Drought stress strengthens the link between chlorophyll fluorescence parameters and photosynthetic traits

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 CO 2 assimilation rate (A n ), the maximum carboxylation rate of Rubisco (V cmax ), the maximum rate of electron transport (J max ), stomatal conductance (g s ) and total chlorophyll content (Chl t ). The ChlF ratio of the leaf level such as maximum quantum efficiency of PSII (F v /F m ) is based on fluorescence kinetics. ChlF intensity ratio (LD 685 /LD 740 ) based on spectrum analysis was obtained. We found that a combination of the stomatal limitation, non-stomatal limitation, and Chl t regulated leaf photosynthesis under drought stress, while J max and Chl t governed the ChlF ratio. A significant link between the ChlF ratio and A n 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.