Silicomolybdate and silicotungstate mediated dichlorophenyldimethylurea-insensitive photosystem II reaction: electron flow, chlorophyll a fluorescence and delayed light emission changes.

We have investigated the possible role of silicomolybdate and silicotungstate as acceptors of electrons in chloroplasts directly from Q, the primary electron acceptor of Photosystem II. Our data show: 1. Either of these compounds can accept electrons directly from Q in a 3-(3', 4'-dichlorophenyl)-1, 1-dimethylurea (DCMU)-insensitive electron transport; however, the DCMU insensitivity is only short-lived, so initial rates must be used exclusively. 2. High concentrations of these silico compounds act as direct chemical quenchers of chlorophyll a fluorescence, but lower concentrations which also mediate O2 evolution affect only the variable component of fluorescence in a manner suggestive of their electron-accepting capabilities. 3. Measurements of delayed light emission confirm the conclusions made from the fluorescence data. Also, they show the role of Q in delayed light emission as hydroxylamine data of other investigations have shown the role of Z, the electron donor of Photosystem II. 4. Silico compounds appear to be acting as electron acceptors and not as simple membrane modifiers allowing other acceptors to support a DCMU-insensitive electron transport.