## 2 Materials and Methods
### Data Processing and Analysis
Error bars denote \(\pm\)SD of the mean. The corn growth stages are depicted with colored lines (blue—young/Yn, green—mature/M, red—early senescence/S1, yellow—late senescence/S2). Refer to Table 1 for measurement units. The set of complementary leaf and canopy measurements at OPE3 occurred at different time intervals: 30 min (flux tower); 10 min (MoniPam); and approximately 1 min (FLoX). Incident PAR was measured by the OPE3 flux tower and at leaf level by the MoniPAM. The FLoX-measured incoming radiance at 750 nm and the MoniPAM PAR measurements were collected at closer time intervals, and over the study period they display a strong linear relationship, which was used to derive PAR estimates for the FLoX footprint. Outliers (i.e., values greater than three median absolute deviations) from this daily linear trend were removed from both data sets. The absorbed in the footprint of the FLoX canopy PAR (APAR, \(\upmu\)mol/m\({}^{2}\)/s), required for computation of the FLoX fluorescence yields, was calculated as:  
\[\text{APAR}=\text{PAR}\times f\text{APAR}, \tag{3}\]  
where PAR is the estimate for the FLoX footprint. The fraction of APAR (_f_APAR) was calculated as a linear function of NDVI [63], experimentally established for corn:  
\[f\text{APAR}=(\text{NDVI}\times 1.251)-0.2528, \tag{4}\]  
where NDVI was derived using the FLoX reflectance measurements. The empirical function was derived based on light bar (Accu-PARceptometer, METER (formerly Decagon), Pullman, WA) measurements of incident and transmitted PAR collected near solar noon on five different clear days covering the growing season, measuring every meter along a 100 m transect through the OPE3 corn field, in conjunction with above canopy spectral reflectance measurements collected using an ASD FieldSpec spectrometer (Malvern Panalytical, Longmont, CO). Using the field observations, _f_APAR\({}_{\text{field}}\) was calculated using the equation:  
\[f\text{APAR}_{\text{field}}=(\text{PAR}_{\text{in}}-\text{PAR}_{\text{trans}}) /\text{PAR}_{\text{in}}, \tag{5}\]  
where PARin is the incident PAR and PARtrans is the PAR transmitted through the canopy measured at ground level [64]. NDVI was calculated from the ASD spectral reflectance using the 670 and 800 nm bands. The NDVI and _f_APAR were averaged to five meter intervals, and data from all dates were combined into the regression along with measurements of bare ground (where _f_APAR = 0) for each day (R\({}^{2}\) = 0.93, standard error of regression 0.056). Canopy SIF yield was calculated as:  
\[\text{SIF}_{\text{y}}=\text{SIF}/\text{APAR}, \tag{6}\]  
for both \(\text{SIF}_{\text{A}}\) (\(\text{SIF}_{\text{Ay}}\)) and \(\text{SIF}_{\text{B}}\) (\(\text{SIF}_{\text{By}}\)) bands, with the assumption that APAR was equally available to both PSII and PSI. To compare all three data sets effectively, the most frequently acquired FLoX data set, screened for outliers as described above and including APAR, was smoothed using a moving Savitzky-Golay filter function to reduce the random noise and enhance the measured steady state signal [65] (MATLAB 2018b), which was linearly interpolated to 10 min intervals and resampled at congruent time steps with the MoniPAM data. The combined MoniPAM and FLoX data sets were then linearly interpolated to extract values at the same 30 min intervals used by the flux data. The PAR values for numerous days across the season were evaluated statistically for significant differences associated with the three sensors, and plotted to confirm that the patterns, variations, and magnitudes of PAR from the \begin{table}
\begin{tabular}{c c} \hline \hline
**Spectral Index [Band Resolution]** & **Reference** \\ \hline NDVI = (R800 \(-\) R670)/(R800 \(+\) R670) [10 nm] & [PERSON] et al., 1974 [7] \\ \(\text{CI}_{\text{red}}=(\text{R}_{\text{795}}/\text{R}_{\text{705}})-1\) [10 nm and 5 nm ] & [PERSON] et al., 2018 [4] \\ PRI = (R531 \(-\) R570)/(R531 \(+\) R570) [2 nm] & [PERSON] et al., 1992 [11] \\ \hline \hline \end{tabular}
\end{table}