EPR study of charge equilibrium at low temperatures in the S₂state of oxygen‐evolving photosystem II particles

The stability of EPR Signal II s in oxygen‐evolving PS II particles in the different S‐states has been studied as a function of storage time at 77 K. Signal II s in dark‐adapted samples retains the same intensity after 1 month storage at 77 K, while that in the S 2 state produced by illumination at 195 K decays with a half‐life time of about 5 days without decay in the manganese multiline intensity observed at 4.5 K. Signal II s , once decayed in the S 2 state, recovers its original intensity upon dark adaptation of the sample at 210 K accompanied by a decrease in the multiline intensity. A model for the charge recombination process for the S 2 state involving Signal II s is discussed.