## 2 Methods
### Spectral Data Processing  
The PRI is usually calculated using two green bands at 531 nm and 570 nm as a normalized index [32,34]:  
\[PRI=\frac{\rho 531-\rho 570}{\rho 531+\rho 570} \tag{2}\]  
In this study, we used canopy reflectance acquired over corn canopies centered at 531.01 nm and 570.08 nm at their native FWHM of 1.5 nm to calculate the PRI. The O\({}_{2}\)-A (760 nm) and O\({}_{2}\)-B (688 nm) absorption features were utilized to derive SIF signals in this study. The fluorescence retrievals were determined using a modified Fraunhofer line depth (FLD) algorithm [66] on the spectral observations over corn canopies. The FLD principle [86] utilizes incident solar irradiance and radiance measurements reflected off vegetation canopies. By comparing the measurements inside and outside (reference band) an absorption feature, the estimate of fluorescence is derived. The modified FLD algorithm [66] utilizes measurements on both shoulders (instead of one, the left shoulder) of an absorption feature to construct the reference band for a more realistic description [54,70]. Detailed description and review of different algorithms to retrieve SIF can be found in previous publications [54,67,70]. In this study, we utilized the bands centered at 685.57 nm, 687.11 nm, and 691.77 nm for red fluorescence retrievals and the bands at 757.86 nm, 760.86 nm, and 772.67 nm for far-red fluorescence retrievals. These bands are at the native sampling resolution of the instrument of approximately 0.2 nm. We use SIF (red) to denote retrievals derived in the O\({}_{2}\)-B feature centered at 688 nm and SIF (far-red) for retrievals within the O\({}_{2}\)-A feature centered at 760 nm. Since values for SIF retrievals are also affected by the intensity of irradiance, we also derived SIF yield, a dimensionless quantity that is independent of the light level. SIF yield was calculated following methods developed in a previous study [57] as the ratio of SIF retrievals and the measurement outside the absorption feature (reference band) from irradiance, denoted as SIF (red) yield in the O\({}_{2}\)-B band region and SIF (far-red) yield for the O\({}_{2}\)-A feature. Normalized Difference Vegetation Index (NDVI)  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multirow{2}{*}{**Planting Date**} & \multirow{2}{*}{**Varieties**} & \multirow{2}{*}{**Maximum LAI**} & \multirow{2}{*}{**Maximum GPP; Date**} & \multirow{2}{*}{**Total Precipitation**} & \multirow{2}{*}{**Average Temperature**} \\  & & & & & **Date** & **(mm)** & **(\({}^{\circ}\)C)** \\ \hline
2008 & 180 & TA 560-00 & 3.27 & 73.98; 214 & 256.54 & 20.80 \\
2010 & 136 & Pioneer 35F37 & 2.48 & 75.17; 199 & 410.22 & 23.22 \\
2011 & 145 & Pioneer 35K09 & 2.79 & 60.81; 192 & 354.08 & 24.51 \\
2012 & 138 & Dekalb 57-67 & 3.42 & 59.66; 208 & 291.09 & 22.97 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Information about crop and environmental conditions in the USDA experimental cornfield in the four growing seasons. Dates are in the format of day of year (DOY). Total precipitation and average temperature are for the time period in Figure 1. Gross primary production (GPP) is in the unit of g CO\({}_{2}\)/m\({}^{2}\)/d.  
was calculated using red (620-670 nm) and near-infrared (841-876 nm) bands similar to the spectral characteristics of MODIS land surface reflectance.