visible in Fig. 1a (full line) and belongs to the intermediate D , i.e. to the tyrosine radical in the 161th position of D 2 protein on the donor side of PS 2 (Debus et al., 1988a) and (ii) EPR signal II very fast (g = 2.0046, B pp = 1.9 mT) which is observable as an increase of signal II in light (Fig. 1a, difference between the dashed and full lines). This signal belongs to the intermediate Z , i.e. to the tyrosine radical in the 161th position of D 1 protein (Debus et al., 1988b) which is also situated on the donor side of PS 2. Further EPR signal is associated with cation-radical of chlorophyll (Chl) a dimer situated in the core of PS 1 (Hoff, 1979).