## 4 Discussion
### Further Understanding about the Relationship of F760 with Growth and Environmental Parameters  
Based on the results mentioned in the previous sections, we know that seasonal and diurnal F760 patterns were completely different (Figures 3 and 5). However, both of them were mainly driven by the variation of SWR. We also noted that the impact of SWR on seasonal F760 was less important than other environmental parameter (i.e., Rh for wheat and Ta for maize) (Figure 6). This may be because measurements for analyzing seasonal F760 were only from part of the whole growth periods since SWR was believed to be a more important environmental parameter in driving the seasonal pattern of F760 in whole-field campaigns (Figure S2). Ta, Rh, and VPD were three other important environmental parameters that could affect F760 variations at seasonal and diurnal scales. This could be explained by the fact that Ta, Rh, and VPD could affect air aridity, and therefore alter transpiration by regulating the water demand of air from vegetation and ultimately affect photosynthesis [45]. However, we should also note that relationships of F760 with Ta, Rh, and VPD were different between wheat and maize (Figures 6 and 7). That is to say, the impact of these parameters on seasonal F760 variation was varied with different plant functional types. For diurnal scales, both of the relationships of F760 with wheat and maize were significant (Figure 7), which means that air aridity was a key factor in controlling diurnal F760 variation. Although SM-induced water stress lowered SIF magnitudes, both at seasonal and diurnal scales (Figure 3a and Figure 5a), SM did not show a considerable relationship with F760 (r values: 0.08 and 0.11 for wheat, \(-\)0.09 and 0.02 for maize, at seasonal and diurnal scales, respectively; Figure 6c,d and Figure 7c,d)  
The seasonal and diurnal patterns of F760 were also affected by growth parameters, which included LAI, Chl, MTA, and RWC. Based on wheat measurements, both LAI, Chl, and MTA played crucial roles in regulating F760, not only at the seasonal scale, but also at the diurnal scale (Figures 6 and 7). This was because LAI, Chl, and MTA were related to the canopy structure that determined light capture by the canopy. However, these three parameters, particularly LAI and Chl, did not show significant impacts on maize SIF at both the seasonal and diurnal scales. This large deviation from the results of wheat may be explained by the more complicated canopy structure of maize than that of wheat. Additionally, the short observation distance between the fiber and canopy may be another explanation for such results, since the short distance results in a small view area of the fiber, and therefore only a partial signal of canopy can be received by the sensor. RWC was also important for F760 variation at seasonal and diurnal scales, which was demonstrated by the relatively high correlation between RWC and SIF observed in maize (r values: 0.42 and 0.21, at seasonal and diurnal scales, respectively; Figures 6d and 8b). The importance of RWC for SIF variation is attributed to the close relationship between RWC and photosynthesis: RWC directly indicates the plant water content, which influences the photosynthesis rate and light capture by leaf wilting and leaf angle adjustments [46]. The leaf angle adjustments by the variation of RWC was supported by high correlation between RWC and MTA based on the measurements of maize (\(-\)0.53 for seasonal scale, \(-\)0.58 for diurnal scale, Figures 6d and 8b). As mentioned above, it can be concluded that F760 was mainly driven by SWR for the environmental parameters and LAI, Chl, MTA, and RWC for the growth parameters, while being less driven by Ta, VPD, Rh, and SM. A similar result has been reported by [PERSON] et al. with the simulated data of the Soil-Canopy Observation of Photosynthesis and Energy (SCOPE) balance model [47]. The contribution of each parameter to SIF is further affected by plant functional types, as revealed by the data from wheat and maize in our study.