A DIFFERENTIAL EFFECT OF 3-(3'4' DICHLOROPHENYL)-1,1 DIMETHYL UREA AND ATRAZINE ON FLUORESCENCE KINETICS IN CHLOROPLASTS

It was found that DCMU had a differential effect at two concentration ranges on variable fluorescence kinetics in isolated chloroplasts. The increase in fluorescence rate at low concentrations of DCMU was abolished by preincubation of chloroplasts with ferricyanide or formate, treatments which were shown to convert Fe in the PS II reaction center (i.e., the FeQ(A) complex) into a non-oxidizable form, but it was not affected by Tris treatment. Increase in fluorescence kinetics (at the initial linear rate) at high concentrations of DCMU was found to be abolished by Tris treatment but it was only marginally affected by ferricyanide or formate treatments. The effect of Tris could be abolished by addition of hydroquinone-ascorbate, which restored electron flow to the pool of secondary acceptors. Contrary to the effect of DCMU, no such differential concentration dependence of the variable fluorescence kinetics was found for atrazine. The increase in fluorescence kinetics (at the initial linear rate) at a low concentration rate of DCMU is presumably restricted to units which contain an oxidizable Fe in the FeQ(A) complex. Increase in fluorescence kinetics (at the initial linear rate) at high DCMU concentration is probably related to the effect of DCMU at the Q(B) site.