High- and Low-Affinity Binding of Photosystem II Herbicides to Isolated Thylakoid Membranes and Intact Algal Cells

Herbicides, Photosynthesis, Highand Low-Affinity Binding, Thylakoids, Algae Highand low-affinity binding o f photosystem II herbicides to isolated thylakoids of Spinacia oleracea and to intact cells o f the unicellular green alga Ankistrodesmus braunii were investigated. Complete mutual displacement o f bound diuron-type herbicides (e.g. diuron, atrazine, terbutryn) by either diuronor phenol-type herbicides (e.g. ioxynil, dinoseb) in thylakoids as well as in intact algal cells was found for herbicide concentrations (< 4 n m o l bound herbicide/m g Chi) which gave almost saturated high-affinity binding. This demonstrates a high degree o f specific binding o f these herbicides towards their receptor sites even in intact algal cells. In contrast, phe­ nol-type herbicides are largely unspecifically bound in algal cells. The mechanism of binding o f all photosystem II herbicides at the high-affinity (specific) bind­ ing site was found to be competitive. W ithin the group o f diuron-type and o f phenol-type herbi­ cides as well as between these two groups, graphical and quantitative analysis o f the LineweaverBurk plot and o f the Dixon plot indicated com petitive binding. From this a common binding site for both types o f herbicides was concluded. The involvem ent o f two different herbicide bindingproteins 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 o f displacement. It is proposed that low-affinity binding represents either a partitioning o f the herbicides into the lipophilic parts o f the membranes or an attachment to distinct receptor sites. Unspecifically bound herbicides might be responsible for several high concentration effects o f the photosystem II herbicides, which are described in the literature. Evidences for the possible existence o f a second binding site o f these herbicides are presented.