## 4 Discussion
### Why Red SIF Tracks GPP Better Than Far-Red SIF under Low Air Temperature  
In this study, we found that both daily \(LUE_{p}\) and \(\Phi_{F\_688}\) were affected by temperature and declined over the overwintering period (Figures 4 and 7), but this was not the case for \(\Phi_{F\_760}\). The \(\Phi_{F\_760}\) remained almost invariant with \(T_{a}\) (Figure 7d). Therefore, our results confirmed from some aspects that SIF at the red band and far-red band contained different information and showed different abilities to track changes in photosynthesis under environmental stresses [21]. This phenomenon can be attributed to more information about PSII contained in red-band \(tSIF\). Photosynthesis has two photosystems. Both photosystems are engaged in the fluorescence spectrum emitted from chloroplasts (Figure 1), which covers a wavelength range of 640-850 nm [74]. However, the fluorescence spectrum emitted from PSII has two peaks, with the amplitude of the left peak (\(\sim\)685 nm) being substantially larger than that of the right peak (\(\sim\)740 nm), whereas the fluorescence spectrum emitted from PSI only has one peak at 740 nm [21; 74]. The far-red \(tSIF\) thus contains contributions from both PSI and PSII. The relative contribution of PSI fluorescence is normally modest compared to PSII fluorescence, and PSI fluorescence generally remains constant under changeable illumination. However, some studies have reported that the PSI activity is enhanced in response to winter cold for overwintering plants [7; 24]. The contribution of PSII \(tSIF\) in the red bands is near 100%, which was significantly higher than that in the far-red SIF, in which PSII \(tSIF\) contributes only about 50% depending on the measurement conditions [74; 75]. Some studies proposed a practical approach to separately estimate the fraction of PSII in the red and far-red regions, and found that the contribution of PSII \(tSIF\) was 86% and 55%, respectively [76]. In addition, many studies assumed that the ratio of APAR allocated to PSII is 0.5 when calculating the PSII \(tSIF\), but, in fact, the ratio of PSII \(tSIF\) changes dynamically [4; 8]. PSII: PSI is approximately 2:1 in C3 plants and approximately 1:1 in C4 plants [77]. Variation in the proportion of \(tSIF\) between PSI and PSII may affect the generalizability of the \(tSIF\)-GPP relationship. [PERSON] et al. [4] proposed a photosynthetic mechanism model which quantitatively describes the coupling relationship between GPP and the total SIF of PSII, but there are still a series of problems to be solved in the calculation of the total fluorescence of PSII. Therefore, it is important for the quantitative estimation of GPP to disentangle the dynamic changes in PSI \(tSIF\) and PSII \(tSIF\), especially under environmental stress conditions. In addition, due to the large variations in sun-target-viewing geometries, the angular effects play an important role in interpreting directional SIF observed at the canopy level. Some studies have reported that the nadir-adjusted SIF was independent of viewing angle effects and showed a similarly good performance in estimating GPP for both red and far-red bands compared to \(tSIF\)[78; 79]. The angular effects in canopy observed SIF could bring some uncertainties in estimating GPP. It should be noted that the correlation between \(LUE_{p}\) and \(f_{esc}\) is the key point to determining the different performances of the nadir-adjust SIF and \(tSIF\) in approximating GPP [79]. Although the \(tSIF\) eliminates the influences of the angle effects, the estimation of \(tSIF\) relies on accurate information about canopy structure parameters [39; 80]. Thus, the estimation of canopy-structure parameters using remote-sensing methods will bring some uncertainties in evaluating the \(tSIF\)-GPP relationship. In our study, we found that \(\Phi_{F\_760}\) exhibited a different pattern in response to low temperature compared to \(\Phi_{F\_688}\) (Figure 4f,i).