Variable fluorescence of closed photochemical reaction centers

Chlorophyll fluorescence induction during 0.4 to 200 ms multiple-turnover pulses (MTP) was measured in parallel with O₂ evolution induced by the MTP light. Additionally, a saturating single-turnover flash (STF) was applied at the end of each MTP and the total MTP +STF O₂ evolution was measured. Quantum yield of O₂ evolution during the MTP transients was calculated and related to the number of open PSII centers, found from the STF O₂ evolution. Proportionality between the number of open PSII and their running photochemical activity showed the quantum yield of open PSII remained constant independent of the closure of adjacent centers. During the induction, total fluorescence was partitioned between F_o of all the open centers and F_c of all the closed centers. The fluorescence yield of a closed center was 0.55 of the final F_m while less than a half of the centers were closed, but later increased, approaching F_m to the end of the induction. In the framework of the antenna/radical pair equilibrium model, the collective rise of the fluorescence of centers closed earlier during the induction is explained by an electric field, facilitating return of excitation energy from the Pheo^- P680⁺ radical pair to the antenna.