## 4 Discussion
### Effectiveness of the Morning-to-Noon Ratio for Monitoring Drought Stress
This may occur because SIFy reflects the synergistic inhibitory effects of water stress on both the vegetation structure and physiology. Our leaf-scale observations also indicated that both the vegetation structure and leaf physiology respond simultaneously to water stress. Compared to non-water stress conditions, \(\Phi_{\text{F}}\), which reflects vegetation physiological status, decreases significantly at noon under water stress, accompanied by noticeable leaf rolling. In contrast, both \(\Phi_{\text{F}}\) and leaf rolling in the morning show minimal differences compared to non-water stress conditions (Figures 5 and 6, respectively). With respect to the components of SIFy, both fPAR and fesc are related to the canopy structure. The diurnal variation trends of LRR and \(\Phi_{\text{F}}\) indicate that both vegetation structure and physiology respond to water stress. However, their responses to drought and their respective impacts on SIFy differ. At the leaf scale, as water stress increases, the midday LRR value exhibits a larger decrease in ratio relative to the morning value compared to the corresponding decrease in \(\Phi_{\text{F}}\). This suggests that structural changes may play a more dominant role. There are also differences in the timing of the daily maximum values of LRR and \(\Phi_{\text{F}}\). As water stress progresses, the peak of \(\Phi_{\text{F}}\) shifts to earlier than 10:00, which differs from the pattern of LRR (Figures 5 and 6d). The highest LRR value typically appears early in the morning due to overnight recovery. Although some studies suggest that plant physiology responds more rapidly to drought in its early stages, the response of plant structure to early drought typically lags behind that of vegetation physiology [49]. However, the maize canopy structure and physiology did not show a scenario in the early stages of drought where the fluorescence efficiency changed while the structure remained unchanged in our data. Therefore, this is insufficient to conclude that fluorescence responds more rapidly than structural changes. Further exploration and research are needed for other vegetation types. The simulation analysis results of the SCOPE model indicated that the LAI was positively correlated with SIFy (Figure 12a). This suggests that, although fesc may increase due to a reduction in the LAI, the product of fPAR and fesc is positively correlated with the LAI. Drought leads to a reduction in the LRR, which indicates a decrease in the LAI (Figure 5). Therefore, water stress exerts an inhibitory effect on SIFy. For the physiological components of SIFy, the relationships between the vegetation Fqe and SIFy values exhibited highly positive correlations (Figure 12b). Leaf-level physiological observations also demonstratedthat water stress caused a decrease in Fs (Figure 6f), indicating that water stress not only suppresses the development of the canopy structure but also suppresses physiological processes. The study of [PERSON] et al. on potatoes also demonstrated that the decline in the SIF under drought conditions is the result of the combined effect of \(\Phi_{\text{F}}\) and LAD [37]. Therefore, from the perspective of drought monitoring, the NMR of remote sensing observations, which comprehensively reflect both the vegetation structure and physiology, may be more effective for monitoring drought. We found that during periods of severe drought, the high Fs values in the morning slightly decreased (Figure 11). This may result in the use of NMR\({}_{\text{SIFy}}\) causing underestimation of water stress under severe drought conditions. This phenomenon could be attributed to the increased NPQ resulting from water stress, which caused a shift in the morning peak of Fs (Figure 12b). Previous studies have shown that \(\Phi_{\text{F}}\) and \(\Phi_{\text{P}}\) are negatively correlated under low-NPQ conditions, whereas the opposite is true under high-NPQ conditions [50].