## 2 Background  
Plants use two photosystems (photosystem I and II, or PSI and PSII), which act in series to carry electrons from water to the final acceptor (NADPH) in the light-dependent reactions of photosynthesis and produce the energy-rich ATP molecule. At the PSI and PSII level, light is absorbed by various photosynthetic pigments (chlorophyll-a, chlorophyll-b, carotenoids), which results in excited states of chlorophyll-a molecules. This energy is then used by photochemical conversion to drive photosynthesis, while a small fraction of the excitation energy is re-emitted back as fluorescence. Under high light and stress conditions, excessive energy can dissipate in a controlled manner via various pathways that are classified as non-photochemical quenching mechanisms (NPQs). Because these three processes all dissipate the excitation energy, they are tightly interrelated. Hence, F can be quenched by photochemistry or NPQ dissipation, the latter being partially under the control of photoprotective mechanisms that help to safely dissipate excess energy [42]. Leaf F is emitted in the red and far-red range (from about 650 nm to 850 nm), with two peaks near 685 nm and 740 nm [43]. PSI and PSII have different antenna compositions and therefore different spectroscopic properties. PSI fluorescence is found to be emitted only in the far-red and stays almost constant, while PSII has two emission peaks in the red and far-red. PSII also exhibits variable fluorescence, mainly in response to the closure of reaction centers following a charge separation (photochemical quenching) and to increased heat dissipation (non-photochemical quenching). Physiologically induced changes in F can be assessed by monitoring the fluorescence quantum yield (FY), defined as the ratio between fluorescence emission and the absorbed light evaluated on the basis of quanta fluxes. Alternatively, the fluorescence efficiency of a given surface can be estimated by the apparent spectral fluorescence yield (ASFY) as the ratio between the spectral fluorescence radiance in quanta units integrated over the emission hemisphere (e.g., in moles of photons m\({}^{-2}\) s\({}^{-1}\) nm\({}^{-1}\) or W m\({}^{-2}\) nm\({}^{-1}\)) to the incident light in the photosynthetic active radiation (PAR) region (e.g., in moles of photons m\({}^{-2}\) s\({}^{-1}\) or W m\({}^{-2}\)). At the leaf level, the ASFY is around \(20\times 10^{-6}\) to \(80\times 10^{-6}\) nm\({}^{-1}\)[44], which gives a total apparent yield of about 0.2-0.8%. At the canopy level, different investigations have revealed values of the same order in the far-red (up to \(30\times 10^{-6}\) nm\({}^{-1}\)) with generally lower values in the red (up to \(8\times 10^{-6}\) nm\({}^{-1}\)) [45; 46]. The level of the F signal can also be assessed by the fraction of fluorescence in the total vegetation radiance (emitted and reflected) at the same wavelengthunder natural sunlight. Outside the absorption bands, these fractions are found to be around 1% in the far-red and 10% in the red, the latter value being due to vegetation's strong absorption in the red [47]. In order to detect weak F signals, telluric and solar (so-called Fraunhofer lines) atmospheric absorption features are generally exploited. Because fluorescence adds to the reflected surface radiance, it induces a subtle but detectable change in the spectral signature of the absorption lines. Over the many absorption lines that overlap the F emission, the ones in the O\({}_{2}\)-A and O\({}_{2}\)-B with a maximum absorption at 760 and 687 nm are particularly suitable for F detection, as they include a large number of close lines and correspond to the two F peaks. As described in [40], retrieval methods that are used to quantify F are subdivided into two major categories: reflectance-based and radiance-based approaches.