## 4 Discussion and Conclusions
[38] showed that the GPP of C3 crops exhibited so-called'midday depression', a stable or even decreasing trend in photosynthesis at high light, whereas an increase in the SIF curve was observed. [PERSON] et al. [39] reported a strongly non-linear relationship between GPP and SIF in an evergreen needleleaf forest based on tower observations, where GPP exhibited a saturation pattern under high-light conditions, while SIF did not. We propose that these findings are not contradictory to our results. In the application of the herbicide, the electron transport chain was blocked such that carbon assimilation decreased and SIF emissions were triggered as one of the de-excited paths to quickly dissipate absorbed excessive light energy, causing a divergence in the SIF-GPP relationship over short time scales. In contrast, the electron transport was not inhibited in the ABA treatment, and the feedback between light and dark reactions still remained normal. Although [PERSON] et al. [38] used one single spectrometer in their field experiment, avoiding wavelength mismatch between \(E_{\text{I}}\) and \(E_{\text{O}}\)[27], the uncertainties in their SIF observations due to the too long basic set sampling time (30 min) may very likely have caused the impact of the dark reactions to be indistinguishable. It is worth noting that [PERSON] et al. [39] also reported that leaf-level \(\Phi_{\text{ChIF}}\) exhibited a negative response to light intensity. This somewhat counterintuitive linkage between decreasing photosynthesis, increasing SIF, and decreasing SIF yield could be explained by the different sensitivity of \(\Phi_{\text{PSII}}\) and \(\Phi_{\text{ChIF}}\) to light levels: \(\Phi_{\text{PSII}}\) shows a much steeper decline than \(\Phi_{\text{ChIF}}\) in response to increasing stress levels such as light intensities [36]. As a consequence, the small decrease in \(\Phi_{\text{ChIF}}\) may be overwhelmed by the large increase in PAR, with the net effect of an increase in SIF emissions. Taken together, a well-designed SIF-observing system should have the potential to detect the contributions of the dark reactions of photosynthesis under field conditions. A tower-based canopy SIF system called FAME, developed very recently by [PERSON] et al. [27], significantly improved both the hardware and software designs, and their measurements showed that canopy-leaving directional SIF emissions saturated or even decreased at high light. In a recent high-temperature experiment, [PERSON] et al. [30] found that both light use efficiency and SIF yield decreased with increasing heat stress when PAR was not a limiting factor. These two studies highlighted the role of the dark reactions in the partitioning of absorbed solar radiation to the different pathways. Our experiments confirm that SIF had the capacity to track changes in the dark reactions of photosynthesis--in other words, the gas exchange and carbon assimilation of plants. A well-designed measurement system with the ability to measure \(E_{\text{I}}\) and \(E_{\text{O}}\) almost simultaneously is needed to fully realize its potential. This study highlights the role of the dark reactions that have been ignored or misinterpreted in previous studies and will support our understanding of how elevated CO\({}_{2}\) concentrations and warming under future climate change influence photosynthesis activities.