## 4 Discussion
### Different Sensitivities of SIF and GPP to Air Temperature  
In our study, we explored the changes in SIF and GPP in response to low temperatures in winter wheat. Based on the measurements in the whole growth period (Figure 4), we found that both \(cSIF_{688}\) and \(cSIF_{760}\) could track GPP well, and they recorded the reduction in GPP in the overwintering period (Figure 4a,d,g). The results show that low temperature simultaneously inhibited both photosynthesis and fluorescence, which was in accordance with earlier research [8; 15]. However, SIF and GPP responded differently to air temperature in this study. From Figure 8, we can see that both \(\frac{LUE_{p}}{cSIF_{yield}}\) and \(\frac{LUE_{p}}{\Phi_{F}}\) increased with \(T_{a}\), which indicates that the temperature sensitivity of fluorescence and photosynthesis was different. For red band SIF, the increases in \(\frac{LUE_{p}}{cSIF_{yield,68}}\) and \(\frac{LUE_{p}}{\Phi_{F,688}}\) with \(T_{a}\) can be attributed to \(LUE_{p}\) being more sensitive to temperature than \(cSIF_{yield\_688}\) and \(\Phi_{F,688}\), although they also responded to air temperatures (Figure 8a,c). For far-red band SIF, the increases in \(\frac{LUE_{p}}{cSIF_{yield,700}}\) and \(\frac{LUE_{p}}{\Phi_{F,700}}\) with \(T_{a}\) can largely be interpreted by the response of \(LUE_{p}\) to temperature (Figures 4c,e and 7b,d). This study only used a linear model to predict changeable trends of variables and to study the relationships between SIF and GPP and the different responses to temperature (Figures 7 and 8). In the future, a large number of leaf level measurements collaborated with the canopy-level observations should be conducted to better investigate the effects of temperature on the SIF-GPP relationship. Assuming that temperature responses of canopy-level and photosystem-level SIF represented the true physical processes, the potential factors affecting the different sensitivities of \(LUE_{p}\) and \(cSIF_{yield}\) or \(\Phi_{F}\) in response to air temperature could be due to their different physiological mechanisms [5; 6]. Under environmental stress conditions, light reactions might operate on distinct time scales from those that carbon reactions operate on photosynthesis [4; 72]. The low temperature in winter generally slows down the activities of the carboxylation process. Thus, \(LUE_{p}\), which is related to carbon reactions, would rapidly respond to air temperature. If low-temperature stress persists, plants will further decompose photosynthetic proteins (e.g., LHC). SIF is directly related to light reactions. Thus, the different physiological changes between the light reaction and carbon reaction might be attributed to the different sensitivities of \(LUE_{p}\) and \(cSIF_{yield}\) or \(\Phi_{F}\) in response to temperature. The absorbed light cannot be completely utilized for CO\({}_{2}\) fixation under low temperatures. If the extra absorbed light cannot be released effectively, it will damage the photochemical tissue. Therefore, NPQ is a crucial photoproduction mechanism to protect PSII from damage caused by cold temperatures and high light [11]. [PERSON] et al. [8] found that SIF tracked GPP well over the course of the overwintering period for evergreen needle forests (ENF) and pointed out that the extra absorbed light can be released by NPQ under chilling. According to [PERSON] et al. [6], seasonal fluctuations in leaf-level fluorescence quantum efficiency were found to be in agreement with the quantum yield of photochemistry for ENF over winter [7; 8]. In ENF during the fall transition, [PERSON] et al. [15] found that the daily mean \(LUE_{p}\) showed saturated phenomena in high air temperatures, but \(\Phi_{F}\) increased gradually with air temperature. NPQ is an important regulated mechanism in the energy distribution, and photochemistry and fluorescence might decrease simultaneously when NPQ increases [6; 8].