## 3 Results
### Results of Spectroradiometric and Eddy Covariance Measurements on the Ground  
To illustrate the temporal behavior, the diurnal courses of APAR, GPP and several other parameters are shown in Figure 4. F\({}_{760}\) values show a distinct diurnal behavior mainly following APAR for both species (Figure 4(A1-A4,D1-D4)). F\({}_{760\text{-}yield}\) follows the incoming radiation until solar noon, but stay rather relatively high after solar noon (Figure 4(E1-E4)). The PRI shows a much less  
Figure 3: Airborne based F\({}_{760}\) and PRI measurements acquired on DOY 183 (1 July 2008) over sugar beet (colored circles) superimposed over a land cover map. Left column: measurements taken before solar noon. Right column: measurements taken after solar noon. Top row (**A1,A2**): Sun-induced fluorescence at 760 nm (F\({}_{760}\)). Bottom row (**B1,B2**): Photochemical reflectance index (PRI). Black triangle and dot mark the location of the ground observations. pronounced diurnal behavior for both species (Figure 4(F1-F4)) and remains relatively constant during the course of the day, in part a consequence of the scaled procedures. Figure 4: Diurnal behavior of canopy variables measured on the ground in a winter wheat field on DOY 127 and 176 and in a sugar beet field on DOY 183 and 253. Top row (**A1–A4**): Photosynthetically active radiation absorbed by the canopy (APAR); Second row (**B1–B4**): Gross primary productivity derived from the eddy covariance (EC) measurements (GPPEC); Third row (**C1–C4**): Light-use efficiency of photosynthetic CO2 uptake derived from the EC data (LUEEC); Fourth row (**D1–D4**): Sun-induced chlorophyll fluorescence (F\({}_{760}\)) obtained from ground-based spectroscopy measurements; Fifth row (**E1–E4**): Sun-induced chlorophyll fluorescence yield (F\({}_{760\text{-}yield}\)) obtained from ground-based spectroscopy measurements; Bottom row (**F1–F4**): Photochemical reflectance index (PRI) obtained from ground-based spectroscopy measurements. Vertical dashed lines indicate the solar noon. Vertical error bars indicate the standard deviation (\(n=200\) single measurements within a 30 min time window, rows **D** to **F**). GPPEC showed clear diurnal behavior with winter wheat reaching its maximum near solar noon (Figure 4(B1,B2)) and sugar beet--before solar noon (Figure 4(B3,B4)). GPPEC of sugar beet decreased from DOY 183 to DOY 253, which can partly be explained by a decrease in the incoming radiation towards fall. LUEEC generally decreased for both species from morning to noon and increased again in the afternoon (Figure 4(C1-C4)), with a more pronounced diurnal course in the young sugar beet crop. Leaf-level measurements of sugar beet showed an increase of maximum stomatal resistance (Figure 5(B3,B4)) and a reduced CO2 uptake rate (Amax in Figure 5(A3,A4)) in the afternoon, which may indicate stomatal closure. However, stomatal closure was not observed for the single leaf measurements in winter wheat (Figure 5(A1,A2,B1,B2)) Additional measurements of Amax in wheat and sugar beet confirm the observations of the single measurements days (Figure S2 supplementary materials). Correlation between Sun-Induced Fluorescence Yield, Photochemical Reflectance Index, and Photosynthetic Light-Use Efficiency  
We found an overall inverse relationship between F760-yield and LUEEC for both species (Figure 6(A1-A4)). For sugar beet, the relationship is almost linear (Figure 6(A3,A4)), whereas for winter wheat the relationship between F760-yield and LUEEC changed over the day and is described by a hysteresis (Figure 6(A1,A2)). With increasing PAR in the morning, F760-yield increased, whereas LUEEC decreased (negative correlation). During midday and high light intensities, LUEEC and F760-yield were positively correlated, only in winter wheat. In the afternoon, with decreasing incoming radiation, LUEEC and F760-yield again showed a negative correlation for both species. We found no overall consistent relationship between PRI and LUEEC for both species (Figure 6(B1-B4).