Inhibition of Photosynthetic Electron Flow by Phenol and Diphenylether Herbicides in Control and Trypsin-Treated Chloroplasts

Alkyl substituted 2-halogen-4-nitro-phenols (like bromonitrothymol) are potent inhibitors of photosynthetic electron flow in chloroplasts. There inhibition site is identical to that of DCMU, though the phenols do not contain their essential chemical element responsible for inhibition nor do they follow their correlation of chemical structure and biochemical function. Binding of labelled metribuzin on the chloroplast membrane and its replacement by the phenol inhibitors support the notion of identical binding sites, however. In trypsin treated chloroplasts, the prim ary acceptor of photosystem II becomes accessible to ferricyanide, whose photoreduction is therefore DCMU insensitive. After trypsin treatment also the sensitivity of ferricyanide reduction to phenol inhibitors is lost as is the binding of labelled m etri­ buzin. The dinitrophenylethers of bromonitrothymol and of ioxynil and other diphenylethers are potent inhibitors of photosynthetic electron flow. Their site of inhibition is after the function of plastoquinone, i. e. they are DBMIB analogues.