## 3 Results
### Results of Spectroradiometric and Eddy Covariance Measurements on the Ground
Potential disturbing effects such as structural changes or changes in the pigment content were expected to be negligible in the course of single days and reflectance anisotropy effects as described by [11] are expected to have similar impacts in the morning and afternoon due to comparable solar illumination angles. We used a relatively simple empirical modeling approach based on the principles of [PERSON]'s LUE concept. Recent studies theoretically and experimentally demonstrate the close link between F\({}_{760}\) and APAR as well as a secondary sensitivity to LUE [45; 46; 47]. Our results are in line with these findings and provide further evidence that APAR and LUE can be successfully approximated with F\({}_{760}\), allowing an improved modeling of GPP diurnals. However, various effects were discussed causing F\({}_{760}\)--GPP relationships to be ecosystem specific [45], eventually hindering a more universal use of F\({}_{760}\) to constrain GPP. In fact, there is an ongoing discussion about how sun-induced fluorescence can best be assimilated in existing model formulations to use it to constrain GPP across ecosystems and scales. According to our knowledge only the newest version of the SCOPE model provides an explicit fluorescence interface [48; 49], as well as a modified version of the community land model 4 (CLM-4) [50]. Both models facilitate a diagnostic use of sun-induced fluorescence for a GPP assessment across ecosystems and scales. New model formulations and developments are, however, needed to fully exploit the information content inherent in the fluorescence signal (i.e., full emission shape, peak emissions at 687 nm and 740 nm). Such information will soon become frequently available from future satellite sensors (e.g., ESA's TROPOMI onboard Sentinel-5 [51], FLORIS onboard FLEX [52]). Further, it became clear that not only F\({}_{760}\) but also the incorporation of the PRI improved the prediction of GPP. Diurnal changes in PRI were already shown to be related to diurnal adaptation of photosynthesis [53]. F\({}_{760}\) is emitted from the core of both photosystems and, thus, is related to the efficiency of photochemical energy separation. The PRI, in contrast, is related to the degree of non-photochemical energy dissipation (NPQ). F\({}_{760}\) and PRI are often inversely correlated (e.g., [53; 54]). However, it cannot be assumed that they are simply or linearly correlated. In our study, the use of a combination of F\({}_{760\text{-}yield}\) and PRI provided the best input to predict diurnal variations in GPP, particularly for the sugar beets with functional limitation of carbon uptake, where NPQ is likely present at a high level of expression (Table 1). We investigated the spatio-temporal variability of vegetation fluorescence using medium resolution airborne data that were acquired over a large area. We demonstrate a substantial functional variability occurring at two different spatial scales, namely within single fields and between different fields. This indicates that management practices, soil properties or seed material are of considerable importance in determining spatial patterns of vegetation carbon fixation. Again large differences between morning and afternoon measurements of F\({}_{760}\) and PRI were observed in sugar beet (Figure 3), which underlines our interpretation of the leaf-level regulation and the functional fingerprint in these two physiological remote sensing parameters.