Abstract
The effect of the plastoquinone antagonist, dibromothymoquinone, on the photoreduction of ferricyanide and plastocyanin by maize mesophyll, maize bundle-sheath and Euglena gracilis chloroplasts has been investigated. Maximum inhibition of FeCN and plastocyanin reduction by mesophyll chloroplasts was obtained at dibromothymoquinone concentrations of 5 × 10 −7 m. At higher concentrations dibromothymoquinone acted as an electron shuttle, increasing the rate of reduction of both substrates. In contrast, little inhibition of FeCN and plastocyanin reduction by bundle-sheath chloroplasts occurred at 5 × 10−7 m dibromothymoquinone, and above this concentration of inhibitor, the extent of inhibition increased, with no shuttle effect being observed. Euglena chloroplasts showed a response intermediate between that of mesophyll and bundle-sheath chloroplasts. The presence of a shuttle effect caused by dibromothymoquinone appears to be directly related to the presence of a proton pump in the chloroplast preparations. Plastocyanin is reduced by photosystem 2 alone and shows some of the properties of a class III electron acceptor, although the rates of reduction observed were much lower than those observed with lipophilic class III acceptors.
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