## 2 Materials and Methods
### Measurements  
At the OPE3 site, a 10 m tall eddy covariance tower measured canopy level CO\({}_{2}\) assimilation at 30 min intervals continuously throughout the growing season [56] (Figure 1). The measured net CO\({}_{2}\) flux (i.e., net ecosystem productivity) is partitioned into GPP (the carbon used by photosynthesis) and ecosystem respiration [56; 28; 57]. During the summer of 2017 we collected complementary leaf and canopy measurements (Table 1) for C4 corn canopy (_Zea mays_ L) to capture the dynamics in photosynthesis, reflectance, and chlorophyll fluorescence (F and SIF) through the growing season. Leaf-level fluorescence and photosynthetic efficiency were characterized using an automated MoniPAM Data Acquisition system (MONI-DA, Heinz Walz GmbH, Effeltrich, Germany) outfitted with emitter-detector probes. The system continuously measured chlorophyll fluorescence in the presence of ambient sunlight (i.e., light-adapted), from several leaf samples simultaneously [24; 38; 47]. By DOY 190, when the corn canopy was established, canopy closure was approximately 85%, and no additional agricultural treatments were planned, the MoniPAM emitter-detector probes were mounted on fully developed leaves (i.e., on the 4 th leaf from the top of the plant). Measurements were collected during the growing season, continuously re-locating the probes as new leaves emerged. Five emitter-detector probes were operated, measuring in parallel light-adapted steady state F (F\({}_{\text{s}}\)), a broadband measurement of chlorophyll fluorescence in the 710-800 nm range, and the associated PAR. Three probes were positioned to measure fully sunlit leaves from the upper canopy, while the remaining two probes collected measurements on shaded leaves at varying illumination levels and positions in the canopy. Since the leaf signal available for remote sensing of the canopy is primarily associated with the illuminated leaf surfaces, we further analyzed the measurements collected on sunlit leaves. With each measurement, the following parameters were collected: steady state and maximum F (F\({}_{\text{s}}\), relative units), operating photosynthetic efficiency of PSII electron transport under sunlight or yield of photosystem II (YII, right axis), relative electron transport rate (ETR, left axis), and incident PAR (Table 1, Figure 2). Figure 1: Eddy covariance measurements collected at OPE3, Greenbelt, MD in 2017, including incident photosynthetically active radiation (PAR, \(\upmu\)mol/m\({}^{2}\)/s) and gross primary production (GPP, mg/m\({}^{2}\)/s)). The Y axes indicate the measured parameters. The days with corresponding MoniPAM and FLoX measurements are depicted in bright green. Days 192–209 demonstrate the dynamics in GPP during the corn young (Yn) growth stage, days 220–235 during the mature (M), and days 236–272 (S) during the senescing (S1 early and S2 advanced) growth stages. Figure 2: Diurnal and seasonal variations in canopy gross primary production (GPP, A), incident and absorbed photosynthetically active radiation (PAR and APAR, B), reflectance indices (NDVI and Cl\({}_{\text{red}}\), C; and PRI, E), leaf-level photosynthetic parameters (YII and ETR, D), and canopy solar induced fluorescence (SIF, F). The Y axes of each panel indicate the measured parameters. The data were collected contemporaneously at time intervals ranging from 1–30 minutes, they were filtered, smoothed and interpolated to the GPP observations (i.e., 30 minutes), resulting in a set of corresponding measurements for 58 days, depicting the diurnal dynamics across the growing season. Canopy diurnal measurements including reflectance and SIF (mW/m\({}^{2}\)/nm/sr)--both SIF\({}_{\text{B}}\) in the O\({}_{2}\)B and SIF\({}_{\text{A}}\) in the O\({}_{2}\)A bands, were collected using a field spectroscopy system FLoX (Dual FLUorescence boX; JB Hyperspectral Devices UG, Dusseldorf, Germany) [55]. The FLoX optical cables for measurements of down-welling radiance were mounted on at the top of a tower at approximately 3 m height.