DCMU inhibits photosynthesis by blocking electron flow from Photosystem II. Also called diuron, DCMU binds at the Q_B plastoquinone-binding site of the Photosystem II D1 protein, preventing energized electrons from being transferred into the plastoquinone pool.
Photosystem II normally extracts electrons from water, releasing oxygen and protons, and passes those electrons through plastoquinone into the wider photosynthetic electron-transport chain. DCMU interrupts that route after the primary quinone acceptors, so linear electron flow and the production of downstream reducing power are severely impaired. The plant can no longer efficiently convert light energy into chemical energy.
DCMU is a useful laboratory inhibitor because its action is relatively specific: it blocks Photosystem II electron flow without directly stopping Photosystem I, light absorption, or the Calvin cycle’s enzymes. Atrazine is a common source of confusion because it also inhibits Photosystem II at the Q_B region, but the stored answer remains the canonical compound named in this question. DCMU’s chemical name is 3-(3,4-dichlorophenyl)-1,1-dimethylurea.