## Materials and Methods
### Parameters Derived from Spectroradiometric Measurements  
F at 760 nm (F\({}_{760}\)) was retrieved from ground and airborne radiance measurements using the 3 FLD approach described in detail in [34]. In short, the retrieval method uses the wide O\({}_{2}\)-A absorption band to decouple the emitted fluorescence radiance from the reflected radiance by using two radiance measurements, \(L_{i}\) inside (i, 760 nm) and \(L_{0}\) outside (o) of the O\({}_{2}\)-A band. They can be expressed as:  
Figure 1: Flight pattern on both observation days: (**A**) Sugar beet field in Merken (DOY 183), overflow 68 times, at 7:30–10:30 UTC and 11:30–15:30 UTC; (**B**) winter wheat field in Selhausen (DOY 176), overflow 46 times at 11:15–15:15 UTC. The color indicates the time of measurements. \[L_{j}=\ L_{j}^{p}+\frac{\left(E_{j}^{g}\ \frac{\rho_{j}}{\pi}+F_{j}\right)\tau\ \uparrow_{j}}{1-S_{j}\cdot\rho_{j}},\ j=i,o \tag{1}\]  
where \(L^{p}\) is the path scattered radiance, \(E^{g}\) is the global irradiance (including direct and diffuse irradiance components) arriving on the surface, \(\rho\) is the surface reflectance, \(\tau\uparrow\) is the upwelling transmittance, and \(S\) is the spherical albedo. The atmospheric variables were calculated for each individual observation with MODTRAN5 (i.e., \(L^{p}\), \(E^{g}\), \(\tau\uparrow\), \(S\)). Reducing the number of unknowns (i.e., \(\rho_{i}\), \(\rho_{o}\), \(F_{i}\), \(F_{o}\)) to only two is needed to eventually solve the system of equations (Equation (1)). We therefore used the 3 FLD approach [35] and linearly related \(\rho\) and \(F\) inside and outside of the O\({}_{2}\)-A band. With this, \(F_{760}\) can be retrieved as:  
\[F_{760}=F_{i}=\ B\left[\frac{X_{i}\left(E_{o}^{*}+X_{o}\cdot S_{o}\right)-AX_{ o}\left(E_{i}^{*}+X_{i}\cdot S_{i}\right)}{B\left(E_{o}^{*}+X_{o}\cdot S_{o} \right)-A\left(E_{i}^{*}+X_{i}\cdot S_{i}\right)}\right],\ \text{with} \tag{2}\]  
\[X_{j}=\frac{\left(L_{j}-L_{j}^{p}\right)}{\tau\uparrow_{j}},\ E_{j}^{*}=\frac{ E_{j}^{g}}{\pi}\ j=i,o,\ \text{and} \tag{3}\]  
\[\left. \begin{array}{c}\rho_{i}=A\rho_{o}\\ F_{i}=BF_{o}\end{array} \right\} \tag{4}\]  
\(X_{j}\) equals the top-of canopy (ToC) radiance leaving the surface. \(A\) is the factor relating \(\rho_{i}\), and \(\rho_{o}\) and was derived from linear interpolation of \(\rho\) using the left (753 nm) and right (771 nm) O\({}_{2}\)-A band shoulders with:  
\[A=\frac{\rho_{753}\omega_{1}+\rho_{771}\omega_{2}}{\rho_{753}} \tag{5}\]  
\[\omega_{1}=\frac{771-760}{771-753},\ \text{and}\ \omega_{2}=\frac{760-753}{771-753} \tag{6}\]  
B is a factor relating F inside and outside the O\({}_{2}\)-A band and was fixed to a value of 0.8, justified by simulations and experiments. Since the product of S and \(\rho\) (Equation (1)) can be assumed as \(<<\)1, \(S\) was eventually set to zero for the F retrieval. Further, slight uncertainties of the atmospheric modeling and remaining spectral shift artifacts can cause uncertainties in F retrievals. We therefore applied a semi-empirical correction coefficient derived over non-fluorescence targets to account for such inaccuracies and to increase the precision of F retrievals (see [34] for a detailed description of this approach). For the retrieval of F\({}_{760}\) from ground measurements, \(E^{g}\) was determined with a measurement of the reference panel. Further, we assumed \(L^{p}\) = 0 and \(\tau\uparrow\) = 1, justified by the short distance between surface and sensor (1 m). The yield of F\({}_{760}\) (F\({}_{760\text{-yield}}\)) is theoretically related to the LUE and is calculated by dividing F\({}_{760}\) by the light absorbed by the green plant material (APAR\({}_{\text{green}}\)):  
\[\text{F}_{760-\text{yield}}=\frac{\text{F}_{760}}{\text{APAR}} \tag{7}\]  
In this study, we estimated APAR as the integrated difference between the WR radiance signal and the radiance measure over the vegetation canopy over the wavelength range of 400-700 nm.