Recent Advances in the Mode of Action of Diphenyl Ethers and Related Herbicides

Several hypotheses have been proposed to explain the light-dependent phytotoxicity of diphenyl ethers and related herbicides: inhibition of photosynthesis; activation of herbicides by light, by the photosynthetic electron transfer chain or by excited forms of carotenoids; and interaction with the biosynthesis of tetrapyrrole pigments. It is shown that the most likely mode of action consists in inhibiting the enzyme protoporphyrinogen oxidase. As a consequence, protoporphyrinogen is oxidized non-enzymatically to protoporphyrin IX. The latter molecule is a powerful photosensitizer, able to generate singlet oxygen in the light and thus to induce peroxidative destruction of membrane lipids.