## 2 Materials and Methods
### Passive Remote Sensing of SIF and Reflectance  
#### 2.3.1 Instrumental Setup and Retrieval of Fluorescence  
Vegetation radiance was recorded with the TriFLEX instrument described in [32].The instrument was fixed at the top of the crane to observe vegetation from nadir. At the working height (21 m), the diameter of the observed area on top of the canopy was about 2 m. Given the crop homogeneity, this measuring configuration can be considered as representative of true remote sensing conditions. Briefly, TriFLEX is a fluorosensor that uses two identical spectro-radiometers (HR2000+, 10 \(\upmu\)m entrance slit, full width at half-maximum (FWHM) 0.4 nm at 687 nm and 0.5 nm at 760 nm, Ocean Optics, Dudenin, FL, USA) to simultaneously record the vegetation radiance and irradiance spectra in the chlorophyll emission band from 630 nm to 815 nm with a 0.09 nm/pixel sampling interval. With this setting, the time delay between vegetation and irradiance spectrum is reduced, enabling kinetic measurements of SIF. A third spectro-radiometer (HR2000+, 50 \(\upmu\)m entrance slit, FWHM 2 nm) alternatively measured vegetation radiance and solar irradiance on a broader spectral range from 300 nm to 900 nm. Irradiance spectra were assessed by measuring radiance over a white reference surface made with a frosted polyvinyl chloride panel. The reflectance spectrum of the reference panel was determined in the laboratory against a reflectance standard (Spectralon, Labsphere, North Dutton, NH, USA), and further checked at the end of the measurements campaign to account for possible shifts induced by prolonged outdoors conditions. Each spectro-radiometer was connected to a fibre optic, which looks down to the vegetation at nadir with a field of view corresponding to a measured area of 2 m diameter. Vegetation and reference spectra were atmospherically corrected for the difference in altitude between ground and the reference panel using MODTRAN 4 [36]. The signal acquired on the vegetation was corrected to compensate for the absorption of the atmosphere along the path from vegetation to sensor, and the signal acquired on the reference board was corrected for the lack of absorption of incident light from the reference altitude to the ground. The two measured signals were corrected from these effects using MODTRAN 4, which computes the two transmission factors [37]. The variation of fluorescence flux induced by this atmospheric correction was about 5% for the O\({}_{2}\)-A absorption band and 1.3% for the O\({}_{2}\)-B band. Raw data were aggregated to a limited number of spectral channels designed to capture the essential features of each absorption band (three channels at 757.86, 760.51, and 770.46 nm for the A band, four channels at 683.14, 685.00, 686.97, and 697.06 nm for the B band), and were automatically saved on a remote server. Bandwidths of integration were from 0.1 nm to 0.89 nm, according to channel (see [32] for details). SIF retrieval was performed with the algorithm proposed in [32] and described in Appendix A. In this method  
Figure 2: Time course of environmental conditions during the experiment: photosynthetically active radiation (PAR, top, \(\upmu\mathrm{mol}\cdot\mathrm{m}^{-2}\cdot\mathrm{s}^{-1}\)), air temperature (middle, \({}^{\circ}\)C), and daily precipitations (bottom, mm). (called \(n\)FLD hereafter, where \(n\) is the number of measuring channels), fluorescence radiance at a given absorption band (SIF687 in the O\({}_{2}\)-B band, SIF60 in the O\({}_{2}\)-A band) is retrieved using a limited number (\(n\)) of measuring channels in each O\({}_{2}\) band and specific models of fluorescence and reflectance that apply only in the vicinity of each oxygen absorption band. We used three channels in the O\({}_{2}\)-A band, and a polynomial of degree one to model the reflectance (i.e., a linear model), and four channels in the O\({}_{2}\)-B band with a polynomial of degree two to model reflectance.