## 5 Conclusions  
Our results show that the response of fluorescence emissions to drought is smaller than those of either stomatal conductance or net photosynthetic carbon assimilation. At the canopy scale and beyond, however, structural dynamics dominate the spatial variationof canopy SIF in response to water stress, explaining the strong drought response of SIF retrieved from space. As drought becomes more severe, the shifts in energy allocation towards decreasing photochemistry and increasing fluorescence emission tend to push plants into the PQ phase, enhancing the nonlinearity in the overall relationship between photochemistry and fluorescence. We confirm that SIF alone has a limited ability to predict drought-induced declines in photosynthetic parameters. Alternatively, the rMLR model, using SIF as one important input variable, demonstrates a satisfactory performance in reproducing declines in stomatal conductance and net photosynthetic carbon assimilation. The rMLR model has good potential for applications at regional and global scales, and thus provides the basis for using SIF mechanistically to estimate GPP under the scenario of increasing intensity and the extent of droughs in the twenty-first century. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/plants12193365/s1](https://www.mdpi.com/article/10.3390/plants12193365/s1)]([https://www.mdpi.com/article/10.3390/plants12193365/s1](https://www.mdpi.com/article/10.3390/plants12193365/s1)); Notes S1: Calculation of \(\Phi_{\rm P}\), \(\Phi_{\rm F}\), \(\Phi_{\rm N}\), \(\Phi_{\rm D}\), NPQ, \(\Phi_{\rm Pmax}\) and \(J_{\rm a\_PAM}\); Figure S1: Variations in \(\beta_{\rm S}\) and \(\beta_{\rm B}\) for the water stress (WS, diamonds) treatments under 12 days of progressive drought stress; Figure S2: Average absorbed photosynthetically active radiation (APAR, \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)) under changing light intensity over the duration of the drought; Figure S3: The responses of full-band chlorophyll fluorescence emission at the photosystem level (ChIF\({}_{\rm P\_F}\), \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)) to changing light intensity during the period between 14 and 28 days (WS14 to WS28) after withholding water; Table S1: List of retrieved parameters, their initial values, lower boundaries (LB), upper boundaries (UB) and constraints. **Author Contributions:** [PERSON] and [PERSON] planned and designed the research. [PERSON] developed model assumptions and scenarios. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] performed experiments. [PERSON] analyzed data. [PERSON] and [PERSON] wrote the manuscript. [PERSON], [PERSON] and [PERSON] edited the manuscript. All authors have read and agreed to the published version of the manuscript. **Funding:** The research was supported by the National Natural Science Foundation of China (Grant No. 41901293 to Z.L., and 42071328 to X.L. ), the China Postdoctoral Science Foundation (Grant No. 2019M663828 to Z.L. ), and the Chinese Universities Scientific Fund (24520212452021125 to X.L.). **Data Availability Statement:** The data are available on request from the authors. The data supporting the findings of this study are available from the corresponding author, [PERSON], upon request. **Acknowledgments:** The authors would like to thank the anonymous reviewers for their valuable comments and suggestions. The authors gratefully thank [PERSON] for helping with the experiment. **Conflicts of Interest:** The authors have no conflict of interest to declare.