Application of Simultaneous Active and Passive Fluorescence Observations: Extending a Fluorescence-Based qL Estimation Model.

The fraction of open Photosystem II (PSII) reaction centers (qL) is critical for connecting broadband PSII fluorescence (ChlFPSII) with the actual electron transport from PSII to Photosystem I. Accurately estimating qL is fundamental for determining ChlFPSII, which, in turn, is vital for mechanistically estimating the actual electron transport rate and photosynthetic CO2 assimilation. Chlorophyll fluorescence provides direct physiological insights, offering a robust foundation for qL estimation. However, uncertainties in the ChlFPSII-qL relationship across different plant functional types (PFTs) limit its broader application at large spatial scales. To address this issue, we developed a leaf-level instrument capable of simultaneously measuring actively and passively induced chlorophyll fluorescence. Using this system, we measured light response, CO2 response, and temperature response curves across 52 species representing seven PFTs. Our findings reveal the following: (1) a strong linear correlation between ChlFPSII derived from passively induced fluorescence and that from actively induced fluorescence (R2 = 0.85), and (2) while the parameters of the ChlFPSII-qL relationship varied among PFTs, ChlFPSII reliably modeled qL within each PFT, with the R2 ranging from 0.85 to 0.96. This study establishes quantitative ChlFPSII-qL relationships for various PFTs by utilizing passively induced fluorescence to calculate ChlFPSII. The results demonstrate the potential for remotely sensed chlorophyll fluorescence data to estimate qL and strengthen the use of fluorescence-based approaches for mechanistic GPP estimation at large spatial scales.