## 1 Introduction  
Photosynthetic CO\({}_{2}\) uptake in terrestrial vegetation ecosystems mediated by plant photosynthesis is commonly referred to as gross primary production (GPP) and constitutes the largest flux of the global carbon cycle [1; 2]. Variations in photosynthetic carbon fixation are identified as the main sources of terrestrial carbon cycle dynamics [2]. GPP accounts for approximately 120 Gt C per year, which is 15 times higher than the global carbon production from combustion of fossil fuels in human industrial processes [1]. This means that even relatively small changes in the terrestrial photosynthetic carbon fixation may lead to significant deviations and uncertainties in estimating regional and global carbon fluxes. This spatio-temporal variability of ecosystem photosynthesis is not sufficiently understood yet, as GPP is determined by complex processes related to plant phenology and physiological activities [2]. Therefore, substantial efforts are being undertaken to better measure this process and eventually provide more precise constrains for mechanistic carbon models which are frequently used to predict GPP. Different modeling strategies can be applied to describe how solar irradiance is converted into dry matter via photosynthesis [3]. However, most models include the light-use-efficiency (LUE) approach according to [4; 5] where GPP is expressed as a product of incident photosynthetically active solar radiation (PAR), its absorbed fraction (\(f_{\text{APAR}}\)), and an efficiency term for conversion of absorbed energy into carbon fixation. This latter term is usually called LUE. In modeling approaches, a maximum LUE for each Plant Functional Type (PFT) is assumed that is down regulated by environmental parameters. In plant sciences, however, it is well established that photosynthetic light conversion represents a complex biophysical and biochemical process, which is influenced by a variety of environmental factors, depends on the ontogeny of the single plant, and is ruled by the genetic plasticity of a particular species or variety [6]. Moreover, the environmental conditions of our planet are being altered by, for example, greatly increasing the amount of biologically available nitrogen resources via artificial fertilizer and replacing natural plant ecosystems by managed vegetation consisting of new species and their compositions. Therefore, modern agricultural crops may appear to have fundamentally different photosynthetic characteristics than naturally evolved species. As a result, it is unclear how precise current modeling assumptions are for predicting future photosynthetic carbon uptake by terrestrial vegetation. GPP rates cannot be measured directly and precisely using remote sensing (RS) approaches. Available RS approaches rely on the measurement of vegetation greenness which is related to the amount of the photosynthetic active pigment chlorophyll, thus, being sensitive to potential photosynthesis only. There have been some attempts to exploit small reflectance changes around 531 nm due to xanthophyll pigment changes to quantify photosynthetic efficiency [7]. The resulting photochemical reflectance index (PRI), however, was developed at the leaf level [8] and is known to be largely affected by the structure of the canopy [9], pigment pool sizes [10] and illumination effects [11; 12]. Nevertheless, the PRI is currently applied to ecosystem and it remains an open debate whether or not canopy-level processes can be quantified by such RS approaches [13]. A novel RS approach recently suggested to advance estimates of GPP is the sun-induced chlorophyll fluorescence signal (F). The weak fluorescence signal emitted from the cores of the two photosystems can be detected passively across scales [14] using high-resolution spectrometers in combination with the Fraunhofer line depth (FLD) principle [15].