## Materials and Methods
### Monitoring Leaf Biochemistry with Field Spectrometer Data  
The canopy-scale SIF is not merely the sum of the SIF emissions of the individual leaves. Instead, considering the canopy scattering and reabsorption is vital when moving from leaf-level to canopy-level SIF, the behaviour of scattering and reabsorption are wavelength-specific. Consequently, SIF at a certain wavelength \(\lambda\) observed in a certain direction \(\omega\) (\(\mathrm{SIF_{\lambda,\omega}}\)) is affected by two wavelength-specific probabilities; the fluorescence emission efficiency \(\epsilon_{\lambda}\) and the photon's escape probability \(f_{\mathrm{esc},\lambda,\omega}\) (Equation (2)). Both \(f_{\mathrm{esc},\lambda,\omega}\) and \(\epsilon_{\lambda}\) are expected to be reactive to water availability. This paper aims at describing these two factors and their relation to drought stress.  
\[\mathrm{SIF_{\lambda,\omega}}=\mathrm{PAR}\cdot\mathrm{fPAR_{\mathrm{Chl}}} \cdot\epsilon_{\lambda}\cdot f_{\mathrm{esc},\ \lambda,\omega} \tag{2}\]  
Isolating \(\epsilon_{\lambda}\) from the Equation (1) allows extracting the biochemical information from the spectrometer-based \(\mathrm{SIF_{\lambda,\omega}}\). [PERSON] et al., (2020) [30] developed the Fluorescence Correction Vegetation Index (FCVI; Equation (3)), which is a reflectance-based surrogate factor for the canopy absorption and canopy scattering. This index is formulated as the difference between the near-infrared reflectance \(\mathrm{R_{NIR}}\) between 767 and 773 nm and the broadband visible reflectance \(\mathrm{R_{VIS}}\) between 400 and 700 nm, assuming that the reflectance observations are taken from the same direction as the SIF measurements. This approach is only valid in the absence of any reabsorption, which is why it is only meaningful for SIFA.  
\[\mathrm{FCVI}=\mathrm{R_{NIR}}-\mathrm{R_{VIS}}\approx f_{\mathrm{esc},760} \cdot\mathrm{fPAR_{\mathrm{Chl}}} \tag{3}\]  
The biochemistry can be assessed with three variables: fluorescence emission efficiency at 760 nm (\(\epsilon_{760}\): Equation (4)), the SIFB yield (SIFBY: Equation (5)) and the photochemical reflectance index (PRI: Equation (6)). The \(\epsilon_{760}\) contains the SIFA normalized by the PAR and FCVI and integrated over a hemispherical space. The PRI is a reflectance-based, narrowband vegetation index, related to the de-epoxydation of xanthophyll, which is linked to the NPQ. A more negative PRI should indicate a higher NPQ [16], and therefore a higher stress level. Similar to SIF, PRI is affected by a biochemical and a structural component [31]. The epoxidation of xanthophyll causes only a change in very limited parts of the absorption spectrum, which is why a bandwidth of 2 nm is chosen in Equation (6).  
\[\epsilon_{760}=\frac{\pi\cdot\mathrm{SIFA}}{\mathrm{FCVI}\cdot\mathrm{PAR}} \tag{4}\]  
\[\mathrm{SIFBY}=\frac{\mathrm{SIFB}}{\mathrm{PAR}} \tag{5}\]  
\[\mathrm{PRI}=\frac{R_{531}-R_{570}}{R_{531}+R_{570}} \tag{6}\]  
To assess the sensitivity to PAR of the leaf biochemistry, the daily correlation coefficient \(\rho\) was calculated. To do so, the Pearson's correlation coefficient between the \(\epsilon_{760}\) and SIBY was used on one hand and, on the other hand, PAR was calculated for each day individually,resulting in a daily correlation coefficient. The daily correlation coefficient only considered data taken between 9 AM and 3 PM, since light-limited photosynthesis is expected in early morning or late afternoon, due to the lower solar irradiations at these times of the day.