## 4 Discussion
Second, the measuring flashes used for the determination of the F\({}_{0}\)-level fluorescence would cause some PSII closure, and thus the real F\({}_{0}\)-level fluorescence would be overestimated to different degrees ([PERSON] et al., 2016). Besides, it has been reported in several literatures that the F\({}_{0}\)-level fluorescence quantum efficiency obviously changes with different vegetation species, chlorophyll contents, and exposure of leave surfaces to the sun. [PERSON] and [PERSON] ([PERSON] and [PERSON], 2016) measured the chlorophyll fluorescence characteristic at 77 K in 44 species of vascular plant, and found that the F\({}_{0}\)-level fluorescence signal between two different C\({}_{3}\) species can be 2-fold variation. The variation of F\({}_{0}\)-level fluorescence signal between the lower and upper leaf surfaces or the shaded and sun leaves were also remarkable. [PERSON] and [PERSON] ([PERSON] and [PERSON], 2016) and [PERSON] et al. ([PERSON] et al., 2016) also observed declines in F\({}_{0}\)-level fluorescence signal with increased chlorophyll content, probably due to an increase in the proportion of fluorescence that is reabsorbed. Moreover, as shown in [PERSON] et al. ([PERSON] et al., 2016), the cinnamic acid stress will significantly reduce the efficiency of \"open\" PSII reaction centers in the dark-adapted state, and a tendency of increase in F\({}_{0}\)-level fluorescence was observed during 2 th and 4 th days. These laboratory measurements can to some degree support and account for the variation of our FQE estimations with the growth of wheat. However, the influence of different factors on FQE variations is complicated and remains unsettled. Our results can provide a reference for the parameterization of FQE values with the winter wheat in the field. More control experiments with models and in the field need to be conducted for a better understanding of the variation of FQE values. Further opportunities are available to investigate the simulated results of MD12 and SCOPE version 1.7 compared with this study's results. In this work, the TB12 biochemical module is generated in the SCOPE model to implement the simulations. For this module, SIF emission efficiency depends on the empirical calibration of a number of datasets collected in field and laboratory experiments. The MD12 module has a more explicit parameterization of fluorescence quenching mechanisms, while it needs two additional inputs (KNPQs and qLs) to express the intermediate conditions, which cannot be derived from our in situ measurements. Given the uncertainty in PSII-PSI fluorescence emission curves and corresponding fqe1 and fqe2 values, the PSI-PSII separation is explicitly avoided in the last version 1.7 of the SCOPE model. In the future, the SIF simulated results of this version can be evaluated and compared with this study's results. More field observations and theoretical simulations are required to verify the results presented in this paper. At present, we conducted the experiments only on winter wheat, and the data sets are limited. Whether the seasonal patterns of FQE variation can be applied to other species has not been ascertained. In the future, the spectral measurements should be conducted across various species and plant functional types (PFT), with more frequent time series, at different locations, in multi-angle mode, along with the observations of vegetation parameters and flux exchanges. This research could become a reality with the achievement of our automatic fluorescence observation network. Moreover, active fqe1 and fqe2 measurements with PAM should be conducted in the field and on vegetation at different growth stages, which can further verify the seasonal cycles of our variable FQE values.