## 1 Introduction  
Nearly all lives on Earth ultimately depend on energy from the sun. Photosynthesis is the only process that can convert light energy to chemical energy [1]. Photosynthesis consists of two sets of reactions: light reactions and dark reactions (the carbon reactions). In the light reactions, the photon energy absorbed by photosystem II (PSII) is consumed in three different pathways: photochemistry, fluorescence, and heat loss (nonphotochemical quenching, NPQ) [2, 3]. In the photochemistry pathway, the absorbed light energy is used to remove an electron from H\({}_{2}\)O, generating linear electron transport from PSII to photosystem I (PSI) [4]. During this electron transport process, absorbed light energy is converted into chemical energy, producing NADPH (the reduced form of nicotine adenine dinucleotide phosphoric acid) and ATP (adenosine triphosphate) [5]. Meanwhile, about 1-2% of absorbed light energy is re-emitted as chlorophyll \(a\) fluorescence (ChIF) [6, 7]. The probability of partitioning an absorbed photon in a given pathway is dependent on the amount of absorbed light and the efficiency of plant photochemical processes (e.g., the photochemical quantum yield of PSII (\(\Phi_{\text{PSII}}\)) and the fluorescence quantum yield (\(\Phi_{\text{ChIF}}\))) [7]. In the dark reactions, the energy stored in ATP and NADPH is then consumed to assimilate CO\({}_{2}\) and produce carbohydrates, primarily glucose [8]. The processes in the dark reactions do not directly require light but are strongly regulated by temperature, water, nutrients, and CO\({}_{2}\) from the environment [8]. Advances in observing solar-induced chlorophyll fluorescence (SIF) from space [9; 10] have documented promise for improved mapping of global terrestrial gross primary productivity (GPP) [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Numerous studies have already reported that SIF and GPP are well correlated for a variety of terrestrial ecosystems. For example, [PERSON] et al. [22] showed that space-borne SIF observations exhibited good performance in estimating crop photosynthesis across the US Corn Belt region. [PERSON] et al. [23] showed that ground-based SIF measurements exhibit a strong linear relationship with GPP at a deciduous forest site. It should be noted that fluorescence emanates from the light reactions, whereas the assimilation of CO\({}_{2}\) is accomplished in the dark reactions of photosynthesis [7]. The difference in light and dark reactions naturally raises an important question: does ChIP respond to the efficiency of the dark reactions in photosynthesis? If the answer is no, it might suggest that the strong SIF-GPP correlations found in many previous studies are only because they have a shared driver, namely, absorbed photosynthetically active radiation (APAR). If the answer is yes, it indicates that SIF does provide direct insight into the processes occurring in the dark reactions, including gas exchange and carbon assimilation. This has important implications for better understanding the SIF-GPP relationship when APAR is not a major limiting factor. For instance, it can inform our interpretation of satellite SIF observations commonly acquired around noontime (high light) when the dark reactions of photosynthesis on top of the canopy tend to become more prominent. This is also particularly relevant in the context of rising temperatures and atmospheric CO\({}_{2}\), as more climate extremes are likely to occur, but the variability of incident PAR will be relatively small [24]. Previous studies on this question have provided inconsistent results. [PERSON] et al. [25] found a significant decrease in photosynthetic CO\({}_{2}\) assimilation as stomata closed, but no clear trend in fluorescence measurements, suggesting that SIF did not directly track carbon assimilation. [PERSON] et al. [26] also reported that water stress caused a clear and fast decline in both net photosynthesis and stomatal conductance but had a smaller effect on SIF emission.