3-(3,4-Dichlorophenyl)-1,1-dimethylurea Effect on Cytochrome b(559) Photooxidation and Q Reduction at Temperatures Near 0 C.

The position and interaction of cytochrome b with the electron transport chain in green plant photosynthesis has remained obscure. Levine et al. (13) and Levine and Gorman (12) first demonstrated the presence of cytochrome b. in the photosynthetic electron transport chain. Their results (12, 13), showing that photosystem II reduced cytochrome b, but that it was oxidized by PS I' in a DCMU-insensitive reaction at room temperature, suggest that cytochrome b. is closely associated with PS II. Vernon et al. (14), Boardman and Anderson (3) and Huzisige et al. (9) have isolated particles having PS II properties which have cytochrome b. as an integral part. Our recent findings (8) showing that cytochrome b,.,, is oxidized by P. (PS II reaction center) in a temperature-insensitive reaction at low temperature point clearly to the conclusion that cytochrome b,,. can interact with PS II under certain conditions. Knaff and Arnon (11) discovered a PS II driven decrease in absorbance with a peak at 550 nm. Erixon and Butler (6) recently observed, using potential poising, a component having a peak at 550 nm. The midpoint potential of this component was -50 mv and prompted them (6) to suggest that the 550 nm component was Q, the acceptor of PS II. I report here that in chloroplasts maintained near 0 C, cytochrome b,, photooxidation by 680 nm light is inhibited by DCMU with the concomitant appearance of a 550 nm component. These results can be explained by cytochrome b., photooxidation and Q reduction by PS II reaction center.