The action of photosynthetic inhibitors has traditionally been monitored in vitro by measuring the so - called ‘Hill Reaction’ . This is the ability of a thylakoid preparation to evolveoxygen in the presence of a suitable electron acceptor. The natural acceptor, NADP+, iswashed free from isolated thylakoids, so artificial acceptors (A) are used experimentally.2 2 2 H O HA AH O 2 2 2