Effect of the transmembrane electric field on the photochemical and quenching properties of photosystem II in vivo.

The intermediate phase of fluorescence relaxation (lms-ls) (Joliot, P., Joliot, A., Bouges, B, and Barbieri, G. (1971) Photochem. Photobiol. 14, 287-305), following a single saturating flash, is shown to be controlled by a slow phase of the reoxidation of Q- by a secondary acceptor and, in vivo, by the transmembrane electric field. The kinetics of reoxidation of Q- are slowed by lowering the pH. This slowing effect is interpreted in terms of the reversible formation at low pH of QH which is not oxidizable by the secondary acceptor. The electric field transforms Photosystem II centers into a non-quenching photochemically inactive state that cannot be attributed to an accumulation of Q-. Centers are unequally sensitive to the field. A critical field strength can be defined for each center above which that center is blocked and below which the center is photochemically active. The transformation from the active to inactive state occurs over a narrow range of field strength. Sensitive centers are blocked by the field in less than 1 ms and become active again in less than 10 ms as the field strength falls. Two hypotheses are proposed for the mechanism of blockage of centers by the field: (1) a field induced conformational change in the centers, (2) the formation or suppression of a dipole critical to the function of a center. The activity of the ATP synthetase, determining the rate of relaxation of the field, was controlled by a light-dark treatment or by a chemical method using p-benzoquinone.