## 4 Discussion  
### Fluorescence Retrieval in the O\({}_{2}\)-A and O\({}_{2}\)-B Bands  
SIF was retrieved from canopy radiance by using the principle of in-filling of oxygen absorption bands in the radiance spectrum by fluorescence emission. Using this principle, the band profile of the vegetation radiance spectrum was compared to the corresponding profile of a reference spectrum acquired on a white flat panel. In the FLD (Fraunhofer Line Discrimination) method initially introduced by [PERSON] [7], substantial errors can result from the spectral variation of reflectance over the absorption band [35; 46; 47]. Improved retrieval methods were designed to account for the reflectance and fluorescence spectral shapes in the absorption bands region, such as 3 FLD [48], iFLD [46], cFLD [13; 49], SFM (Spectral Fitting Method) [11; 47], or SVD-based (Singular Vector Decomposition) [50]. In the method used in this study and formerly introduced in [32], three or four spectral channels are used to sample radiances in order to better take into account the shape of the reflectance spectrum of the vegetation. Near the O\({}_{2}\)-A band, reflectance varies almost linearly and, therefore, three channels are used (3 FLD). In the O\({}_{2}\)-B band, four channels are required to characterize the curvature of reflectance (4 FLD). The performance of this method has already been tested against direct measurements of leaf fluorescence and same values of emission ratio were found within the experimental error [35]. In Appendix A, we evaluated this method on simulated radiance spectra with known reflectance and known fluorescence levels. In the O\({}_{2}\)-A band, and using three channels for the retrieval, the mean difference over the simulated database between the retrieved fluorescence and the true value (\(\overline{STF_{bias}}\)) is 0.015 umol\(\cdot\) m\({}^{-2}\cdot\) sr\({}^{-1}\cdot\) nm\({}^{-1}\) under an irradiance of 1400 umol\(\cdot\) m\({}^{-2}\cdot\) s\({}^{-1}\). It represents about 1.2% of the experimental level observed on the fully developed crop (median value of SIF760 is 1.3 umol\(\cdot\) m\({}^{-2}\cdot\) sr\({}^{-1}\cdot\) nm\({}^{-1}\) under the same irradiance). In the total error budget, the \(\overline{STF_{RMSE}}\)--which represents the dispersion of the error over the simulated cases of the database used in Appendix A--should also be considered. We cannot certify that the whole database of the simulated cases corresponds exactly to the experimental cases that we encountered in our study, even if we tried to use input parameters in SCOPE simulations that are as close as possible to the real ones. However, we can consider that \(\overline{STF_{bias}}\pm\overline{STF_{RMSE}}\) will give a good estimate of the error in most cases. Reported to a level of 1.3 umol\(\cdot\) m\({}^{-2}\cdot\) sr\({}^{-1}\cdot\) nm\({}^{-1}\) for the median value of SIF760, this error will not exceed 3%. In the O\({}_{2}\)-B band, the corresponding retrieved bias is -0.038 umol\(\cdot\) m\({}^{-2}\cdot\) sr\({}^{-1}\cdot\) nm\({}^{-1}\) with four channels and 0.13 umol\(\cdot\) m\({}^{-2}\cdot\) sr\({}^{-1}\cdot\) nm\({}^{-1}\) with three channels. Therefore, SIF687 is on the average underestimated by the 4 FLD method, while it is largely overestimated by the 3 FLD method. Given the lower value of SIF687 compared to SIF760, this will lead to larger relative errors on SIF687. Reported to the median value of SIF687 retrievals of the experimental dataset at the same irradiance level (0.5 umol\(\cdot\) m\({}^{-2}\cdot\) sr\({}^{-1}\cdot\) nm\({}^{-1}\) at 1400 umol\(\cdot\) m\({}^{-2}\cdot\) s\({}^{-1}\)), an averaged bias of 6% is expected.