The stability of EPR Signal IIs in oxygen-evolving PS II particles in the different S-states has been studied as a function of storage time at 77 K. Signal IIs in dark-adapted samples retains the same intensity after 1 month storage at 77 K, while that in the S2 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 IIs, once decayed in the S2 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 S2 state involving Signal IIs is discussed.
