## 1 Introduction
However, in their study, fluorescence measurements were acquired only at solar noon, and no information is provided about the diurnal dynamic of fluorescence and its interaction with GPP. In another study performed at ground level on a maize crop over four seasonal cycles, a much lower correlation between SIF760 and GPP was found (\(r^{2}=0.30\)) [23], which indicates that the correlation between SIF and GPP is subjected to changes, while the factors that control these variations remain unknown. [PERSON] et al. [24] analyzed the full diurnal cycle of SIF and GPP, but their investigation was limited to a short period in the season, with no major phenological vegetation changes. In another study, the relationship between airborne SIF and GPP was found to be asymptotic and ecosystem-specific [25]. Spaceborne SIF is also used to constrain process-based vegetation models like SCOPE [26], with better simulations of GPP than other approaches based on vegetation indices [27; 28]. However, the factors that drive the SIF-GPP relationship in different experimental conditions according, for example, to irradiance or phenology remains unclear, as studieson this topic are still rare. More specifically, the question remains as to what extent this relationship is due to absorbed photosynthetically active radiation (APAR) and/or light use efficiency (LUE) [29]. Moreover, most studies are limited to far red SIF (e.g., SIF760, measured in the O\({}_{2}\)-A band at 760 nm), and very little information is available on the diurnal and seasonal dynamics of red SIF (RSIF, e.g., SIF687, measured in the O\({}_{2}\)-B band at 687 nm) [23]. Compared to far red fluorescence, red fluorescence--produced dominantly by photosystem II (PSII)--is the most variable part of ChIF [30]. Hence, RSIF would be a preferential choice to assess physiologically-induced ChIF variations, and the detection of both peaks would provide complementary information on photosystem I (PSI) and PSII emissions [31]. In this study, we used the field platform for continuous measurements of fluorescence [32] located in Avignon, France to investigate both diurnal and seasonal dynamics of RSIF and FRSIF over a wheat crop during a complete vegetative cycle. SIF was continuously measured and retrieved from the O\({}_{2}\)-B (SIF687) and O\({}_{2}\)-A (SIF760) bands. Carbon fluxes were simultaneously monitored using eddy covariance techniques [33]. In most previous ground studies, SIF measurements were taken at a very low altitude (around 1 m above the canopy, see [22; 23; 24]). These experimental conditions are not truly representative of a remote sensing viewing configuration, because it implies either a limited sampled area (which is not representative of the whole canopy), or a large field of view, which encompasses different viewing angles. Here, we performed nadir measurements from the top of a crane at an altitude of 21 m with a relatively narrow low field of view (5\({}^{\circ}\)) to limit the extent of the viewing angles. Fluorescence was also assessed at leaf level using active measurements to derive information on fluorescence and photochemical yields. The aims of the study are:  
* to investigate both diurnal and seasonal dynamics of RSIF and FRSIF during a complete vegetative cycle of a crop,
* to analyze the relationship between SIF and GPP according to emission wavelength, time scale, and phenological stage,
* to investigate the factors that potentially control the relationship between SIF and GPP using independent measurements of APAR on one hand, and of fluorescence and photochemical yield by active methods on the other hand.