Abstract
Under steady-state photosynthetic conditions in low light, 830-nm absorption (A830) was very close to that in darkness, indicating that the primary donor P700 in the reaction center of photosystem I (PSI) was in non-oxidized (reduced) form. However, P700 was not fully oxidized by applying a strong white light pulse. This result indicates the presence of a population of PSI centers with reduced P700 that remains thermodynamically stable during the application of a pulse of saturating light. This population of reduced (and photochemically-) inactive P700 disappeared in intermediate light, but appeared in strong light. To investigate the reason for the accumulation of reduced-inactive P700 in low light, P700 oxidation by a pulse of saturation light was analyzed under various conditions. We concluded that the reduced-inactive P700 in low light was produced as a result of a limitation of electron flow at the acceptor side of PSI.
