The slow rate of proton consumption at the reducing side of Photosystem I is limited by the rate of redox reactions of extrinsic electron acceptors, but not by a diffusion barrier for protons

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Absorption changes of cresol red at 575 nm which indicate proton consumption in the external bulk phase were measured under repetitive flash excitation of pea thylakoids. In the presence of methyl viologen as terminal electron acceptor, proton consumption due to the activity of both photosystems was slow with a half-rise time of 130 ms. If either superoxide dismutase was added or the natural acceptor system was complemented by addition of ferredoxin and NADP +, the rise of the alkalization was accelerated and two phases became distinguishable: a slow phase which was due to Photosystem II activity ( t 1 2 ≈ 100 ms ) and a fast phase which was due to Photosystem I activity. In the presence of the artificial electron acceptor methyl viologen, the rise of the fast phase was accelerated in proportion to the amount of added superoxide dismutase to a limit of t 1 2 = 10 ms . From these results we concluded that the slow rate of proton consumption at the reducing side of Photosystem I was limited by the nature of the supplied acceptor system, but not predominantly by a diffusion barrier for protons.