Abstract
High- and low-affinity binding of photosystem II herbicides to isolated thylakoids of Spinacia oleracea and to intact cells of the unicellular green alga Ankistrodesmus braunii were investigated.
Complete mutual displacement of bound diuron-type herbicides (e.g. diuron, atrazine, terbutryn) by either diuron- or phenol-type herbicides (e.g. ioxynil, dinoseb) in thylakoids as well as in intact algal cells was found for herbicide concentrations (< 4 nmol bound herbicide/mg Chl) which gave almost saturated high-affinity binding. This demonstrates a high degree of specific binding of these herbicides towards their receptor sites even in intact algal cells. In contrast, phenol-type herbicides are largely unspecifically bound in algal cells.
The mechanism of binding of all photosystem II herbicides at the high-affinity (specific) binding site was found to be competitive. Within the group of diuron-type and of phenol-type herbicides as well as between these two groups, graphical and quantitative analysis of the Lineweaver- Burk plot and of the Dixon plot indicated competitive binding. From this a common binding site for both types of herbicides was concluded. The involvement of two different herbicide binding- proteins is discussed. Low-affinity (unspecific) binding was found to be irreversible in contrast to the easily reversible high-affinity binding. Irreversibility was indicated by a lack of displacement. It is proposed that low-affinity binding represents either a partitioning of the herbicides into the lipophilic parts of the membranes or an attachment to distinct receptor sites. Unspecifically bound herbicides might be responsible for several high concentration effects of the photosystem II herbicides, which are described in the literature. Evidences for the possible existence of a second binding site of these herbicides are presented.
