## 1 Introduction  
Terrestrial vegetation plays a major role at the Earth's surface by harvesting sunlight and converting photons' energy into a redox potential available to the rest of the biosphere. This basic process of energy conversion occurs in the antennae pigments of photosystems from the excited state of chlorophyll a (Chl a). In optimal conditions, the process is highly efficient, although a small part of the absorbed energy is re-emitted as fluorescence. This emission occurs in two broad bands in the red and far red parts of the spectrum, with maxima near 685 and 740 nm, respectively [1]. As fluorescence emission competes with photochemical conversion and heat dissipation of the excess energy, chlorophyll fluorescence (ChlF) has long been considered as a convenient non-intrusive probe to assess photosynthetic processes in chloroplasts and leaves (for a comprehensive review, see [2; 3; 4]). However, assessment of ChlF at larger scales has long been limited by the power of light sources that can be used to excite fluorescence from remote distances [5]. In the last decade, new methods for the remote sensing of ChlF at canopy level were developed to disentangle sun-induced fluorescence (SIF) from reflected light. Shortly, these methods take advantage of narrow absorption features of the solar spectrum--the so-called Fraunhofer lines--in which incident radiation is significantly reduced [6; 7]; for a review, see [8]. As vegetation fluorescence is added to reflected light, it induces a small but detectable alteration of the absorption features, allowing the quantification of fluorescence. This \"in-filling\" approach has been successfully applied in the atmospheric absorption bands (namely, O\({}_{2}\)-A and O\({}_{2}\)-B) or in the solar Fraunhofer lines to detect vegetation fluorescence from various platforms at ground level [6; 9; 10; 11; 12], from an airborne platform [13; 14], from an unmanned aerial vehicle (UAV) [15], and from space [16; 17]. Recently, the European Space Agency (ESA) selected the FLEX (FLUorescence EXplorer) mission to be the 8 th Earth Explorer in the framework of the Living Planet Program [18]. Using data from the GOSAT-TANSO (Greenhouse gases Observing SATellite-Thermal And Near-infrared Sensor for carbon Observation) instrument, a linear relationship between far red SIF (FRSIF) and gross primary production (GPP) was found on a monthly or yearly basis [16; 19]. On a monthly basis, FRSIF from croplands in the US corn-belt strongly correlated with GPP estimated from flux towers, but deviated from model-based GPP. Hence, agricultural productivity is suspected to be severely underestimated by global GPP models [20]. According to these observations, SIF appears as a powerful indicator that can help to improve the assessment of plant productivity on a global basis. However, a better understanding of the complex interactions between the factors that contribute to canopy fluorescence is required to go beyond a simple empirical observation of an association between GPP and fluorescence radiance. So far, ChlF has been extensively studied at the chloroplast or leaf level, but observation of ChlF at the canopy level and its relationship with photosynthesis in natural conditions is still in its infancy [21]. In this context, outdoor ground studies based on continuous ChlF measurements over well-characterized ecosystems are essential experimental tools to capture the complex dynamic of SIF at both diurnal and seasonal scales. Using a high spectral resolution spectrometer, [PERSON] et al. [22] found a strong linear relationship (\(r^{2}=0.80\)) between SIF at 760 nm measured in the O\({}_{2}\)-A band (SIF760) and GPP measured with eddy covariance (EC) techniques on a rice crop in an analysis over two complete vegetative cycles. This value of the correlation coefficient is close to the value found from space data in [16].