## 3 Results
### Seasonal Patterns of Fluorescence  
Figure 6 gives an overview of the time course of the fluorescence signals during the whole measurements campaign, compared to NDVI, GPP, and fAPAR. NDVI increased at the beginning of the measurement period from a value of about 0.4-0.45, and reached a steady state level above 0.9 which lasted from DOY 100 to DOY 150, and then decreased slightly at the beginning of the senescent phase. It should be noted that during the steady state phase of NDVI, the crop still grew up, as can be seen from canopy height and biomass measurements (Figure 1; Table 1). During the measurement period, SIF687 varied from 0.2 to 0.8 \(\mathrm{mW\cdot m^{-2}\cdot sr^{-1}\cdot nm^{-1}}\), while SIF760 varied from almost 0 to 2 \(\mathrm{mW\cdot m^{-2}\cdot sr^{-1}\cdot nm^{-1}}\). SIF760 showed large variations in amplitude at the beginning of the measurement period during crop growth, even when NDVI reached a steady state level (Figure 6c,d). A strong decrease of SIF760 was also observed at the end of the experiment, associated with the senescent period and a decrease of leaves' chlorophyll content (see Figure 1). Compared to SIF760 (Figure 6c), SIF687 (Figure 6b) showed more limited changes in intensity, despite crop growth and senescence. These contrasting evolution patterns between fluorescence bands are better evidenced by the time course of the emission ratio (Figure 6e). SIF687/SIF760 showed large variations during crop growth and senescence, and remained almost stable with low values between 0.25 and 0.6 during crop maturity. Normalization of SIF687 and SIF760 with PAR reduced diurnal and day-to-day variations, as can be seen from the seasonal patterns of ASFY687 and ASFY760 (Figure 6f,g). Again, it can also be observed that the two fluorescence bands exhibited very different patterns. ASFY687 showed less variations than ASFY760 during the different phases of the crop development (apart from some peaks associated with cloudy periods), while ASFY760 showed a regular increase from the beginning of the experiment and a strong decrease at senescence.  
Figure 6 shows that fAPAR increased during the development stage (until DOY 120) and then remained stable from DOY 120 to DOY 154 at a high level (>0.93) until the beginning of senescence. During senescence, fAPAR decreased slightly, in association with a concomitant lowering of Chl content (Figure 1). fAPAR presented daily variations, which were up to 20% at the beginning of the growth phase (DOY < 110), but were greatly reduced as soon as fAPAR reached its highest values (\(\geq\)2%).  
Figure 5: Effect of diffuse fraction of PAR on apparent spectral fluorescence yield of canopy (ASFY687, ASFY760). Only days between DOY 120 (30 April) and DOY 157 (6 June)—when the crop was in a steady state—are considered here to avoid canopy growth effects.  
\begin{table}
\begin{tabular}{c c c} \hline  & **Clear Sky** & **Overcast** \\ \hline Diffuse fraction of PAR & \(\leq\)0.2 & \(\geq\)0.9 \\ n samples & 221 & 93 \\ Median PAR (\(\mathrm{\SIUnitSymbolMicro mol\cdot m^{-2}\cdot s^{-1}}\)) & 1620 & 494 \\ Median \(ASFY687\times 10^{6}\) (\(\mathrm{nm^{-1}}\)) & 6.1 & 9.3 \\ Median \(ASFY760\times 10^{6}\) (\(\mathrm{nm^{-1}}\)) & 18.7 & 21.1 \\ \hline \end{tabular}
\end{table}
Table 3: Changes in mean ASFY upon sky conditions. Data taken from Figure 5.  
At the seasonal scale, GPP presented a regular increase during crop growth from DOYs 60 to 120, and a strong decrease at senescence. The seasonal patterns of GPP and SIF760 were quite similar, but large differences were observed between the patterns of GPP and SIF687.