## 3 Discussion
These studies are consistent with our findings. Our results also show that the fluorescence response to water stress occurs later, and at a smaller magnitude, which appears to contradict studies using satellite SIF, suggesting a strong negative response [26; 27]. Such a dichotomy can be resolved by assessing the relative importance of the structural and physiological contributions to the drought response of fluorescence at different scales. Fluorescence variations reported in leaf-scale studies are driven by changing fluorescence efficiencies alone (unless absorbed PAR is unchanged) which, as shown here, has a relatively muted sensitivity to water stress. However, at the whole-plant or canopy scales, both structural and physiologic components may regulate TOC SIF, and so a drought-induced decline in absorbed PAR dueto canopy structural changes (i.e., changes in leaf area or leaf angles) dominates negative anomalies observed in satellite SIF [27; 45]. It is worth noting that these seemingly inconsistent results should not necessarily be viewed as a failure of SIF for monitoring plant water stress, as the changes in canopy structure may also directly reflect the plant water status. The inconsistency also highlights the need to consider/normalize canopy structure factors during drought. Otherwise, we may run the risk of wrongly assigning physiological causality to variance in TOC SIF due to changes in the canopy structure [28]. Equation (3) also contains other parameters (for example, \(K_{\text{DF}}\), \(K_{\text{mc}}\), \(K_{\text{mc}}\), \(\Gamma^{\ast}\) and \(\Phi_{\text{Pmax}}\)). These additional parameters are often assumed to be constant in the literature but may actually vary with water stress. The actual \(K_{\text{DF}}\) value is currently unknown. [PERSON] et al. (2019) [31] set \(K_{\text{DF}}\) to 19, while [PERSON] et al. (2022) [46] assumed \(K_{\text{DF}}\) to be 9. According to Equation (3), using these two \(K_{\text{DF}}\) values will directly cause \(A_{\text{net}}\) to change by a factor of two. \(K_{\text{DF}}\) remains unchanged at the same temperature, but there is currently no relevant research on \(K_{\text{DF}}\) under water stress. \(K_{\text{mc}}\), \(K_{\text{mc}}\) and \(\Gamma^{\ast}\) depend on the partial pressure of oxygen and temperature, which are related to the specificity factor of Rubisco [47]. Furthermore, \(\Phi_{\text{Pmax}}\) decreases under water stress conditions. Considering these potential effects of water stress on the parameters involved in our theoretical equations, more future measurements are needed to quantitatively examine their relationships under water conditions. We performed a sensitivity analysis on the MLR model. The input variables are \(C_{\text{i}}\), \(K_{\text{co}}\), \(\Gamma^{\ast}\), \(\Phi_{\text{Pmax}}\), \(q_{\text{L}}\), and \(\text{ChlF}_{\text{P-F}}\), respectively. The sensitivity analysis results show that the main parameters affecting \(A_{\text{net}}\) variation are leaf physiological parameters, \(q_{\text{L}}\) and \(\text{ChlF}_{\text{P-F}}\). These two parameters explain more than 80% of the \(A_{\text{net}}\) variation, while other leaf physiological parameters with greater influence such as \(C_{\text{i}}\), \(K_{\text{co}}\), and \(\Phi_{\text{Pmax}}\) account for more than 15% of the total variation. However, the parameter \(\Gamma^{\ast}\) plays a small role in explaining the \(A_{\text{net}}\) variation. [PERSON] et al. (2022) [47] studied the relationship between SIF and GPP through the MLR model and showed that the \(q_{\text{L}}\) is sensitive to light intensity and can be expressed by the exponential equation of two parameters (\(aq_{\text{L}}\) and \(bq_{\text{L}}\)) between the \(q_{\text{L}}\) and PAR. However, we found that in the uniform plant functional types, the parameters \(aq_{\text{L}}\) and \(bq_{\text{L}}\) vary greatly. Therefore, we use the NPQ/\(\Phi_{\text{P}}\) version of the \(J_{\text{a}}\)-ChlF\({}_{\text{P-F}}\) equation. The NPQ/\(\Phi_{\text{P}}\) version mixes photochemistry and non-photochemistry.