Photosystem II - mediated cyclic photophosphorylation.

DCMU-sensitive synthesis of ATP can be shown to continue in KCN-treated chloroplasts after cessation of O₂ evolution. The catalyst for this reaction, $\text{p}$-phenylenediamine, also stimulates synthesis of ATP in NH₂OH-treated chloroplasts, but at much higher rates. This ATP synthesis can be observed in the presence of the quinone antagonist dibromothymoquinone, and under the appropriate conditions it is completely sensitive to DCMU. Since neither uptake nor evolution of O₂ can be observed during illumination, these results are interpreted as evidence for catalysis of cyclic photophosphorylation by photosystem II.