pH DEPENDENCE OF THE EFFECTS OF DIURON, ATRAZINE AND DINOSEB ON THE LUMINESCENT PROPERTIES OF THYLAKOID MEMBRANES

The effect of the external pH in the range of 5.5 to 8.0 on the activity of three PSII herbicides (diuron, atrazine and dinoseb) was studied in isolated thylakoid membranes by means of prompt and delayed fluorescence induction kinetics. Well-expressed pH dependence was established for prompt fluorescence parameters (Fv/Fm, Fi/Fm) as well as for delayed fluorescence parameters (I1, I2). Two types of pH dependence of the herbicide effect were observed. The apparent resistance to herbicides of the urea/triazine type (diuron and atrazine) was found to be maximal at pH 6–6.5, although the amplitude of the herbicide effect was different between diuron and atrazine. The pH dependences for thylakoids treated with the phenolic herbicide, dinoseb, showed a minimum in the herbicide effect at pH 7.5. The mechanism by which pH influences the herbicide sensitivity of thylakoid membranes is supposed to involve pH-induced conformational changes of the QB-binding site on the D1 protein of PSII, resulting in alteration of the affinity to the herbicide molecule. The conformation of the QB-site at a certain pH is characterized by maximal herbicide resistance. The optimal value of pH depends on the type of herbicide, specifically the location of the Hbond between the herbicide molecule and D1. The abrupt distinction in the behaviour of the different types of herbicides with respect to pH could serve as a basis to develop a test method to discriminate between different families of herbicides and to classify a given herbicide as belonging to one or another family.