The effect of temperature on the formation and decay of the multiline EPR signal species associated with photosynthetic oxygen evolution

We have investigated the effects of temperature on the formation and decay of the light-induced multiline EPR signal species associated with photosynthetic oxygen evolution (Dismukes, G.C. and Siderer, Y. (1980) FEBS Lett. 121, 78-80). (1) The decay rate following illumination is temperature dependent: at 295 K the half-time of decay is about 40 s, at 253 K the half-time is approx. 40 min. (2) A single intense flash of light becomes progressively less effective in generating the multiline signal below about 240 K. (3) Continuous illumination is capable of generating the signal down to almost 160 K. (4) Continuous illumination after a preilluminating flash generates less signal above 200 K than at lower temperatures. Our results support the conclusion of Dismukes and Siderer that the S₂ state gives rise to this multiline signal; we find that the S₁ state can be fully advanced to the S₂ state at temperatures as low as 160 K. The S₂ state is capable of further advancement at temperatures above about 210 K, but not below that temperature.