}\)-A absorption, that is, two spectrometers were used to measure incoming solar irradiance (\(E_{\text{I}}\)) and outgoing radiance from the canopy (\(E_{\text{O}}\)). As described by [PERSON] et al. [27], however, it is virtually impossible to perform a precise cross-calibration between the upwelling and downwelling channels, and therefore \(E_{\text{I}}\) and \(E_{\text{O}}\) measurements are not sufficiently compatible for retrieving SIF emissions. To analyze the SIF response to factors other than incident PAR, it is important to reduce (or normalize) the impact of the variations in incoming irradiance. Although [PERSON] et al. [25] tried to minimize the impact of varying PAR by performing the pre/post-treatment measurements at nearly the same time (but on a different date), their strategy may not effectively rule out uncertainties such as cloud passages that may result in fluctuating light conditions. Considering these issues in their experiment, it is unlikely that they could accurately identify the response of the dark reactions of photosynthesis from their measurements. [PERSON] et al. [26] suggested that SIF was mainly controlled by the light reactions, and the fluorescence response occurred much later than the stomatal response. However, most of SIF and SIF yields in their study showed a large standard deviation of about 30% of their mean values, most likely caused by large differences among the samples. Thus, the simultaneous SIF response to stomatal closure might be overwhelmed by their large standard deviations. Our experiments also showed that changes in \(\Phi_{\text{ChIF}}\) and \(\Phi_{\text{PSII}}\) are largely congruent under constant light, suggesting that both of them have a similar response to changes in the dark reactions. This appeared to be inconsistent with previous studies that reported the decoupling of SIF emissions and the carbon reactions of photosynthesis under certain conditions. For example, SIF emissions showed a rapid increase shortly after the application of a herbicide, resulting in a large reduction in CO\({}_{2}\) assimilation rates [37]. [PERSON] et al.