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

The fraction of open Photosystem II (PSII) reaction centers (<i>q</i>_L) is critical for connecting broadband PSII fluorescence (ChlF_PSII) with the actual electron transport from PSII to Photosystem I. Accurately estimating <i>q</i>_L is fundamental for determining ChlF_PSII, 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 <i>q</i>_L estimation. However, uncertainties in the ChlF_PSII-<i>q</i>_L 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 ChlF_PSII derived from passively induced fluorescence and that from actively induced fluorescence (<i>R</i>² = 0.85), and (2) while the parameters of the ChlF_PSII-<i>q</i>_L relationship varied among PFTs, ChlF_PSII reliably modeled <i>q</i>_L within each PFT, with the <i>R</i>² ranging from 0.85 to 0.96. This study establishes quantitative ChlF_PSII-<i>q</i>_L relationships for various PFTs by utilizing passively induced fluorescence to calculate ChlF_PSII. The results demonstrate the potential for remotely sensed chlorophyll fluorescence data to estimate <i>q</i>_L and strengthen the use of fluorescence-based approaches for mechanistic GPP estimation at large spatial scales.