Herbicide-induced peroxidation: Influence of light and diuron on protoporphyrin IX formation

Abstract

Formation of ethane and protoporphyrin IX as indicators of peroxidizing activity of oxyfluorfen and chlorophthalim have been assayed comparing heterotrophic, dark-grown, glucose-supplemented Scenedesmus with autotrophic cells grown in pure mineral medium. Within 4 hr, protoporphyrin is formed in the dark by heterotrophic cells, as observed to a lesser extent with autotrophic cells, provided glucose is added. Under a nitrogen atmosphere very little protoporphyrin is produced, and the uncoupler CCCP strongly decreases porphyrin formation. Under culture conditions for 4 hr in the light, autotrophic cells also form substantial amounts of the porphyrin without glucose added, and in heterotrophic cells the level vs the dark sample is at least doubled. Diuron inhibits formation of protoporphyrin IX in illuminated autotrophic cells and prevents formation of the light-induced additional porphyrin in heterotrophic cells. Accordingly, under air, diuron completely inhibits peroxidation in autotrophic but not in heterotrophic cells. Using nitrogen or oxygen atmosphere, it can be shown that the peroxidation-inhibiting effect of diuron is not due to depletion of oxygen but to impaired photosynthesis. Photosynthesis (autotrophic cells) or metabolism of external carbohydrate (heterotrophic cells) ensure formation of protoporphyrin IX. Although autotrophic and heterotrophic cells attain different levels of protoporphyrin IX, induced by either oxyfluorfen or chlorophthalim, the amount of peroxidative ethane formation is found to be similar. Autotrophic cells need comparatively low light intensity (50 μEm −2sec −1) for herbicide-induced peroxidation while heterotrophic ones exhibit maximum peroxidative activity at about 400 μEm −2sec −1. This cannot be explained by the different photosynthetic capacity of both cell types, but possibly reflects differences in the light-mediated peroxidation process itself.