## 4 Discussion
### Uncertainties of the GPP Estimation Based on PRI and SIF
[PERSON] et al. [47] found that the stomata responded sensitively to the water deficit, as demonstrated by the stomatal conductance and the net photosynthetic carbon assimilation that decreased, whereas the SIF responded to water stress in eastern cottonwood less strongly. [PERSON] et al. [46] reported that both fluorescence quantum efficiency and stomatal conductance declined in response to water stress and NPQ exhibited an upregulated trend [15]. [PERSON] et al. [19] found that the SIF yield showed a substantial reduction in the winter due to sustained NPQ and deactivation of the photosystems. Under stress, chlorophyll molecules typically are unable to fully utilize the solar energy they have taken in for CO\({}_{2}\) fixation. The photochemical activities will be damaged if the additional absorbed energy cannot be efficiently dissipated [31]. NPQ generally upregulated in response to environmental stress, while Gs showed a down-regulated trend under stress conditions [35]. The rate at which the products of electron transport--ATP and NADPH--are consumed in the dark processes is influenced by the CO\({}_{2}\) partial pressure in the stroma, which is determined by CO\({}_{2}\) diffusional resistances in the stomata and mesophylls. Feedback from the dark to light processes may therefore have an impact on how much energy is distributed across PQ, SIF, and NPQ [6]. It indicates that NPQ not only regulates the quantum yield of photochemistry and fluorescence quantum efficiency occurring in light reactions, but also can reflect some information of dark reaction in response to stress conditions. In this study, the ratio of \(GPP_{meas}\) to \(GPP_{meas}\) was related to Gs for corn (r = 0.25, \(p<0.01\)) and soybean (r = 0.13, \(p<0.05\)) at half-hourly timescales (Figure 10). Therefore, the relationships of GPP to SIF and PRI are affected by the plant physiological status in response to stress conditions, especially the incongruity of dark reactions and light reactions. The regulated NPQ was affected by the xanthophyll cycle, which can be detected by canopy reflectance of 531 nm at short-time timescales [22]. However, the seasonal variations in PRI were largely influenced by changes in pigment pools (e.g., chlorophyll content) and the reliability of using PRI to indicate NPQ needs to be further investigated. [PERSON] et al. [19] reported that the plant adaptation to winter not only corresponds to the changes in the xanthophyll cycle, but also variation of a variety of carotenoids (e.g., lutein and beta-carotene), which may play a photoprotective role for plants in response to low temperature. The intricate interplay of environmental factors (e.g., air temperature, light intensity, and water availability) have an impact on how SIF and PRI are linked to GPP [33; 47]. Moreover, the canopy reflectance was influenced by the solar viewing geometry, canopy structure and soil background, which further affects the calculation of the PRI and the linkage of PRI to NPQ [49]. In this study, we found that the slopes of the \(SIF_{t}\)-GPP relationship change with PRI using tower-based measurements (Figure 4). PRI can be used to improve the \(SIF_{t}\)-based GPP estimation (Figure 9). Nevertheless, the potential underlying mechanisms for linking PRI to NPQ at canopy scale need to be explored further.