Photodestruction of Pigments in Higher Plants by Herbicide Action: I. THE EFFECT OF DCMU (DIURON) ON ISOLATED CHLOROPLASTS

3-(3',4'-Dichlorophenyl)-1',1'-dimethyl urea (DCMU) inducedthe photobleaching of chlorophylls and carotenoids in isolatedchloroplasts of Hordeum vulgare. In chloroplasts illuminatedin both the absence and presence of DCMU (5.0 mmol m^–3),the destruction of carotenoid preceded that of the chlorophylls.The rate of photodestruction was accelerated by the presenceof DCMU. After only 2 h illumination the rates of loss of ß-caroteneand of the epoxyxanthophylls, neoxanthin and violaxanthin, weresimilar (approximately 40–50% loss in the presence of5–0 mmol m^–3 DCMU) but weremuch greater than thatof lutein (25% loss). Analysis of the individual pigment-proteincomplexes, isolated from chloroplasts following such treatment,showed that whilst pigment destruction had occurred in all complexes,the relative content of the LHCP2/CPa complexes (containingthe PSII core) had fallen to the greatest extent. Further illuminationof the chloroplasts, for up to 22 h, resulted in far greaterbleaching but showed a similar pattern of pigment loss, withDCMU again accelerating the rate at which this loss occurred.ß-Carotene-5,6-epoxide was identified as a productof such photo-oxidative conditions. Key words: DCMU, carotenoids, chlorophylls, photobleaching, ß-carotene-5,6-expoxide