## 5 Conclusions  
In this paper, we investigated diurnal and seasonal dynamics of red and far red SIF over a full seasonal cycle on a wheat crop, and its relationship with carbon uptake assessed by eddy covariance techniques. Using active techniques, fluorescence and PSII photochemical yield were measured at leaf level and compared to TOC measurements. We found a low diurnal variability of the apparent fluorescence yield (ASFY) during clear sky days, although differences between red and far red SIF can be observed. Diurnal changes in ASFY760 are in agreement with active leaf measurements of the mean stationary fluorescence level \(\overline{F}s\), while ASFY687 shows variations in opposite direction. Under cloudy sky, ASFY687 shows a noticeable increase, while ASFY760 does not change. On the seasonal time scale, far red SIF appears to indicate better canopy growth and changes in chlorophyll content during senescence than red SIF. We observed a strong linear relationship between far red SIF and GPP (\(r^{2}\) ranging from 0.5 to \(>\)0.9, \(p<\) 0.001). This relationship is stronger during the phase of canopy growth or changes in chlorophyll content. It is also stronger when only clear sky days are considered, and for daily integrated values compared to 30 min integrated ones. In all cases, the relationship between red SIF and GPP is weaker than the corresponding one between far red SIF and GPP. The red SIF-GPP relationship is also the weakest during canopy growth. When canopy is in a steady phenophase, SIG687 and SIF760 are almost as effective as incoming PAR in predicting GPP. In view of these results, the use of SIF to directly assess GPP with simple linear regression presents some limitations. We showed that the relationship between SIF and GPP is highly variable depending on the emission wavelength, the time scale of observation, and/or the phenological stage of the crop. A part of the loss in correlation can be attributed to re-absorption effects, as for RSIF, which poorly correlates with GPP in all cases. Another part can be attributed to the non-proportional relationship between _Fyield_ and LUE that was evidenced in our study by active fluorescence measurements. Our results suggest that using simple linear relationships to infer GPP from SIF can only be applied in specific cases: with FRSIF, a high dynamic range of green biomass and a low variation range of LUE. In other cases, approaches using more sophisticated algorithms that exploit the full range of the available optical information, including RSIF, PRI (Photochemical Reflectance Index) [64; 65], and reflectance should be considered. It is important to note that these results have been obtained on a single wheat crop over a single season. Further experiments must be conducted over multiple seasons on different crops and species before drawing general conclusions on the relationship between SIF and GPP. This work was supported by the \"Programme National de Teledetection Spatiale\" through the \"Plateforme de test pour capteurs de fluorescence satellites ou avionnes\" project, the \"Agence National de la Recherche\" (ANR) through the CALSIF project (ANR No.12-BS06-0006-01) and the CNES through the \"Terre, Ocean, Surfaces Continentales, Atmosphere\" (TOSCA) program (LASVEG and ACTIPASS projects). [PERSON] was granted by a CNES contract through the TOSCA program. The authors also want to thank the \"Unite Experimentale Environnement et Agronomie d'Avignon\" for the support in crop management and measurements facilities, [PERSON] for his support during the measurement campaign and [PERSON] for the measurements of crop characteristics and data processing. We thank four anonymous reviewers for their valuable comments. **Author Contributions:** [PERSON], [PERSON], [PERSON] and [PERSON] conceived and designed the instrumental setup. [PERSON] and [PERSON] conceived and designed the experiments. [PERSON], [PERSON], [PERSON] and [PERSON] performed the fluorescence experiments. [PERSON] performed the gas exchange experiments. [PERSON] and [PERSON] analyzed the data.