Inhibition of Chloroplast Electron Transport Reactions by Trifluralin and Diallate

Abstract

The herbicides trifluralin (.alpha.,.alpha.,.alpha.-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and diallate (S-[2,3-dichloroallyl] diisopropylthiocarbamate) inhibit electron transport, ATP synthesis and cytochrome f reduction by isolated spinach (Spinacia oleracea L.) chloroplasts. Both compounds inhibit noncyclic electron transport from H2O to ferricyanide more than 90% in coupled chloroplasts at concentrations < 50 .mu.M. Neither herbicide inhibits electron transport in assays utilizing only photosystem I activity, and the photosystem II reaction elicited by addition of oxidized p-phenylenediamine or 2,5-dimethylquinone is only partially inhibited by herbicide concentrations which block electron flow from H2O to ferricyanide. Inhibition of ATP synthesis parallels inhibition of electron flow in all noncyclic assay systems, and cyclic ATP synthesis catalyzed by either diaminodurene or phenazine methosulfate is susceptible to inhibition by both herbicides. These results indicate that trifluralin and diallate both inhibit electron flow in isolated chloroplasts at a point in the electron transport chain between the 2 photosystems.