Chlorophyll fluorescence (ChlF) has been used to understand photosynthesis and its response to climate change, particularly with satellite-based data. However, it remains unclear how the ChlF ratio and photosynthesis are linked at the leaf level under drought stress. Here, we examined the link between ChlF ratio and photosynthesis at the leaf level by measuring photosynthetic traits, such as net CO2 assimilation rate (An), the maximum carboxylation rate of Rubisco (Vcmax), the maximum rate of electron transport (Jmax), stomatal conductance (gs) and total chlorophyll content (Chlt). The ChlF ratio of the leaf level such as maximum quantum efficiency of PSII (Fv/Fm) is based on fluorescence kinetics. ChlF intensity ratio (LD685/LD740) based on spectrum analysis was obtained. We found that a combination of the stomatal limitation, non-stomatal limitation, and Chlt regulated leaf photosynthesis under drought stress, while Jmax and Chlt governed the ChlF ratio. A significant link between the ChlF ratio and An was found under drought stress while no significant correlation in the control, which indicated that drought stress strengthens the link between the ChlF ratio and photosynthetic traits. These results suggest that the ChlF ratio can be a powerful tool to track photosynthetic traits of terrestrial ecosystems under drought stress.

Ecosystem stability is the ability of an ecosystem to maintain or restore its structure and function, representing the reliability of the earth to supply the normal function (De Keersmaecker, Lhermitte, Tits, et al., 2015;Pimm, 1984). Climate change and biodiversity loss had dramatically affected the terrestrial ecosystem (Geng et al., 2019). For example, global warming and extreme drought had been demonstrated to influence the primary productivity and species interactions which had a potential risk to ecosystem stability (Huang & Xia, 2019). Likewise, biodiversity loss might decrease ecosystem stability since abundant species composition could respond to climate change in various ways (Gross et al., 2014). Thus, the evaluation of terrestrial ecosystem response to disturbances and the quantification of the ecosystem stability is important to predict future ecosystem structure and functions.

Chlorophyll fluorescence (ChlF) is the key to studying the physiological mechanisms of plant photosynthesis, quantifying the spatiotemporal pattern of vegetation photosynthesis, and accurately understanding the productivity of terrestrial ecosystems under the background of climate change. However, few studies have been conducted on combined observations of actively and passively induced ChlF. Here, we compared the advantages and disadvantages of active and passive observations of ChlF and showed the instrument composition of the combined observations of actively and passively induced ChlF at leaf and canopy scales. The application prospects of joint observations of actively and passively induced ChlF focus on exploring energy distribution among photosynthesis, fluorescence and heat dissipation at the chloroplast-leaf-canopy scale, clarifying the mechanism underlying the relationship between ChlF and gross primary productivity, verifying satellite-based sun-induced chlorophyll fluorescence and interpreting the shape of the ChlF spectrum. Our work suggests that the combined observation of actively and passively induced ChlF is essential to reveal the mechanisms underlying the relationships between fluorescence and photosynthesis at various scales and to improve vegetation productivity models at the global scale.