Herbicide-induced oxidative stress in photosystem II

Some herbicides act by binding to the exchangeable quinone site in the phot osystem II (PSII) reaction centre, thus blocking electron transfer. In this article, it is hypothesized that the plant is killed by light-induced oxid ative stress initiated by damage caused by formation of singlet oxygen in t he reaction centre itself. This occurs when light-induced charge pairs in h erbicide-inhibited PSII decay by a charge recombination route involving the formation of a chlorophyll triplet state that is able to activate oxygen. The binding of phenolic herbicides favours this pathway, thus increasing th e efficiency of photodamage in this class of herbicides.