Superoxide formation in pea chloroplasts by a dioxathiadiaza‐2,5‐pentalene derivative, a new lipophilic

Abstract

The dioxathiadiaza-2,5-pentalene derivative, HEP II, has herbicidal effects similar to those of methyl viologen. HEP II promotes superoxide formation when added to illuminated pea chloroplasts. Superoxide dismutase, but not catalase, diminished formation of the superoxide whereas cyanide and azide enhanced its formation, presumably by inhibiting the endogenous superoxide dismutase activity. DCMU, which inhibits photosynthetic electron transport, inhibited superoxide formation. Rates of superoxide formation and oxygen uptake were very similar when equal concentrations of methyl viologen or HEP II were added. At subsaturating concentrations of electron acceptor, Mg2+ decreased the rate of oxygen uptake with methyl viologen but not with HEP II, probably reflecting differences in their interactions with the Photosystem I electron donation site. It is likely that HEP II, by analogy with methyl viologen, is reduced by chloroplast Photosystem I and reoxidised by molecular oxygen, generating superoxide. Photosynthesis