## 1 Introduction  
Solar-induced chlorophyll fluorescence (SIF) is an optical signal that is emitted by chlorophyll a molecules when absorbed solar energy exceeds the requirements of the photochemical reaction [1]. It has attracted the attention of botanists, ecologists, and agricultural scientists since SIF gives an insight into the photosynthetic machinery of terrestrial vegetation [2]. However, it is difficult to decouple SIF from reflected solar flux, which is mainly due to the much lower contribution of SIF emission to the reflected signal (1-5% in the near-infrared) [3]. In order to overcome this problem, the in-filling method using two dark bands in telluric oxygen (O\({}_{2}\)) absorption lines (O\({}_{2}\)-A, 760 nm and O\({}_{2}\)-B, 687 nm) has commonly been used for SIF retrieval because of the more comparable signal of SIF relative to reflected flux in these two O\({}_{2}\) bands [3; 4]. These two O\({}_{2}\) bands are also close to the terrestrial SIF spectrum peaks centered at \(\approx\)685 and \(\approx\)740 nm, known as red and far-red fluorescence [5]. While the O\({}_{2}\)-B band coincides almost entirely with the red fluorescence, its depth is much lower and width is much smaller than that of the O\({}_{2}\)-A band [6]. Therefore, the O\({}_{2}\)-A band has largely been used to estimate SIF from tower [7; 8; 9], aircraft [10; 11], and satellite platforms [12; 13; 14; 15]. It has been demonstrated that SIF can be considered as a potential indicator of photosynthesis at different scales. It was reported that, at the leaf and canopy level of crops, relative SIF (the ratio of SIF and radiance at O\({}_{2}\) band) is inversely related to photosynthetic light-use efficiency (LUE), and the correlation of SIF with LUE is better at 760 nm than that at 687 nm [16]. At regional scales, the apparent SIF yield (SIF is divided by incident photosynthetically active radiation) also correlates well with the maximum carboxylation rate (Vcmax), a key parameter of leaf biological traits [17]. These results provide us with further understanding about global photosynthesis, which drives the global carbon cycle and fixes carbon dioxide (CO\({}_{2}\)) as organic compounds [18]. This carbon flux at the ecosystem level is known as gross primary production (GPP) [19], which plays an important role in the global food security and carbon budgets [20]. In recent studies, a strong linear relationship was found between SIF and GPP in specific ecosystem types [21; 22; 23], suggesting that SIF could serve as a good proxy for GPP estimation. Moreover, early information of stress-induced changes in photosynthesis could also be detected by the measurements of SIF on the ground [7; 10] or from space [12; 14; 15]. Therefore, it might be concluded that SIF could provide accurate and more direct information for functional status of vegetation, carbon assimilation estimate, and stress detection over large scales [24]. Although the feasibility of SIF in these above aspects has been widely discussed, it remains challenging to use SIF as a direct indicator of photosynthesis due to the fact that some growth and environmental parameters could also affect SIF emissions at different time scales. For example, solar radiation is a critical environmental parameter that drives the variation of SIF at seasonal and diurnal scales [9; 25]. Air temperature and vapor pressure deficit (VPD) also affect the seasonal patterns of SIF over different land covers [9; 12; 26] and exhibit different relationships with SIF under the influence of water stress [10]. SIF also shows a great sensitivity to growth parameters, such as chlorophyll content, leaf area index, and leaf inclination angles, because these parameters could affect the absorption of solar energy and ultimately result in the variation of LUE in the end [10; 27; 28]. Different plant types and phenological stages could also induce the variation of growth parameters and thus cause the difference of SIF emissions [27; 29; 30].