###### Abstract  
Solar-induced chlorophyll fluorescence (SIF), which can be used as a novel proxy for estimating gross primary production (GPP), can be effectively retrieved using ground-based, airborne and satellite measurements. Absorbed photosynthetically active radiation (APAR) is the key bridge linking SIF and GPP. Remotely sensed SIF at the canopy level (\(SIF_{canopy}\)) is only a part of the total SIF emission at the photosystem level. An SIF-based model for GPP estimation would be strongly influenced by the fraction of SIF photons escaping from the canopy (\(f_{esc}\)). Understanding the response of \(SIF_{canopy}\) to the absorbed photosynthetically active radiation absorbed by chlorophyll (\(APAR_{chl}\)) is a key step in estimating GPP but, as yet, this has not been well explored. In this study, we aim to investigate the relationship between remotely sensed \(SIF_{canopy}\) and \(APAR_{chl}\) based on simulations made by the Soil Canopy Observation Photosynthesis Energy fluxes (SCOPE) model and field measurements. First, the ratio of the fraction of the absorbed photosynthetically active radiation absorbed by chlorophyll (\(\text{fPAR}_{chl}\)) to the fraction of absorbed photosynthetically active radiation absorbed by green leaves (\(\text{fPAR}_{green}\)) is investigated using a dataset simulated by the SCOPE model. The results give a mean value of 0.722 for Cab at 5 \(\upmu\)cm\({}^{-2}\), 0.761 for Cab at 10 \(\upmu\)cm\({}^{-2}\) and 0.795 for other Cab content (ranging from 0.71 to 0.81). The response of \(SIF_{canopy}\) to \(APAR_{chl}\) is then explored using simulations corresponding to different biochemical and biophysical conditions and it is found that \(SIF_{canopy}\) is well correlated with \(APAR_{chl}\). At the O\({}_{2}\)-A band, for a given plant type, the relationship between \(SIF_{canopy}\) and \(APAR_{chl}\) can be approximately expressed by a linear statistical model even for different values of the leaf area index (LAI) and chlorophyll content, whereas the relationship varies with the LAI and chlorophyll content at the O\({}_{2}\)-B band. Finally, the response of \(SIF_{canopy}\) to \(APAR_{chl}\) for different leaf angle distribution (LAD) functions is investigated using field observations and simulations; the results show that \(f_{esc}\) is larger for a planophile canopy structure. The values of the ratio of \(SIF_{canopy}\) to \(APAR_{chl}\) are 0.0092 \(\pm\) 0.0020, 0.0076 \(\pm\) 0.0036 and 0.0052 \(\pm\) 0.0004 \(\upmu\)cm\({}^{-1}\) sr\({}^{-1}\) for planophile vegetables/crops, planophile grass and spherical winter wheat, respectively, at the O\({}_{2}\)-A band. At the O\({}_{2}\)-B band, the ratios are 0.0063 \(\pm\) 0.0014, 0.0049 \(\pm\) 0.0030 and 0.0033 \(\pm\) 0.0004 \(\upmu\)cm\({}^{-1}\) sr\({}^{-1}\), respectively. The values of this ratio derived from observations agree with simulations, giving values of 0.0055 \(\pm\) 0.0002 and 0.0068 \(\pm\) 0.0001 \(\upmu\)cm\({}^{-1}\) sr\({}^{-1}\) at the O\({}_{2}\)-A band and 0.0032 \(\pm\) 0.0002 and 0.0047 \(\pm\) 0.0001 \(\upmu\)cm\({}^{-1}\) sr\({}^{-1}\) at the O\({}_{2}\)-B band for spherical and planophile canopies, respectively. Therefore, both the simulations and observations confirm that the relationship between \(SIF_{canopy}\) and \(\text{APAR}_{chl}\) is species-specific and affected by biochemical components and canopy structure, especially at the O\({}_{2}\)-B band. It is also very important to correct for reabsorption and scattering of the SIF radiative transfer from the photosystem to the canopy level before the remotely sensed \(SIF_{canopy}\) is linked to the GPP.  
**Keywords:** solar-induced chlorophyll fluorescence (SIF); absorbed photosynthetically active radiation (APAR); chlorophyll; SCOPE model