Photodependent generation of reactive oxygen species by a phytotoxic pyridine derivative

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LS 82556 [(α methyl benzyl) carbamoyl 5 propionyl -2,6 lutidine] is an experimental herbicide discovered by Rhōne-Poulenc. This compound induces symptoms quite similar to those of diphenyl ethers (1,2) and is active only in the light (3,4). It induces photodependent pigment bleaching and rapidly destroys the aerial parts of higher plants (1,5,6). Its action seems associated to insatured fatty acid peroxidation leading to membrane disruption (1,5,7). Several authors have found some effects on the redox photosynthetic reaction (8,9) while others suggest a requirement of the mitochondrial electron transfer chain (2,10). However, LS 82556 and DPE are powerful inhibitors of tetrapyrrole biosynthesis, leading to porphyrin accumulation and to at least one part of their phytotoxic effect under light (11,12). From the point of view of photochemistry the phytotoxic effect of this compound could be associated either with the photodependent formation of active oxygen species (10,13) or with the degradation of endogenous antioxidants such as vitamin C or E (5,7,10,14,15,16). The aim of this work was to study the in vitro formation of superoxide and singlet oxygen, by direct irradiation of LS 82556 used as a photosensitizer.