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

The fraction of open Photosystem II (PSII) reaction centers () is critical for connecting broadband PSII fluorescence (ChlF) with the actual electron transport from PSII to Photosystem I. Accurately estimating is fundamental for determining ChlF, which, in turn, is vital for mechanistically estimating the actual electron transport rate and photosynthetic CO assimilation. Chlorophyll fluorescence provides direct physiological insights, offering a robust foundation for estimation. However, uncertainties in the ChlF- 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, CO response, and temperature response curves across 52 species representing seven PFTs. Our findings reveal the following: (1) a strong linear correlation between ChlF derived from passively induced fluorescence and that from actively induced fluorescence ( = 0.85), and (2) while the parameters of the ChlF- relationship varied among PFTs, ChlF reliably modeled within each PFT, with the ranging from 0.85 to 0.96. This study establishes quantitative ChlF- relationships for various PFTs by utilizing passively induced fluorescence to calculate ChlF. The results demonstrate the potential for remotely sensed chlorophyll fluorescence data to estimate and strengthen the use of fluorescence-based approaches for mechanistic GPP estimation at large spatial scales.