The herbicide commonly used in experiments to block Photosystem II is DCMU. Its full chemical name is 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and it is also known as diuron.
DCMU acts at the QB plastoquinone-binding site of Photosystem II, a protein complex embedded in the thylakoid membranes of chloroplasts. By preventing electrons from moving from Photosystem II into the plastoquinone pool, it interrupts the photosynthetic electron-transport chain. This makes DCMU a useful laboratory tool for isolating or studying specific steps in photosynthesis.
A common mix-up is confusing DCMU with paraquat. Paraquat primarily disrupts Photosystem I by diverting electrons and generating reactive oxygen species, whereas DCMU is characteristically associated with Photosystem II inhibition. Glyphosate works through a different metabolic pathway involving aromatic-amino-acid synthesis.
DCMU was introduced commercially by Bayer in 1954 under the name Diuron. Although it is called a herbicide, researchers often use it at carefully controlled concentrations to investigate electron flow, photophosphorylation, and light-reaction physiology.