## 4 Discussion  
### The Different Responses of GPP and SIF to the Environment  
In this study, we investigated the responses of GPP and SIF to the different environmental variables from a corn tower-based SIF and TROPOSIF based on the binning method. The difference between GPP and SIF light responses had been reported in previous studies [21; 23]. Our results also found that at both site and satellite scales, there were divergencies in the light response speeds between GPP and SIF which was affected by Ta and EF though their light response curves were similar. When Ta and EF were lower, SIF increased faster than GPP. With the increase in Ta and EF, the difference between increase speeds of GPP and SIF was gradually reduced. Additionally, the sensitivities of GPP and SIF to radiation, temperature, and water availability were different. SIF was more sensitive to radiation than to Ta and EF, and the sensitivity of GPP to radiation was more regulated by the Ta and EF, which was consistent with studies by [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [21]. The \begin{table}
\begin{tabular}{c c c} \hline \hline  & **GPP-SIF (EF bins)** & **GPP-SIF (Ta bins)** \\  & **GPP=**\(\frac{\alpha\times\text{SIF}}{\textbf{p+SIF}}\)** & **GPP=**\(\frac{\alpha\times\text{SIF}}{\textbf{p+SIF}}\)** \\ \hline \([-2,-1.5)\) & a = 0.52, b = 0.35, R\({}^{2}\) = 0.18 & a = 0.45, b = 0.2, R\({}^{2}\) = 0.23 \\ \([-1.5,-1.0)\) & a = 0.71, b = 0.51, R\({}^{2}\) = 0.38 & a = 0.54, b = 0.29, R\({}^{2}\) = 0.26 \\ \([-1.0,-0.5)\) & a = 1.03, b = 0.68, R\({}^{2}\) = 0.42 & a = 0.76, b = 0.40, R\({}^{2}\) = 0.39 \\ \([-0.5,0)\) & a = 1.065, b = 0.56, R\({}^{2}\) = 0.56 & a = 0.93, b = 0.44, R\({}^{2}\) = 0.40 \\ \([0,0.5)\) & a = 1.21, b = 0.73, R\({}^{2}\) = 0.58 & a = 1.37, b = 0.94, R\({}^{2}\) = 0.48 \\ \([0.5,1.0)\) & a = 1.36, b = 0.80, R\({}^{2}\) = 0.63 & a = 1.45, b = 0.92, R\({}^{2}\) = 0.50 \\ \([1.0,1.5)\) & a = 1.55, b = 0.98, R\({}^{2}\) = 0.76 & a = 1.45, b = 0.97, R\({}^{2}\) = 0.47 \\  & & a = 1.49, b = 1.15, R\({}^{2}\) = 0.44 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The fitted equations and parameters for Figure 12.  
Figure 12: The half-hourly GPP–SIF relationship at DM site in different EF (**a**) and Ta (**b**) bins. above results highlighted the fact that, although light energy is the common driver for both GPP and SIF, the sensitivities of GPP and SIF to radiation were different and relied on the environmental conditions related to temperature and water availability. [PERSON] et al. [48] found that the regulation mechanisms of light and dark reactions were different. Light reactions are more sensitive to radiation but relatively less sensitive to temperature and water availability. In contrast, dark reactions are totally determined by plenty of biochemical reactions, which are more sensitive to the parameters that influence the enzyme activity with related to temperature. In addition, carbon fixation is also influenced by intercellular CO2 concentration, which is affected by stomatal conductance related to water availability. GPP is the result of light and dark reactions, while chlorophyll fluorescence is more involved in light reactions. Therefore, GPP light response curves were more influenced by temperature and EF than SIF light response curves, as our results show in Section 3.1, indicating that SIF was more sensitive to radiation and less sensitive to EF and Ta than GPP. Additionally, the lower sensitivity of SIF to Ta and EF may result in the decoupling of GPP and SIF, especially in the lower EF and Ta, which provides important evidence that SIF is no longer a strong proxy of GPP when the temperature and water status were not suitable for the vegetation growth. In the aspect of light energy utilization, SIF is one of the energy excitation pathways used to balance the absorbed and utilized energy in photosynthesis and to avoid light damage in the photosystem (e.g., high light intensity but in cold or dry environments) [49; 50; 51; 4, 20].