###### Abstract
However, the linkage between SIF and GPP is not set in stone, and it can be influenced by a complex canopy structure, growth stages, and environmental conditions. With a complex canopy structure, the emitted SIF is scattered and reabsorbed by leaves until it is intercepted by sensors [15]. Because of the limited field of view (FOV) of sensors, hemispherical SIF emission can only be captured in the observation direction [15]. The observed SIF is only a small part of the total SIF and contains information from both photosystem I (PSI) and photosystem II (PSII) [16; 17; 18; 19; 20]. To obtain more findings about the physiological linkage between SIF and GPP, the impacts of the observed angle, the canopy structure, and the contribution of PSI SIF should be eliminated from the observed canopy SIF to obtain the total SIF at the full band (Figure 1) [20; 21]. Although the total SIF is directly linked to photosynthesis, the relationship between SIF and GPP remains uncertain. The SIF-GPP relationship might be decoupled under mild stress conditions [22], and SIF shows less sensitivity in response to stress than GPP does. Environmental variables, such as air temperature, soil moisture, and light intensity, have an  
Figure 1: Photosynthesis process and PSI and PSII chlorophyll fluorescence emission mechanisms. influence on the SIF-GPP relationship. Despite the fact that APAR drives both SIF and GPP, some studies found that there is a stronger relationship between the APAR and SIF than between the APAR and GPP and that the ratio of GPP to SIF decreases with light intensity. These findings may be related to the differences between light and dark reactions or the irregularly varied proportion of energy allocated to SIF and photosynthesis [23; 24]. Despite the fact that SIF can monitor the downregulation of GPP in response to drought conditions at both the leaf and canopy levels, a water deficit also affects the link between SIF and GPP. Likewise, the SIF-GPP relationship is also somewhat impacted by photochemical activities and the kinetics of enzymes involved in dark processes, which are both affected by temperature [25]. Additionally, the SIF-GPP relationship can be altered by the dynamic energy distribution among three energy dissipation pathways in response to various temperature conditions. Therefore, SIF, a single energy dissipation pathway, is insufficient to effectively account for the dynamic fluctuations in GPP under stress. As chlorophyll molecules become excited as a result of APAR, three processes--photochemical quenching (PQ), non-radiative decay or non-photochemical quenching (NPQ), and fluorescence--can be used to dissipate the excitation energy [26]. The three energy dissipation pathways compete with one another and display various patterns in response to the constantly changing external environment. As one of three pathways for consuming the solar energy absorbed by leaves, NPQ may distribute the energy between fluorescence and PQ [27]. The capacity of vegetation to digest APAR via photochemistry is often reduced by biotic circumstances, but NPQ normally occurs, which could potentially change the link between SIF and GPP [28]. However, it is currently unknown how SIF and GPP correlate with one another under various environmental circumstances and how NPQ mediates this relationship. As another energy dissipation pathway, NPQ is important for SIF-based GPP estimation models. It is critical to quantify the contributions of NPQ to absorbed energy under stressful conditions as the response of SIF to stress is not unique, and NPQ co-occurs during fluorescence processes [11]. The downregulation of photosynthetic efficiency at diurnal timescales is often correlated with the degree of epoxidation in the xanthophyll pigments, which increases under stress in leaves [13]. Under stress, xanthophyll de-epoxidation typically causes a rise in NPQ activity. However, NPQ is generally difficult to obtain at the canopy scale, which limits its application in GPP estimation based on SIF.