Activity of Photosystem II Herbicides Is Related with Their Residence Times at the D 1 Protein

The reversible binding kinetics o f atrazine, diuron and ioxynil were measured via their bind­ ing and release parameters during steady state inhibition o f electron transport. The parameters were determined in isolated chloroplasts o f peas and o f triazine-resistant and -susceptible bio­ types o f Chenopodium album using a kinetic model. This model is based on the flash-induced oxygen evolution patterns o f isolated broken chloroplasts. It was found that the binding parameters were always significantly higher in the case o f an oxidized acceptor quinone complex as compared with a semi-reduced complex. Triazine resist­ ance seems to originate from a significant increase o f the release kinetics. The release parame­ ters could be used to calculate the residence times o f the herbicides at the D 1 protein. The values o f these residence times were always much higher for the herbicides than for QB; this explains the inhibition o f electron transport. The only exception was the residence time o f atra­ zine in the resistant biotype, where the value was close to that o f QB. It is concluded that the “on” kinetics o f a compound to its binding environment at the D 1 protein are determined principally by the accessibility o f the niche to the compound. The dif­ ferences in activity between herbicides are mainly due to variations in the release kinetics.