Mode of Action of the Herbicide DCMU When chloroplasts are treated with 3-(3,4- dichlorophenyl)-1,1-dimethylurea (DCMU, or diuron), a potent herbicide, O2 evolution and photophosphorylation cease. Oxygen evolution, but not photophosphorylation, can be restored by addition of an external electron acceptor, or Hill reagent. How does DCMU act as a weed killer