EFFECTS OF DARK-INDUCED AND LIGHT-INDUCED PROTON GRADIENTS IN THYLAKOIDS ON THE Q-THERMOLUMINESCENCE AND B-THERMOLUMINESCENCE BANDS

Thermoluminescence experiments have been carried out to study the effe ct of a transmembrane proton gradient on the recombination properties of the S-2 and S-3 states of the oxygen evolving complex with Q(A)(-) and Q(B)(-), the reduced electron accepters of Photosystem II. We firs t determined the properties of the S(2)Q(A)(-) (Q band), S(2)Q(B)(-) a nd S(3)Q(B)(-) (B bands) recombinations in the pH range 5.5 to 9.0, us ing uncoupled thylakoids. Then, a proton gradient was created in the d ark, using the ATP-hydrolase function of ATPases, in coupled unfrozen thylakoids. A shift towards low temperature of both Q and B bands was observed to increase with the magnitude of the proton gradient measure d by the fluorescence quenching of 9-aminoacridine. This downshift was larger for S(3)Q(B)(-) than for S(2)Q(B)(-) and it was suppressed by nigericin, but not by valinomycin. Similar results were obtained when a proton gradient was formed by photosystem I photochemistry. When Pho tosystem II electron transfer was induced by a flash sequence, the red uction of the plastoquinone pool also contributed to the downshift in the absence of an electron acceptor. In leaves submitted to a flash se quence above 0 degrees C, a downshift was also observed, which was sup pressed by nigericin infiltration. Thus, thermoluminescence provides d irect evidence on the enhancing effect of lumen acidification on the S -3 --> S-2 and S-2 --> S-1 reverse-transitions. Both reduction of the plastoquinone pool and lumen acidification induce a shift of the Q and B bands to lower temperature, with a predominance of lumen acidificat ion in non-freezing, moderate light conditions.