## 4 Discussion
### The Link between \(\mathrm{SIF}_{canopy}\) and \(\mathrm{APAR}_{chl}\)  
In this study, we investigated the relationship between \(\mathrm{SIF}_{canopy}\) and \(\mathrm{APAR}_{chl}\) using both simulated and field-measured datasets. \(\mathrm{APAR}_{chl}\) is a key parameter linking \(\mathrm{SIF}\) to GPP and so the results of this study may serve as a reference for the estimation of GPP using \(\mathrm{SIF}\). It is an important issue in studies of applications using \(\mathrm{SIF}\) and it is still a matter of debate [46; 47]. According to [PERSON] et al. [29], the emitted \(\mathrm{SIF}\) contains contributions from both photosystem I (PS I) and photosystem II (PS II). Compared to the \(\mathrm{SIF}\) in the far-red part of the spectrum, the \(\mathrm{SIF}\) in the red band is more closely linked to the activity of PS II. The \(\mathrm{SIF}\) yield of PS II is considered to be more sensitive to variations in illumination and physiological or biochemical parameters [30; 48]. Therefore, at the photosystem level, it is reasonable to believe that the \(\mathrm{SIF}\) in the red part of the spectrum is more closely related to the GPP. However, in remote sensing applications at the canopy level, only the \(\mathrm{SIF}\) signal that escapes from the canopy can be detected; the \(\mathrm{SIF}\) is reabsorbed by chloroplasts during the radiative transfer process within the leaves and canopy [49; 50; 51; 52] and so the characteristics of \(\mathrm{SIF}_{canopy}\) are different to those of \(\mathrm{SIF}_{PS}\).  
In this study, the simulated data showed that the relationship between \(\mathrm{SIF}_{canopy}\) at O\({}_{2}\)-A band and \(\mathrm{APAR}_{chl}\) is relatively insensitive to the LAI and Cab content compared to the O\({}_{2}\)-B band and can be expressed by a unique linear relationship at the O\({}_{2}\)-A band. However, at the O\({}_{2}\)-B band, this relationship is significantly influenced by the LAI and Cab content. The effect of the Cab content on the ratio of \(\mathrm{SIF}_{canopy}\) to \(\mathrm{APAR}_{chl}\) is larger than that due to the LAI. This can be interpreted as meaning that the fluorescence photons emitted at the photosystem level can be reabsorbed by chlorophyll within the leaf or during the radiative transfer process from the leaf to the canopy. This reabsorption occurs because the fluorescence emission spectrum overlaps the chlorophyll absorption spectrum [30]. The absorption of chlorophyll at the red band is much stronger than that at the far-red band and the rate of reabsorption of red fluorescence can reach 90% [53]. As a result, \(\mathrm{SIF}_{canopy}\) at the O\({}_{2}\)-B band is strongly influenced by pigments and the correlation between \(\mathrm{SIF}_{canopy}\) and \(\mathrm{APAR}_{chl}\) here is more sensitive to the Cab content than at the O\({}_{2}\)-A band.  
As [PERSON] et al. [30] stated, at the red band, reabsorption effects dominate the amount of fluorescence escaping from the canopy. As verified by both simulations and field experiments, the ratio of \(\mathrm{SIF}_{canopy}\) to \(\mathrm{APAR}_{chl}\) at both the O\({}_{2}\)-A and O\({}_{2}\)-B bands varies with the leaf angle distribution function, with more horizontal canopies having a larger ratio of \(\mathrm{SIF}_{canopy}\) to \(\mathrm{APAR}_{chl}\). This indicates that the large uncertainty in \(f_{esc}\) can weaken the relationship between the remotely sensed red \(\mathrm{SIF}\) and \(\mathrm{APAR}_{chl}\) or GPP.