## 4 Discussion
### Relationship of GPP-SIF Under Drought Stress  
We observed a significant non-linear correlation between GPP and SIF within a daily cycle, consistent with the findings from previous studies [23; 42]. Although the strength of this relationship decreased with increases in the drought stress level, the R\({}^{2}\) of GPP-SIF was above 0.30 on all observed days (Figure 5). The non-linear correlation between \(\Phi\)F and LUE became stronger as the drought stress level increased. This strong physiological connection highlights how SIF can be used to estimate GPP, in contrast with previous research reporting weak GPP-SIF correlations in cropland and forests under drought  
Figure 11: The R\({}^{2}\) of **(a)** GPP-SIF\({}_{760}\) and **(b)** GPP-SIF\({}_{687}\), and RMSE of **(c)** GPP-SIF\({}_{760}\) and **(d)** GPP-SIF\({}_{687}\) with and without the VPD effect within a daily cycle in the verification data. The whiskers represent the standard error (SE). conditions [21; 23], demonstrating that SIF remains a powerful tool for the monitoring of GPP in temperate deciduous broad-leaved forests under drought stress. However, the reduction in the GPP-SIF correlation due to drought stress was not negligible. Our results reveal a relationship between GPP and SIF in the form of \"\(GPP=K\times In(SIF)+b\)\"; the K and R\({}^{2}\) within a daily cycle decreased with the increase in the drought stress level (\(\text{PWSI}_{\text{bin}}\)). These results are consistent with those reported in previous studies [12; 20; 22]. We believe that the phenomena observed are attributable to the inconsistency in the responses of SIF and GPP to drought stress. It has been reported that SIF exhibits a slightly weaker response than GPP under drought and heatwave conditions [21; 22; 43]. Our results also emphasized the asymmetric impact of drought stress on GPP and SIF (Figures 2 and 3). GPP continuously decreased as \(\text{PWSI}_{\text{bin}}\) increased. However, there was an inflection point for SIF, which was approximately within the range of 0.4-0.5 for \(\text{PWSI}_{\text{bin}}\). This result is consistent with our previous findings [33]. After the turning point, SIF continued to decrease, but the rate of decrease in SIF was still lower than that for GPP (Table S1). The response of GPP to drought stress was more intense than that of SIF. The results of the SEM analysis showed that the decrease in K was more dependent on GPP than on SIF (Figure 8). Under drought stress, the stomatal response occurs faster than SIF-associated photosynthetic electron transport, leading to a decrease in the net photosynthetic carbon assimilation rate due to an insufficient supply of CO\({}_{2}\). Therefore, GPP was directly affected by drought stress. When the water supply is limited, plants employ various strategies to regulate photosynthesis, such as redistributing energy between photosystems via the thermal dissipation of excitation energy [11; 27], which results in decreased GPP and SIF. The rise in NPQ and the decline in the fraction of open PSII reaction centers (q\({}_{L}\)) offset the reduction in photochemical activity, thus alleviating the effect on SIF [20]. Thus, the response of the light reaction may be more muted than the stomatal response. The midday/afternoon depression in photosynthesis caused the peak time of GPP to advance under moderate to severe drought stress, resulting in morning values that were higher than the afternoon values (Figure 3c). However, the peak time of SIF did not change significantly, even though SIF showed more significant variation in the afternoon under a higher level of drought stress (Figure 3a,b). This observation aligns with the findings of [PERSON] et al. [10], who reported that the SIF levels were higher in the morning than in the afternoon. The midday/afternoon depression in photosynthesis may be due to differences in vegetation's daily water allocation and use during drought.