Two mechanisms of photoenergy regulation revealed by kinetic behaviors of chlorophyll fluorescence during light adaptation

Photochemical reactions were analyzed and modeled to observe photoenergy regulation mechanisms in PSII from measured chlorophyll fluorescence (ChlF) kinetics. Two pH-driven mechanisms were revealed: One is the PsbS-mediated non-photochemical quenching (NPQ) of activated antennae (A*), as commonly understood, and the other is a disruption of energy transfer from A* to P680. Representing the latter with a conformational change from complex formation of zeaxanthin and lutein with antennae closely described measured ChlF from initially dark-adapted state to light-adapted state and measured NPQ variations. Analysis based on the model indicates that zeaxanthin and lutein lead to slow protracted reductions in P680* and ChlF via the second mechanism without strong influence by pH or PsbS. Protonation of PsbS plays a major role via the first mechanism in responding to fast changes in illumination. The research provides insights into the mechanisms for photoenergy regulation in PSII and a kinetic model with broadened applicability.