## 4 Discussion  
### Effectiveness of the Morning-to-Noon Ratio for Monitoring Drought Stress  
[PERSON] et al. introduced the NMR concept and preliminarily validated its effectiveness in assessing daily water stress conditions [PERSON] et al. (2019). Building on this foundation, this study offers a more comprehensive understanding of the mechanisms underlying the effectiveness of the NMR for monitoring water stress in maize canopies, which is supported by systematic observations at both the leaf and canopy scales. Our results demonstrated that remote sensing technology can be used to observe changes in the vegetation structure and physiology in response to drought, which together influence the diurnal dynamics captured by the NMR (Figures 5 and 6, respectively). With respect to the canopy structure, our observations of maize at the leaf scale revealed that the most severe leaf rolling occurred at noon because of water stress, with the severity of rolling progressively increasing with increasing water stress. Furthermore, leaf rolling recovered the next day following nocturnal rehydration, and the leaves remained unrolled throughout the day (Figure 5). At the canopy scale, the midday NDVI values associated with the vegetation structure also exhibited similar changes (bb). These findings indicate that the midday values of the NDVI related to the vegetation structure can  
Figure 16: The Relationships between \(\Delta\mathrm{T}_{\mathrm{noon}}\) and NMR **(a)**, \(\Delta\mathrm{T}_{\mathrm{noon}}\) and AMR **(b)** for the two plots during the clear-sky observation period. The solid lines in different colors denote the linear regression fits for the various types of the NMR and AMR, with regression slopes (a) and correlation coefficients (R\({}^{2}\)) shown in the lower-left corner. effectively reflect rapid changes in the water stress status. However, differences in the vegetation LAI and chlorophyll content caused by historical stress can also lead to declines in the midday NDVI values, potentially interfering with the assessment of water stress. Therefore, relying solely on abnormal declines in the midday NDVI values to assess water stress conditions may lead to inaccuracy. Dividing by the measured morning NDVI values may effectively mitigate the interference caused by this issue. As shown in Figure 5, when the minimal environmental stress occurred in the morning, leaves remained relatively unfolded under the different water stress levels. This occurs because nocturnal transpiration is associated with less water consumption, allowing vegetation to absorb soil moisture and restore the leaf water potential to a relatively high level during the day [46; 47]. At the canopy scale, the NDVI in the morning also exhibited similar response characteristics to water stress (b, respectively). The NDVI values measured in the morning provide information on the LAI and chlorophyll content, which are minimally affected by water stress on a given day. Therefore, when the morning NDVI values are employed as a baseline, NMR\({}_{\text{NDVI}}\) can effectively reduce the influence of historical stress and reflect daily changes in water stress levels (Figures 15 and 16). In addition to leaf rolling, vegetation may employ other methods to adjust the canopy structure. [PERSON] et al. (2021) reported changes in the LAD under drought conditions in potatoes, with smaller increases in the LAD in the morning and the greatest increases observed at noon [37]. This suggests that the NDVI of potato canopies may also exhibit diurnal variations similar to those observed in the maize canopy. Therefore, the NMR of canopy remote sensing observations, which reflect the diurnal variation in the vegetation structure, can be used to effectively monitor daily water stress levels in vegetation. In terms of vegetation physiology, our observations at the leaf scale revealed that the extent of the decline in \(\Phi_{\text{F}}\) caused by water stress varied significantly throughout the day.