## 2 Materials and Methods
### Data Processing  
#### Calculation of Vegetation Indices and Photosynthetic Observations at the Canopy Scale  
We calculated the reflectance via measurement data from the Ocean Optics HR 2000+ spectrometer and derived vegetation indices such as the NDVI, FCVI, and NIRv on the basis of the calculated reflectance. Due to the wide [30; 31] application of these vegetation indices in characterizing vegetation structure, three representative vegetation indices were selected. The specific equations used are detailed in Table 3.  
Figure 3: Images of leaves under well-watered and water-stressed conditions, along with their corresponding measurement times and LRR values. Owing to a superior signal-to-noise ratio and spectral resolution, QE Pro spectrometer measurement data were selected for calculating the SIF. The far-red SIF was extracted via the spectral fitting method (SFM) [35]. The SFM was employed within the O\({}_{2}\)A absorption band to mathematically model the spectral shapes of the true reflectance (r) and fluorescence (F). \[\mathrm{L}(\lambda)=\frac{\mathrm{r}_{\mathrm{mod}}(\lambda)\times\mathrm{E}( \lambda)}{\pi}+\mathrm{F}_{\mathrm{mod}}(\lambda)+\varepsilon(\lambda) \tag{3}\]  
where L is the upward radiance, and E is the downward irradiance. Moreover, \(\mathrm{r}_{\mathrm{mod}}\) denotes the function describing the spectral shape of the true reflectance within the O\({}_{2}\)A band, \(\mathrm{F}_{\mathrm{mod}}\) is the function used to describe the spectral shape of F, and \(\varepsilon\) denotes the modeling error. A quadratic reflectance fitting function was selected, and a linear function was used for fluorescence fitting. To mitigate the effect of illumination instability, the SIF extracted under sunny weather conditions was utilized for analysis. To analyze the physiological information within the SIF regarding the drought response, we estimated \(\mathrm{\Phi F_{\mathrm{canopy}}}\) via the FCVI and NIRv methods. The \(\mathrm{\Phi F_{\mathrm{canopy}}}\) values obtained via the two methods are denoted as \(\mathrm{\Phi F_{\mathrm{F}}^{\mathrm{FCVI}}}\) and \(\mathrm{\Phi F_{\mathrm{F}}^{\mathrm{NIRv}}}\), respectively [20; 21]. Both methods provide a more detailed understanding of the interplay between the physiological activities and structural attributes of the canopy under varying water stress conditions. \[\mathrm{\Phi F_{\mathrm{F}}^{\mathrm{FCVI}}}=\frac{\mathrm{SIFy}}{\mathrm{ FCVI}\times\mathrm{f}_{\mathrm{chl}}} \tag{4}\]  
\[\mathrm{\Phi F_{\mathrm{F}}^{\mathrm{NIRv}}}=\frac{\mathrm{SIFy}}{\mathrm{ NIRv}\times\mathrm{f}_{\mathrm{chl}}} \tag{5}\]  
\[\mathrm{f}_{\mathrm{chl}}=\frac{\mathrm{fPAR}_{\mathrm{chl}}}{\mathrm{fPAR}_ {\mathrm{canopy}}} \tag{6}\]  
where \(\mathrm{f}_{\mathrm{chl}}\) is the ratio of the chlorophyll-absorbed PAR (\(\mathrm{fPAR}_{\mathrm{chl}}\)) to the total canopy-absorbed PAR (\(\mathrm{fPAR}_{\mathrm{canopy}}\)) and is closely related to the variation in the leaf chlorophyll content [36]. Owing to the spectral range limitations of the Ocean Optics QE65 Pro spectrometer, the FCVI could not be calculated. To minimize the differences in \(\mathrm{\Phi F_{\mathrm{canopy}}}\) calculated via different methods, reflectance data obtained by the Ocean Optics HR2000+ spectrometer were used to compute the FCVI and NIRv. On the basis of previous research, vegetation indices related to the chlorophyll content and canopy structure can be used to determine \(\mathrm{fPAR}_{\mathrm{chl}}\) and \(\mathrm{fPAR}_{\mathrm{canopy}}\), respectively [37; 38; 39]:  
\[\mathrm{fPAR}_{\mathrm{chl}}=1.37\times\mathrm{NDVI}_{\mathrm{rededge}}-0.17 \tag{7}\]  
\[\mathrm{fPAR}_{\mathrm{canopy}}=1.24\times\mathrm{NDVI}-0.168 \tag{8}\]  
SIFy can be calculated as follows:  
\[\mathrm{SIFy}=\frac{\mathrm{SIF}}{\mathrm{PAR}} \tag{9}\]