## 4 Current Approaches to Open Challenges of F Estimations from Proximal to Airborne Scale
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#### 4.3.3 Ancillary Data for Retrieved F Interpretation  
F measured at the leaf level or retrieved at the canopy level is influenced by several physiological drivers and structural (physical) effects (Figure 1). Most importantly, the F signal (or, even more significant, the fluorescence apparent yield) resulting from both emission and reabsorption is strongly modulated by the amount of pigment, whereas the retrieved signal is highly dependent on the pigment pool (leaf level) and internal scattering due to the structure (canopy level). Hence, interpreting F in terms of its physiological meaning requires (1) correction for reabsorption effects (dominant for the red F) [e.g., 55] and (2) normalization for the absorbed PAR (APAR, measurable at the leaf level but to be estimated at the canopy level). The latter allows for the analysis of the vegetation functioning, given the intrinsic leaf constituents (pigments, water content), in response to its environmental conditions [141]. Under light-unsaturated conditions, fluorescence will be mostly driven by PAR, and the relationship between the fluorescence yield and photosynthesis yield will be linear [67; 68]. However, under light-saturated conditions, regulated photoproduction (or non-photochemical quenching, NPQ) mechanisms will be activated and modify the relationship between the fluorescence and photosynthesis yield, which requires further consideration in any setup. The Photochemical Reflectance Index [142]--which uses the reflectance at 531 and 570 nm as the sensitive and reference bands, respectively, for the photochemical changes due to photoproduction--is frequently used to track NPQ. Although several studies have reported good correlations between PRI and NPQ during different short-term light phases [59], some discrepancies have been acknowledged, leading to proposals of other interacting absorbance mechanisms at the leaf level [143] and structural effects in the canopy [144]. Furthermore, stress ambient T/RH, water availability, and wind all drive the energy balance, with evapotranspiration (sensible heat dissipation) on the one hand and the light reactions of photosynthesis on the other hand. They affect the energy dissipation balance within the leaf and hence also the F emission. These key environmental variables form the ancillary data required for further interpretation of the F signal in relation to its dynamic environmental (e.g., light, temperature, nutrients, vapor pressure deficit) [145] and physiological (e.g., pigment content, water content) drivers. Hence, in addition, knowledge of canopy temperature and vapor pressure deficit will be helpful for translating fluorescence into photosynthesis products. However, currently, the first step (i.e., the correction for F reabsorption for the canopy-retrieved F signal) requires more consideration, given the structural drivers that are superimposed at the canopy level [47; 76; 125]. Overall, the key driving variables need to be monitored at a proper spatiotemporal scale [146], and this might not always be straightforward. A complete list of ancillary data and the frequency of data capture will primarily depend on the size of the target, the experimental setup, and the priorities in the research question set. Ancillary data requirements can also be case-dependent (e.g., F measured in a fertilization experiment could require fertilization dose, soil type, etc.). Radiative transfer models (RTMs) can help to identify the mandatory parameters to incorporate into the process (cf. Section 4.5).