## 1 Introduction  
Large uncertainties occur in the estimation of global terrestrial photosynthesis flux, which is the largest component of the global carbon cycle [1; 2]. Solar-induced chlorophyll fluorescence (SIF) is directly related to the light reaction processes of photosynthesis [3], and provides a new way to more accurately map gross primary production (GPP). A large number of studies have found that SIF tracks GPP well at different temporal and spatial scales [4; 5; 6; 7]; however, the SIF-GPP relationship is largely dependent upon the biome and climate [8; 9]. How the relationships between SIF and GPP with respect to different plant functional types (PFTs) are influenced by various environmental conditions remains under-explored. As both SIF and GPP are driven by absorbed photosynthetically active radiation (APAR), the light use efficiency (LUE) model has been successfully used to model the relationship between SIF and GPP [10; 11]. Indeed, a large number of studies have found that the SIF presents a strong linear relationship with GPP at large temporal and spatial scales [12; 13; 14]. However, it has been shown that SIF is more strongly related to APAR than to photosynthesis [15; 16], and the relationship between SIF and GPP is modulated by other processes [17; 18; 19]. The energy absorbed by chlorophyll molecules can be dissipated through photochemical quenching (PQ), non-photochemical quenching (NPQ), and SIF at the leaf level [20]. SIF is directly related to the light reaction, whereas the potential mechanistic link between SIF and the linear electron transfer rate needed for carboxylation processes is the key point linking SIF to GPP [3; 21]. The differences in carbon reaction between C3 and C4 species may result in the different slopes of SIF and GPP [7; 22]. At the canopy level, the canopy-observed SIF is only part of the total SIF emitted from the chlorophyll molecules, due to scattering and reabsorption effects [23; 24; 18; 25]. The canopy structure and chlorophyll content also affect the SIF-GPP relationship in some aspects [26; 27; 28; 29; 30]. Therefore, these interactions make the SIF-GPP relationship complex [3; 21]. The responses of SIF and GPP to varying environmental conditions across ecosystems remain unknown, resulting in uncertainties in the estimation of global GPP based on SIF. The effects of environmental conditions on the relationship between SIF and GPP have become a state-of-the-art topic. As SIF emitted from photosystem II (PS II) is directly linked to the photosynthetic activity, it is strongly affected by environmental conditions [7; 32]. The proportion of energy allocated to each energy-dissipated pathway will vary under different environmental conditions [33; 34]. In addition, the differences in light and carbon reactions in response to changing environmental conditions will also affect the SIF-GPP relationship [35; 36]. A large number of studies have reported that SIF and GPP respond differently to light intensity [21; 37; 38] at both leaf and canopy levels, where the ratio of GPP to SIF decreased with increasing light intensity. Although SIF can partly track the down-regulation of GPP under drought conditions [39; 40], drought stress also affects the SIF-GPP relationship [41; 42]. The incongruity of light and carbon reactions and the complex energy distribution under stress conditions may contribute to the different sensitivities of SIF and GPP in response to drought stress [3; 21]. Recent studies have begun to explore the responses of SIF and GPP to temperature [40; 43]. [PERSON] et al. [44] found that satellite SIF allowed for early detection of the response of winter wheat to heat stress, and could provide a larger physiology-related stress response than APAR and traditional vegetation indices (e.g., the enhanced vegetation index; EVI) during heatwave events. [PERSON] et al.