## 4 Discussion
### The Synergistic Effects of Other Factors on the GPP-SIF Relationship Under Drought Stress  
During drought, there were usually changes in other factors that synergistically impacted the GPP-SIF relationship. The canopy structure is a key factor influencing the SIF-GPP relationship [34], and its response to drought stress would also indirectly affect the GPP-SIF relationship. During the entire observation period, the canopy structure remained relatively stable (Figure S1b), with an NDVI of approximately 0.87, showing no obvious seasonal dynamics in response to changes in PWSI. This indicates that short-term drought stress had a minimal impact on the canopy structure of the cork oak plantation. Therefore, as the level of drought stress increased, the contribution of canopy structure components to SIF and GPP remained almost unchanged, consistently staying at a low level (Figure 5). One possible explanation is that the deep-rooted nature of the forest allows for the preservation of photosynthesis and canopy stability over prolonged periods under short-term water deficit conditions. Another possible reason is that cork oak is a drought-tolerant species with a higher response threshold for morphological changes under drought stress. Although the results of the random forest analysis show that the impact of the canopy structure on the relationship of SIF-GPP was lower than that of meteorological factors, the potential influence of the canopy structure on the relationship under drought stress still needs to be considered. The PWSI still had a significant correlation with the canopy structure parameters on an instantaneous scale (half-hour) (Figure S3). Slight changes in the leaf pigment content and leaf angle caused by drought stress interfere with the observation of photosynthesis and fluorescence, which may have been one of the factors contributing to the uncertainty of our results. Under low drought stress (Figure 3), SIF and GPP exhibited a similar pattern regarding the incoming PAR, rising from morning until noon and decreasing thereafter in the afternoon. However, as the drought stress level increased, the responses of SIF and GPP to PARdid not change proportionally. Except in the early morning, when SIF and GPP showed no obvious drought stress responses, their responses to PAR synchronously decreased, with higher PAR corresponding to lower SIF and GPP due to drought stress. The contribution of PAR to SIF and GPP was also reduced with the increase in drought stress (Figure 6). Moreover, \(\Phi\)F and LUE remained at higher levels in the early morning, potentially because of the lower water demand of photosynthesis under low PAR. Another explanation is that, with lower nighttime transpiration, the water content of vegetation is more derived from soil and the atmosphere [47]. Photosynthesis requires more water as radiation increases, leading to the gradual emergence of the limiting effect of water on SIF and GPP. The GPP-SIF relationship was negatively affected by PAR (Figure S4), a finding that is consistent with previous studies [27; 48]. Drought stress is often accompanied by high PAR, which offsets, to some extent, the negative impact of drought on the physiological components of SIF. This can explain why, although the direct correlation between GPP and SIF decreases under drought stress, a close physiological connection still exists [49]. The reduction in GPP-SIF caused by PAR may have been due to the different responses of SIF and GPP to PAR. Under low-PAR conditions, an increased diffuse light rate enables deeper penetration into the canopy, boosting canopy photosynthesis [50; 51]--a phenomenon referred to as the \"diffuse fertilization effect\", which influences GPP more than SIF. At high PAR levels, light saturation limits GPP, whereas SIF remains unaffected by this constraint [52]. The finding that the direct effect of PAR on SIF was greater than that on GPP in the path analysis (Figure 8) also illustrates this point.