The mode of action of nitrodiphenyl ether and related herbicides

Abstract

Nitrodiphenyl ether (NDPE) and related herbicides cause light and oxygen-dependent lipid peroxidation in susceptible green plants and algae. The exact mode of action of these herbicides has not yet been clearly established. Experiments with the alga Scenedesmus obliquus indicate that the role of photosynthetic electron transport is principally to generate the oxygen required for NDPE toxicity, rather than having a more direct involvement. DPE I an experimental NDPE, did not induce lipid peroxidation or chlorophyll bleaching in the alga Chlamydomonas reinhardtii but the herbicide did inhibit growth of the algal cultures.The toxic effects of DPE I on chrysanthemum petals containing a mixture of carotenoids but no chlorophyll appeared to support the hypothesis that carotenoids are the photosensitisers in NDPE action. The bleaching of chromoplast fractions isolated from chrysanthemum petals, when incubated under light, was enhanced by DPE I. Experiments suggested that the NDPE-enhanced bleaching does not involve singlet oxygen or Fenton chemistry. Structure activity studies suggested that the mechanism of NDPE-induced bleaching of chromoplasts is different to that of the toxic effects induced by these herbicides in leaves. DPE I treatment caused the abnormal accumulation of photodynamic tetrapyrrole compounds in cucumber, barley, Scenedesmus and Chlamydomonas. In C. reinhardtii the major tetrapyrrole accumulating was identified as protoporphyrin IX. Experiments with enantiomers of a phthalide DPE, phthalide DPE III showed that the S(-) isomer induced greater accumulation of tetrapyrroles than the R(+) isomer. This difference was reflected in the herbicidal activities of the S(-) and R(+) isomers and suggests the likely involvement of an enzymic binding process, possibly within the chlorophyll biosynthetic pathway, in the mode of action of the DPE herbicides.