Restoration by sllicotungstic acid of DCMU-inhibited photoreactions in spinach chloroplasts.

The restoration by silicotungstic acid of the reversible light-induced pH rise mediated by pyocyanine in EDTA-treated chloroplasts corresponds to an irreversible fixation of the acid. The proton uptake is linearly related to the amount of fixed acid (4 protons per molecule of acid) as long as the amount of silicotungstic acid does not exceed 200 nmoles/mg of chlorophyll. In the same conditions silicotungstic acid partly restores ferricyanide reduction and O2 evolution in chloroplasts suspensions supplemented with DCMU. These photoreactions are observed only with chloroplasts and these chloroplasts must have an unimpaired water-splitting mechanism. Silicotungstic acid does not impair DCMU fixation on the specific sites. More likely in its presence the properties of the membrane change and ferricyanide can accept electrons from a part of the electron transport chain, between the Photosystem II reaction center and the block of the electron flow by DCMU.