Influence of temperature on photosystem II electron transfer reactions.

The influence of temperature on the rate of reduction of P-680+, the primary donor of Photosystem II, has been studied in the range 5-294 K, in chloroplasts and subchloroplasts particles. P-680 was oxidized by a short laser flash. Its oxidation state was followed by the absorption level at 820 nm, and its reduction attributed to two mechanisms: electron donation from electron donor D1 and electron return from the primary plastoquinone (back-reaction). Between 294 and approx. 200 K, the rate of the back-reaction, on a logarithmic scale, is a linear function of the reciprocal of the absolute temperature, corresponding to an activation energy between 3.3 and 3.7 kcal . mol-1, in all of the materials examined (chloroplasts treated at low pH or with Tris; particles prepared with digitonin). Between approx. 200 K and 5 K the rate of the back-reaction is temperature independent, with t 1/2 = 1.6 ms. In untreated chloroplasts we measured a t 1/2 of 1.7 ms for back-reaction at 77 and 5 K. The rate of electron donation from the donor D1 has been measured in dark-adapted Tris-treated chloroplasts, in the range 294-260 K. This rate is strongly affected by temperature. An activation energy of 11 kcal . mol-1 was determined for this reaction. In subchloroplast particles prepared with Triton X-100 the signals due to P-680+ were contaminated by absorption changes due to the triplet state of chlorophyll a. This triplet state has been examined with pure chlorophyll a in Triton X-100. An Arrhenius plot of its rate of decay shows a temperature-dependent region (292-220 K) with an activation energy of 9 kcal . mol-1, and a temperature-independent region (below 200 K) with t 1/2 = 1.1 ms.