Change of proton motive force across thylakoid membrane in soybean leaf during state transitions

Change of proton gradient across thylakoid membrane in soybean leaves was studied with millisecond delayed light emission (ms-DLE) during the course of state transitions which were indicated by the chlorophyll fluorescence at room temperature and 77 K. When dark-adapted leaves were induced to state I with far-red light, F-m/F-o, F-685/F-735 and the intensity of fast phase of ms-DLE were affected slightly. However, during the induction to state II with red light, both F-m/F-o and F-685/F-735 decreased immediately and the former were quicker than the latter. In this interval, the intensity of fast phase of ms-DLE increased to a maximum and then decreased to a lower value during the transition to state II. Nigericin, an uncoupler which eliminates the proton gradient across thylakoid membrane, inhibited the increase in the intensity of fast phase of ms-DLE during the transition to state II. Another uncoupler, valinomycin, which eliminates the membrane potential, did not affect the changes of the intensity of fast phase. These results suggest that the prompt increase in the intensity of fast phase of ms-DLE at the beginning of transitions to state II is correlated mainly with the proton gradient released from water oxidation in photosystem II.