Site of Action of 2,5-Dimethoxy-3,6-Dichloro-/?-Benzoquinone in the Photosynthetic Electron Transport Chain

Dichlorodimethoxy-/?-benzoquinone (DCDMQ) was tested for its site of action in the photo­ synthetic electron transport chain. Hill reaction mediated by DCDMQ was insensitive to DBMIB (1 nm) but sensitive to DCMU, suggesting its site of action before plastoquinone but after Q the primary electron acceptor of photosystem II. Extraction of freeze-dried chloroplasts with heptane and analyzing their capacity to photo-oxidize water using various Hill oxidants revealed that silicomolybdate (SiMO) and DCDMQ could effectively restore the activity. Diaminodurene (DAD) in the presence of ferricyanide could restore 40% of the activity. But ferricyanide alone failed to restore the ability to photo-oxidize water in heptane extracted chloroplasts. Similarly, Na2S 0 3 which is known to cause a bottleneck in the electron flow at plastoquinone affected the ferricyanide Hill reaction. Hill reactions mediated by SiMO and DCDMQ were insensitive to the addition of Na2S 03, suggesting that both these oxidants intercept electrons before plastoquinone. But 50% of the activity was lost when sulfite was added to the Hill reaction mediated by DADox. DNP-INT, melittin and picrylhydrazyl were recently introduced as photosystem II inhibitors inhibiting the electron flow between Q and the PQ pool. While DCBQ and DCDMQ Hill reactions were insensitive to DNP-INT, ferricyanide was highly sensitive. The quinonediamines TMPD and DADox showed 50% decrease in the electron transport rate, similar to heptane extracted or sulfite inhibited chloroplasts. Melittin increased the electron transport rate when ferricyanide or TMPD was the Hill oxidant, while DCBQ and DCDMQ reduction remained unaffected. However, DADox Hill reaction showed 50% inhibition in the presence of melittin. Picrylhydrazyl — which inhibits the electron flow between Q and the PQ pool — inhibited the Hill reaction of all the PS II electron acceptors except that of DCDMQ. It is possible that there is another site of intercepting electrons between Q and plastoquinone before the site where most of the quinonediamines accept electrons.