## 4 Discussion
### Diurnal Cycles and Short Term Changes in Fluorescence  
Leaves experience important light changes during the day or from day to day, due to change in sun elevation, cloud cover, or occultation by other leaves. If considering other things to be equal, SIF scales linearly with irradiance. To better assess the effect of other factors on SIF, we normalized it by PAR to define an apparent yield fluorescence yield at top of canopy (ASFY687 and ASFY760). Light-induced changes on SIF were evaluated on repetitive clear sky days. It was found that ASFY687 and ASFY760 slightly change during the day (by about 20% and 10%, respectively, Figure 3). Additional differences between red and far red fluorescence were revealed when comparing diurnal cycles of SIF687 with SIF760, as well as ASFY687 with ASFY760, and when investigating the fluorescence ratio SIF687/SIF760 (Figure 3). There is a possibility that at least a part of these differences could be due to fluorescence retrieval errors. SIF retrievals on simulated radiances spectra have demonstrated a RMSE of 0.06 \(\upmu\mathrm{mol}\cdot\mathrm{m}^{-2}\cdot\mathrm{sr}^{-1}\cdot\mathrm{nm}^{-1}\) under a PAR of 1400 \(\upmu\mathrm{mol}\cdot\mathrm{m}^{-2}\cdot\mathrm{s}^{-1}\), which represents about 10-15% of SIF687. These simulation results also showed that the 4 FLD algorithm tends to underestimate SIF by about 0.038 \(\upmu\mathrm{mol}\cdot\mathrm{m}^{-2}\cdot\mathrm{sr}^{-1}\cdot\mathrm{nm}^{-1}\) on average. Hence, an underestimation of SIF687 in the morning and in the evening due to retrieval errors could be a possible cause of the observed difference between SIF687 and SIF760. Another source of errors which is not accounted for by conventional retrieval algorithms (nFLD or SFM) is the effect of bidirectional reflectance of canopies associated with spectral anisotropy of sky radiance. Some authors pointed out that bidirectional properties of canopy reflectance may induce errors in SIF retrieval, depending on the difference between reflectances under diffuse and direct lights on one hand, and on the fractions of diffuse and direct irradiances on the other hand [51; 52; 53]. Because of different optical pathways in the atmosphere, direct and diffuse irradiances show different spectral patterns in the oxygen bands. Specifically, the depth of the band (defined as the ratio between the irradiance at the edge of the band and the irradiance at the bottom of the band) is generally deeper for diffuse than for direct irradiance, because of the larger optical pathway of diffuse light [6]. In addition, the reflectivity of the canopy could differ between direct and diffuse light because of its complex interaction with incident light and angularly-dependent leaf surface polarization. This would result in differences between the radiance spectra of the reference panel and vegetation unrelated to fluorescence emission, which are a possible source of error in the retrieved SIF signal [54]. Here, we cannot exclude a contribution of bidirectional reflectance to the observed differences in the dynamics of SIF687 and SIF760. However, similar differences in the diurnal patterns can be observed between TOC radiances at wavelengths close to the SIF bands (_L_685 and \(L\)755). Unlike SIF, radiances are not subject to retrieval errors. This suggests a common origin for the observed differences in the diurnal patterns between SIF687 and SIF760, and between \(L\)685 and \(L\)755. A possible origin is the difference in the radiative transfer within the canopy layers of the emitted/reflected-transmitted light depending on wavelength, as red light is strongly re-absorbed by chlorophyll and far red is not. Such a pattern with an almost linear increase (and decrease) of SIF687 has been found on pine [10], but not on sorghum [9], which is in line with a possible effect of canopy architecture in relation with radiative transfer.