The Use of O2-Evolving Subchloroplast Particles to Study Acceptor and Inhibitor Sites on the Reducing Side of Photosystem II

Photosystem II particles that retain the ability to evolve O2 have been used to examine acceptor and inhibitor sites in the photosynthetic electron transfer chain between Q and plastoquinone. Employing the water to dichlorobenzoquinone reaction to assay photosystem II activity, we have demonstrated that electron transport in thylakoids and particles is equally sensitive to inhibition by DCMU. dinoseb, metribuzin, HQNO and DBMIB. Based on differential sensitivity to inhibition by DCMU vs. HQNO or DBMIB, we suggest that when synthetic quinones, e.g. 2,6-dichlorobenzoquinone operate as Hill reagents in particles they are reduced primarily by the plastoquinone pool. When synthetic quinones, e.g. 5,6-methylenedioxy-2,3-dimethyl benzoquinone act as autoxidizable acceptors they accept electron from the Q/B complex at a point that is located between the DCMU and HQNO (DBMIB) inhibition sites.