Variable fluorescence of closed photochemical reaction centers.

Abstract

Chlorophyll fluorescence induction during 0.4 to 200ms 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 Fo of all the open centers and Fc of all the closed centers. The fluorescence yield of a closed center was 0.55 of the final Fm while less than a half of the centers were closed, but later increased, approaching Fm 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.