## 3 Results  
### Daily Cycles of Fluorescence  
Illumination conditions in terms of light intensity and incident angle present large variations during the time course of the day, which can impact light absorption and emission. Figure 3 shows mean diurnal patterns of top of canopy (TOC) red and far red radiances (\(L685\), \(L758\)) and fluorescence signals (SIF687, SIF760, ASFY687, ASFY760) on sunny days, when the crop has reached a steady mature state (LAI = 6, see Table 1). PAR showed a typical variation close to a cosine law as a function of sun zenith angle (Figure 3a). Depending on emission wavelength, SIF exhibited different diurnal patterns: SIF687 showed an almost linear increase (decrease) with time before (after) solar noon, while SIF760 had a rather bell-shaped variation, reaching a steady state level around solar noon (Figure 3c,d). TOC radiances daily cycles were also wavelength-dependent (Figure 3e,f). An interesting result is that SIF and radiance patterns were quite similar for nearby wavelengths. At wavelengths strongly absorbed by chlorophyll (i.e., 687 nm for SIF and 685 nm for \(L\)), linear variations with time were observed. At non-absorbed wavelengths (i.e., 760 nm for SIF and 758 nm for \(L\)), the diurnal cycle showed a bell-shaped pattern similar to PAR. Differences in emission wavelengths were also reflected in the mean diurnal course of reflectances and ASFY (Figure 3g,h). ASFY687 reached its highest level around noon, while ASFY760 showed a small decrease compared to its morning value. Diurnal variations of fAPAR--caused, for example, by leaf movements or by the complex interactions of varying incident angles with canopy structure--would affect ASFY diurnal pattern. To evaluate possible fAPAR effects on ASFY, we investigated the fAPAR time course by measuring the transmitted PAR and converted it into fAPAR using Equation (4). During the mature state of the crop, fAPAR showed only small variations, and a small decrease (<2%) could be observed in the middle of the day in clear sky conditions. To better understand the origin of daily changes in ASFY, we compared SIF measurements with active fluorescence measurements performed at leaf level with the PAM-2000. Active measurements provide relative changes in fluorescence quantum yield if measurements are taken on the same leaf and if all experimental parameters are kept constant (e.g., measuring distance, measuring beam intensity). When comparing different leaves, changes in pigment content can result in an additional dispersion. Here we compared the PAM-2000 fluorescence level (Fs) between groups of leaves sampled in the morning and at noon on three different sunny days (Table 2). The analysis was limited to sunny days to ensure repeatability in light intensity and quality between days. No significant differences in Fs were observed between groups taken at the same hour on different days. However, a slight but highly significant decrease (\(p<0.01\)) was observed on six out of nine days when comparing noon groups to morning groups of leaves (\(-12\%\) on average). A significant decrease between morning and noon was also observed on \(F^{\prime}m\) and \(\Phi_{P}\) (\(-31\%\) and \(-32\%\) on average, respectively). The decrease in \(F^{\prime}m\) shows that a significant amount of non photochemical quenching was accumulated at noon, which reduced the photochemical yield of photosystem II. Significant variations of ASFY687 and ASFY760 were also observed from Table 2 (\(+12\%\) and \(-8.7\%\) on average, respectively), but none correspond linearly to the amount of the decrease in photochemical yield. During the day, surface irradiance can also present large changes in the fractions of direct and diffuse light. In order to evaluate the effect of light intensity and directionality on apparent yield, we compared the SIF-PAR relationship between clear sky and overcast days for different development stages of the crop (Figure 4).