Influence of herbicide binding on the yield of singlet oxygen production in photosystem II and on charge recombination kinetics in the T4 mutant of Rhodopseudomonas viridis

摘要

Herbicide binding influences the redox potential (E_m) of the Q_A/Q_A~- redox couple in photosystem II. Phenolic herbicides lower the E_m by 45 mV, while DCMU raises it by 50 mV (Krieger-Liszkay and Rutherford, 1998). Earlier work shows that phenolics increase photodamage, while DCMU protects against it (Pallet & Dodge, 1980; Nakajima et al., 1996). We suggested previously (Krieger-Liszkay & Rutherford, 1998) that the recombination pathway in PSII is modulated by the binding of herbicides to the Q_B binding site. Lowering of the redox potentials by binding of phenolics should increase a true back reaction between P680~+Q_A~- involving the repopulation of the primary charge pair. The P680~+Ph~- radical pair can decay via the formation of the triplet state of P680. This triplet is relatively long-lived and has the potential to react with O_2, forming toxic singlet oxygen, ~1O_2. An increase of the redox potential by DCMU should favour a recombination via a direct safe route, thereby lowering the yield of ~3Chl and subsequent ~1O_2 formation.