Period four oscillations in chlorophyll a fluorescence.

The discovery of period four oscillations of the fluorescence yield under flashing light demonstrated that not only the redox state of the Photosystem II (PS II) electron acceptor Q(A), but also the oxygen evolving cycle (described by the S states) modulates the fluorescence yield of chlorophyll (Chl). The positive charges accumulated on the donor side of PS II act on the fluorescence yield (measured in the Q(A) (-) state during a strong flash) through the concentration of the quencher P(680) (+), the oxidized form of PS II reaction center Chl a. However, the period four oscillations of the fluorescence yield detected 1 s after a strong flash (in the P(680)Q(A) state) have not yet been fully explained.