Redox reactions on the reducing side of photosystem II in chloroplasts with altered herbicide binding properties

Measurements of chlorophyll fluorescence have been used to monitor electron transfer from Q (the primary electron acceptor of photosystem II) to B (the bound quinone which serves as the secondary acceptor) in chloroplasts isolated from atrazine-susceptible and atrazine-resistant pigweed chloroplasts. The Q^- → B electron transfer was at least 10-fold slower in the plastids from resistant plants. Binary oscillations in the rate of Q^- decay after a series of flashes were of opposite phase in the two types. The data are interpreted to indicate that the apoprotein of B is altered in the photosytem II complex of the two types of plants-this is correlated to altered binding affinity of herbicides to this component and may be related to altered redox properties of the bound quinone cofactor.