Photosynthetica X:X | DOI: 10.32615/ps.2026.008

Role of pheophytin in chlorophyll a fluorescence transients

G. GARAB
Department of Physics, Faculty of Science, University of Ostrava, Ostrava, Czech Republic
Institute of Plant Biology, HUN-REN Biological Research Centre, Szeged, Hungary

In this review, I revisit the groundbreaking discovery of pheophytin a (Pheo) in the primary charge separation of photosystem II (PSII) reaction centres (RCs) and draw attention to key observations that concern the origin of the variable chlorophyll a fluorescence (Fv) of PSII as well as the partitioning of the absorbed quanta between different de-excitation pathways. In particular, I recall data from the 1970s and 1980s, which have revealed photoreduction of Pheo in closed PSII (PSIIC) containing the primary quinone acceptor in reduced form, and in photoinhibited PSII with largely suppressed Fv. Taken together with recent experiments on the turnover of the P680+Pheo- radical pair and thoroughly documented large Fv values in PSIIC, these data invalidate the fundamental assumption of the so-called QA model of chlorophyll a fluorescence (ChlF): that PSIIC displays no photochemical activity, and thus the Fv/Fm ratio can be equated with the photochemical quantum efficiency of PSII (Fm, maximum fluorescence yield elicited by a saturating flash). Consequently, the co-dependent quantities - namely, the quantum efficiencies of the alternative pathways - cannot be derived from conventional ChlF measurements. It is proposed that monitoring the redox transients of Pheo and the P680⁺Pheo- radical pair may provide deeper insight into the photochemical activity and structural dynamics of PSII reaction centres, as well as the utilization or dissipation of absorbed excitation energy -thereby improving our understanding of the mechanisms underlying various biotic and abiotic stresses affecting PSII.

Additional key words: chlorophyll a fluorescence induction; nonphotochemical quenching; pheophytin; photochemical quantum efficiency; photoinhibition; QA model.

Received: January 13, 2026; Revised: April 3, 2026; Accepted: April 21, 2026; Prepublished online: May 14, 2026 

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