Inhibition of Photosynthetic Electron Transport in Chloroplasts by UHDBT, A Synthetic Analogue of Cellular Benzoquinones

A new synthetic analogue of plastoquinone, UHDBT (5-n-undecyl-6-hydroxy- 4,7-dioxobenzothiazole) has in one instance been reported as blocking the oxidation of reduced plastoquinone and hence electron transfer from photosystem II to photosystem I, and in another instance as acting like diuron and blocking the reduction of plastoquinone. In reexamining the effect of UHDBT on light-induced electron transport in chloroplasts, by measurements of induced fluorescence and of the redox state of the primary electron acceptor of photosystem II, we found that inhibition by UHDBT resembles that by diuron. However, experiments with trypsin-treated chloroplasts indicated that the site of UHDBT inhibition may be different from that of diuron.