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 O 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 O evolution was measured. Quantum yield of O evolution during the MTP transients was calculated and related to the number of open PSII centers, found from the STF O 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 of all the open centers and F of all the closed centers. The fluorescence yield of a closed center was 0.55 of the final F while less than a half of the centers were closed, but later increased, approaching F 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.