Herbicidal Efficacy of Protoporphyrinogen Oxidase Inhibitors

Compounds which inhibit protoporphyrinogen oxidase (Protox) were known as “photobleaching herbicides” before their site of action was discovered. Photobleaching herbicides cause very strong bleaching of the treated part of higher plants. It was known that a photobleaching herbicide requires oxygen and light to express its herbicidal activity. After intensive investigations on the mode of action of photobleaching herbicide, protoporphyrinogen oxidase has been identified as the molecular target. It is generally accepted that the inhibition of Protox and oxidation of protoporphyrinogen leads to the accumulation of a strong photosensitizer, namely protoporphyrin IX (PP IX), followed by activation of oxygen and lipid peroxidation (Duke et al. 1993). This mechanism explains the necessity of light and oxygen. After discovery of the molecular target of photobleaching herbicides, several chemical structures were known to inhibit Protox. It is possible to classify Protox inhibitors into several groups according to their chemical structure.