## 3 Results
### Results of Spectroradiometric and Eddy Covariance Measurements on the Ground
All approaches using either \(\mathrm{F_{\mathrm{760-yield}}}\) or PRI as approximation of LUE (\(\mathrm{GPP_{C}}\), \(\mathrm{GPP_{D}}\)) improved the modeling of GPP diurnals (Figure 8 and Table 1). Results were statistically similar for \(\mathrm{GPP_{C}}\) and \(\mathrm{GPP_{D}}\). However, improvement in the diurnal course to match that for \(\mathrm{GPP_{EC}}\) was obtained when both PRI and \(\mathrm{F_{\mathrm{760-yield}}}\) were included in the empirical model (modeling approach E). In winter wheat, \(\mathrm{GPP_{EC}}\) was mostly driven by the PAR (Figure 7). However, slight improvements were achieved by including PRI or \(\mathrm{F_{\mathrm{760-yield}}}\) in the model. In sugar beet, better model predictions could be observed after incorporating PRI and \(\mathrm{F_{\mathrm{760-yield}}}\). These improvements were mainly related to a more realistic representation of the afternoon decline of GPP in response to a stomatal closure of sugar beet plants. In general, models \(\mathrm{GPP_{D}}\) and \(\mathrm{GPP_{E}}\) performed best for both species (Table 1). For sugar beet (DOY 183 and 253), model \(\mathrm{GPP_{E}}\), which includes a combination of PRI and \(\mathrm{F_{\mathrm{760}}}\) as a proxy for LUE, allowed the largest improvements. The retrieval of fluorescence is challenging. For airborne retrievals, basically three different approaches are currently available: Singular vector decomposition approaches obtain a series of spectral functions based on a set of training data. Singular vectors represent spectral effects of atmospheric absorption and scattering processes as well as surface properties. Recombining these singular vectors facilitates the atmospheric correction and the fluorescence retrieval within one processing step [40; 41; 42]. Another approach is based on a rigorous physically based atmospheric correction in combination with a full spectral fitting of the fluorescence signal and is currently being developed [43]. In our study, we used a recently adapted version of the FLD principle [34], which works well for medium resolution spectroscopy data as long as non-vegetated reference targets are abundant in the time series. However, for future applications we expect that the spectral fitting method will provide the most robust and consistent fluorescence retrievals, without need for reference surfaces in the scene. It should be noted that many advances have occurred since 2008 so that if this study were repeated in 2016 it would require different spectrometers and different sampling procedures to incorporate recent knowledge and technologies. We continued the pioneering work of [20], who introduced a semi-mechanistic framework to use F\({}_{760}\) measurements for improved modeling of diurnal courses of GPP. We extended the work with this demonstration study and show that a notable model improvement occurs in particular in the presence of a functional regulation of photosynthesis: leaf level measurements at winter wheat indicate no diurnal regulation, resulting in a high stomatal resistance and a constant photosynthetic capacity during the day. In consequence, there was no significant improvement of GPP predictions by using F\({}_{760}\) (Figure 8). In contrast, sugar beet showed a clear stomatal closure in the afternoon and a concomitant reduction of the leaf-level photosynthetic capacity. Thus, GPP models largely improved predictions of GPP in sugar beet when utilizing F\({}_{760}\) (Figure 8) on the two observation days. Comparable results were obtained for F\({}_{760\text{-}\text{yield}}\) and PRI for both species. It must be noted that only the limited dataset of this demonstration study restricts the generalization of the drawn conclusions. Nevertheless, within the limits of the dataset we interpret this as a clear indication that F\({}_{760}\) is indeed related to the functional status of actual photosynthesis as already suggested by [20; 42; 44].