The mode of action of chemically different herbicides (ureas, pyridazinones, phenylcarbamates, triazines, hydroxyquinolines, hydroxybenzonitriles and dinitrophenols) on photosynthetic electron transport was investigated by measurements of oxygen evolution and thermoluminescence. Depending on the particular herbicide used the thermoluminescence band related to Q (the primary acceptor of Photosystem II) appears at +5, 0 or -14°C. It was shown that these three different peak positions can be ascribed to various redox states of Q, the shifts being due to the binding of herbicides to the chloroplast membrane. Both displacement experiments and additive inhibition of herbicide pairs measured by thermoluminescence and oxygen evolution suggested that the sites of action of these herbicides are on the same protein. However, herbicide treatment of trypsinized chloroplasts showed that there were three different binding sites on the same protein, in agreement with the classification of herbicides into three groups based on thermoluminescence measurements. Our results suggest that the primary and secondary acceptors of Photosystem II (Q and B, respectively) are in close proximity and form a common complex with the herbicide-binding protein within the chloroplast membrane.
ASJC Scopus subject areas
- Cell Biology