## 4 Discussion
### Effectiveness of the Morning-to-Noon Ratio for Monitoring Drought Stress
In the absence of water stress, these parameters remained stable throughout the day. In contrast, under water stress, higher values were maintained in the morning, but these values decreased significantly at noon (Figure 6e). Canopy fluorescence remote sensing observations, such as those of SIFy and \(\Phi_{\text{F}}^{\text{FCVI}}\), also exhibited similar diurnal trends (Figure 8b,d). This suggests that in the morning, when the environmental stress is minimal, the impact of water stress on fluorescence is relatively limited, whereas the midday fluorescence shows a heightened sensitivity to water stress. Nevertheless, water stress monitoring solely on the basis of the midday decline in fluorescence is not reliable. During different growing seasons, the energy allocation proportions of vegetation also vary, leading to differences in \(\Phi_{\text{F}}\)[48]. By applying the physiological state of vegetation in the morning, when it is less affected by water stress, as a baseline, the impact of seasonal variations can be reduced. Therefore, the diurnal variation characteristics of canopy fluorescence, represented by NMR\({}_{\Phi_{\text{F}}-\text{FCVI}}\) and NMR\({}_{\Phi_{\text{F}}-\text{FCVI}}\), can be used to effectively monitor changes in water stress at the daily scale (Figures 15 and 16, respectively). Therefore, NMR data of remote sensing observations related to vegetation physiological processes can serve as effective indicator data for minimizing interference from non-drought factors and for accurately monitoring the water stress status over the measurement period. In summary, by using information obtained in the morning of the measurement day, when structural and physiological impacts from water stress are minimal, as a baseline, we can eliminate differences in the vegetation structure and physiology caused by historical stress. The information obtained at noon effectively highlights changes in vegetation due to water stress. Therefore, the NMR can be used to assess the water stress status effectively on the measurement day. Besides the NMR, the AMR, which shifts the calculation time window from noon to afternoon, is also a potential alternative for daily water stress monitoring. We compared the NMR and AMR within the maize canopy, demonstrating that the AMR is less responsive to daily water stress in vegetation (Figures 15 and 16). Evidence from Figures 7 and 8 indicates that water stress is more pronounced at midday than in the afternoon in the maize canopy. These results indicate that the NMR is more effective for monitoring water stress in maize canopies. Further research by [PERSON] et al. analyzed the diurnal variation curves of the far-red SIF, focusing on aspects such as the slopes on either side of the curve, the size of curve opening, and the timing of the peak. They reported that under water stress, the peak timing of far-red SIF shifts earlier, with diminished peak values, an expanded curve, and a decreased slope on the right [16]. However, it is challenging to obtain the high-quality, continuous intraday observations necessary to accurately assess these diurnal variations. Therefore, the NMR may offer a more practical and operationally viable method for drought monitoring, given the current technological capabilities. Sensitivity of the Noon-to-Morning Ratio in Drought Monitoring Across Different Remote Sensing Observations  
Our results indicated that the NMR data of the NDVI, SIF, and \(\Phi_{\text{F}}^{\text{F}\text{F}\text{V}}\) can be used to monitor water stress in maize canopies. However, their sensitivities to changes in water stress differed (Figure 15). Recent studies have revealed that \(\Phi_{\text{F}}\)canopy, which is more closely related to vegetation physiology, may exhibit greater sensitivity to drought compared to SIFy [17]. Our results indicated that NMR\({}_{\text{SIFy}}\), which contains information on both the vegetation structure and physiology, was most sensitive to water stress (Figure 16).