## 3 Results
### Relationship between Fluorescence Yield, Photochemistry, and Light Use Efficiency  
We determined the leaf response to light level by measuring the fluorescence parameters (\(Fs\), \(\Delta F/F^{\prime}m\)) on randomly selected leaves during the maturity stage of the crop (i.e., between DOY 118 and 143). During this period, incident PAR was almost totally absorbed by the canopy, and \begin{table}
\begin{tabular}{c c c c c c c} \hline
**GPP** & **Integration Time** & **30 min** & **30 min** & **30 min** & **6.5 h** & **6.5 h** & **6.5 h** \\ \hline
**Sky Conditions** & **All** & **Clear Sky** & **Clear Sky during the Whole Day** & **All** & **Clear Sky** & **Clear Sky during the Whole Day** \\ \hline data points & 52 & 19 & 10 & 64 & 25 & 15 \\ \hline SIF687 & & & & & & \\ \(r^{2}\) & 0.21 & 0.17 & 0.66 & 0.24 & 0.2 & 0.66 \\ \(p\)-value & 0.001 & 0.08 & 0.004 & \textless{}0.001 & 0.02 & \textless{}0.001 \\ \(t\)-statistics & 3.66 & 1.84 & 3.91 & 4.41 & 2.42 & 5.01 \\ \hline SIF60 & & & & & & \\ \(r^{2}\) & 0.56 & 0.56 & 0.75 & 0.76 & 0.78 & 0.93 \\ \(p\)-value & \textless{}0.001 & \textless{}0.001 & 0.001 & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 \\ \(t\)-statistics & 7.9 & 4.61 & 4.84 & 13.99 & 9.05 & 12.99 \\ \hline \end{tabular}
\end{table} 
Table 5: Statistics of the linear regression between SIF averaged over 30 min between 12:00 and 12:30 UTC and GPP as a function of experimental conditions. GPP integration time: 30 min, GPP is averaged over 30 min between 12:00 and 12:30 UTC; 6.5 h, GPP is averaged between 8:00 and 15:30 UTC.Sky conditions: All, all measured points at 12:00 UTC are included in the analysis; Clear sky, clear sky at the measuring time only; Clear sky during the whole day, only data from completely clear sky days are considered. \(p\)-value and \(t\)-statistics for the probability of the regression slope to be zero are indicated. Figure 9: Coefficient of determination (\(r^{2}\)) of the linear regression between GPP and SIF760, SIF687 and PAR for a moving time window (20, 30, 40, 50, 60, and 70 days). Linear regression was performed between daily averages from Figure 7c,d. DOY denotes the middle of the moving time window. fAPAR remained almost constant at its maximum value with low diurnal variations (<2%, see Figure 6). In order to compare leaf measurements to EC data, leaf data obtained with the PAM-2000 at given times (8:30, 12:00, and 16:00) were averaged over the measured leaves to define a canopy-averaged photochemical yield \(\overline{\Phi}_{P}\) based on \(\Delta F/F^{\prime}m\) and an averaged fluorescence yield \(\overline{Fs}\) (Figure 10). We found that \(\overline{Fs}\) was almost not responsive to light level in the range between 500-1800 \(\upmu\)mol \(\cdot\) m\({}^{-2}\cdot\) s\({}^{-1}\), and no relationship between \(\overline{Fs}\) and LUE could be evidenced (\(r^{2}=0.11\), \(p=0.17\)). Similar results were found by investigating the relationship between LUE and the fluorescence yield defined at canopy level (SFY) (Table 6). Although statistically significant relationships could be found in some cases according to the \(p\)-values, \(r^{2}\) values stayed low, which means that SFY can only <<explain>> a minor part of LUE variation in this case. This contrasts with the relationships found between \(\overline{\Phi}_{P}\) and PAR and between \(\overline{\Phi}_{P}\) and LUE, where an almost proportional relationship was found between \(\overline{\Phi}_{P}\) and LUE (\(\overline{\Phi}_{P}=(0.053\pm 0.18)+(10.8\pm 5.8)LUE\), \(r^{2}=0.63\)). Figure 10: Scatter plots of stationary fluorescence level (\(\overline{Fs}\)) and photochemical yield (\(\overline{\Phi}_{P}\)) measured at leaf level with a PAM-2000 as a function of incident PAR and light use efficiency from EC measurements (\(LUE=GPP/APAR\)).\(r^{2}\) values of the linear regression and \(p\)-values of the slope of the regression line are indicated. Regression line equations are (with \(\pm 99\%\) confidence intervals): (**a**) \(y=(0.368\pm 0.055)\)\(+(33.\pm 44.