Quantifying PSII Open Centers and Multiscale Photosynthetic Electron Transport With Solar-Induced Chlorophyll...

Quantifying photosystem II (PSII) open reaction centers (q_L) and their relationship with electron transport rate (ETR) is crucial for understanding photosynthetic dynamics across spatial scales. However, accurate estimation of these photosynthetic variables remains challenging due to observational constraints and complex environment-ecosystem feedbacks under dynamic conditions. A novel approach to mechanically quantify the q_L and linear electron transport rates (J_PSII) from PSII to photosystem I (PSI) using solar-induced chlorophyll fluorescence (SIF) at leaf and canopy scales was established, termed the SIF-qL redox model. The simulation was validated at two evergreen forest sites (ZGT and DEJU) using continuous pulse-amplitude modulated (PAM) chlorophyll fluorescence and flux measurements. At the leaf-scale, the model demonstrated high accuracy in predicting q_L for needleleaf vegetation in ZGT (R² = 0.82, RMSE = 0.09), but performed poorly in DEJU (R² = 0.45). The model accurately simulated J_PSII dynamics at both sites at the leaf level (R² = 0.92-0.97). When scaled to canopy level, the model maintained reliable predictive capability for J_PSII (R² = 0.62-0.71). The simulations captured distinct q_L dynamics: at ZGT, q_L declined with increasing PAR, particularly at lower temperatures, while DEJU showed minimal temperature-dependent variation. The observed J_PSII- q_L relationships revealed that Ribulose-1,5-bisphosphate (RUBP) generation limitations on J_PSII were dominant at both sites. This study developed a SIF-qL redox model to mechanistically connect chlorophyll fluorescence with photosynthetic dynamics, while validating the cross-scale applicability of leaf-level parameters in canopy-scale simulations. Future refinements should address species- and environment-specific adaptations to enhance the universal applicability of the ecosystem photosynthesis process.