## 3 Results
### Relationship between SIF and GPP  
The relationship between SIF and GPP was investigated by linear regression between the two variables upon different observational cases: (_i_) the emission wavelength (SIF687 vs. SIF760); (_ii_) the time scale of observation (vs. daily averages in the 8:00-15:30 UTC time period) (Figure 7); (_iii_) the diffuse fraction of PAR (clear sky vs. all sky conditions); and (_iv_) the time of observation. A significant relationship was found between half-hourly values of SIF760 and GPP (\(r^{2}=0.58\), Figure 7b). Using daily instead of half-hourly averages significantly increased the coefficient of determination (\(r^{2}=0.83\), Figure 7d), while the slope of the regression line did not change (\(17.6\pm 0.01\)). Figure 6: (**a–i**) Seasonal time course of main canopy variables (PAR, SIF687, SIF760, Normalized Difference Vegetation Index (NDVI), SIF687/SIF760, ASF687, ASF760, GPP, fAPAR) from growth phase to senescence. (**a–c**,**h) Sticks indicate 30 min averaged values, squares indicate average over the day (8:00–15:30 UTC). (**d–g**) 30 min averaged values. (**i**)Squares indicate average over the day (8:00–15:30 UTC) and error bars indicate maximum and minimum fAPAR values during the course of the day,. 0.9 (1 sd) (daily) instead of 17.2 \(\pm\) 0.4 (1 sd) \(\upmu\)moles C \(\cdot\) J\({}^{-1}\cdot\) sr \(\cdot\)\(\upmu\)m (half-hourly)). On the other hand, the relationship between half-hourly SIF687 and GPP was strongly reduced compared to SIF760 (\(r^{2}=0.25\), Figure 7a). Using daily averages improved the relationship, but still gave a coefficient of determination (\(r^{2}=0.34\)) lower than the corresponding one for SIF760 (Table 4). The relationship between half-hourly SIF and GPP slightly weakened if we considered measurements under clear sky, except if we considered only measurements when the sky is clear during the full day (Table 4), where a slight increase of the coefficient of determination was observed. For daily-integrated measurements, limiting the observations to clear sky increased \(r^{2}\) for both SIF687 and SIF760. A critical question in SIF remote sensing is the representativeness of single SIF measurements per day to describe carbon uptake over a long period of time [16; 20]. Sun-synchronous satellites like GOSAT or FLEX can only acquire SIF once a day. For example, the orbit of FLEX allows a single measurement at 10:00 hours (local time descending node, LTDN), which varies slightly with latitude. We evaluated the relationship between SIF measured at a given time in the day and GPP. Two cases were considered for the integration time of GPP: (i) the instantaneous value, integrated over 30 min at the same time as the fluorescence measurement; and (ii) the daily integrated value between 8:00 and 15:30. Slight changes in the coefficient of determination were observed as a function of measurement time, with the strongest relationship occurring around noon (Figure 8). We therefore evaluated the relationship between SIF and GPP for SIF measurements at 12:00 according to different sky conditions: (i) clear and cloudy sky together; (ii) clear sky at the time of SIF measurement; (iii) clear sky during the entire day of measurement (Table 5). It was found that mixing clear and cloudy sky conditions in the regression decreased the coefficient of determination. Additionally, the relationship decreased when we considered instantaneous values of GPP instead of daily integrated ones. The strongest relationship (\(r^{2}=0.93\)) was found for SIF760 when we only included data from clear sky days in the regression computation. This questions the potential of simple linear relationship based on SIF to estimate the fraction of carbon uptake that takes place during overcast days.