RAPID REDUCTION OF P-700 PHOTOOXIDIZED BY A FLASH AT LOW TEMPERATURE IN SPINACH CHLOROPLASTS

Abstract—Excitation of chloroplasts at low temperature (down to 4.2 K) by short laser flashes causes largely reversible absorption changes which are attributed to the photooxidation of P-700, the primary electron donor of Photosystem I. At temperatures below 100 K the dark re-reduction of P-700⁺ is biphasic. with half-times of about 122μs and 1.7 ms. The relative contribution of the two phases varies with temperature with the fast phase becoming dominant at the lowest temperatures (˜90% at 5 K). The results are interpreted in terms of tunnelling of the electron from one or two primary accepting sites back to P-700, a process which is largely dominant over the process of charge stabilisation.