## 4 Discussion
### Relationship of GPP-SIF Under Drought Stress
Under drought conditions, physiological water stress is commonly relieved overnight, and plants tend to exhibit greater drought sensitivity in the afternoon, leading to photosynthetic productivity being higher in the morning than in the afternoon [10; 44; 45]. The midday/afternoon depression does not affect the reduction in synchrony between SIF and GPP in the afternoon; however, the advancement of the GPP peak time disrupts the synchrony between SIF and GPP in the morning, leading to a more pronounced reduction in the R\({}^{2}\) of GPP-SIF within a daily cycle. We observed a LUE-\(\Phi\)F relationship that increased as \(\text{PWSI}_{\text{bin}}\) increased within a daily cycle (Figures 4 and 5), reflecting the strong interdependence between \(\Phi\)F and LUE and, notably, the close link between photosynthetic light reactions and carbon assimilation pathways under drought stress. This observation serves as the theoretical basis for the use of SIF in tracking GPP under drought conditions [20; 22; 23; 27]. Our results showed that the LUE-\(\Phi\)F relationship varied non-linearly (quadratically) within a daily cycle, with a positive linear relationship in the morning and a negative linear relationship in the afternoon (Figure 4c,d). This relationship pattern illustrates the link between fluorescence and photosynthesis at the photochemical stage. Under drought stress, increasing NPQreduced the efficiency of fluorescence emission and photochemical reactions [46]. The ETC and photochemistry, limited by absorbed light energy and NPQ, showed a simultaneous decrease; consequently, \(\Phi\)F and LUE showed a synchronous decline in the morning. Due to the midday/afternoon depression, the stomata closed, carbon assimilation substrates became insufficient, the photochemical pathway was limited, the NPQ pathway became saturated, and more light energy was allocated to fluorescence emission, resulting in asynchronous changes between \(\Phi\)F and LUE in the afternoon. There was always a close correlation between LUE and \(\Phi\)F, regardless of whether the fluctuations were in the same or opposite directions. We quantified the importance of the physiological and non-physiological components of SIF and GPP by performing a logarithmic transformation on the light use efficiency model (Figure 6). The results showed that the importance of the non-physiological components decreased. In contrast, the importance of the physiological components increased under drought stress. This finding explains the increasingly close LUE-\(\Phi\)F relationship under drought stress, indicating that SIF remains a good indicator of photosynthesis. There were also differences in the relationship between GPP and different SIF bands. We observed that, under the same drought stress level, the relationship between GPP and the red SIF was closer than that with the far-red SIF (Figures 5 and 6). This was attributed to the fact that the red SIF carries more detailed information about PSII, whose fluorescence emission is commonly utilized to reflect the photosynthetic capacity. At the photosystem level, the red SIF is expected to provide more accurate estimates of GPP. We also observed that the relationship between GPP and SIF\({}_{687}\) was less sensitive than that with SIF\({}_{760}\) to drought stress (Figure 6). Therefore, the red SIF is a stable proxy for photosynthesis under drought stress.