Inhibition of pigment synthesis by 1,3-dimethyl-4-(2,4-dichlorobenzoyl)-5-hydroxypyrazole, norflurazon, and new herbicidal compounds in radish and flatsedge plants

Abstract The pigment formation in radish ( Raphanus sativus L. cv. Tokinashi) and flatsedge ( Cyperus microiria Steud.) plants was investigated under the influence of 1,3-dimethyl-4-(2,4-dichlorobenzoyl)-5-hydroxypyrazole (DTP), norflurazon [San 9789, 4-chloro-5-methylamino-2-(3-trifluoromethylphenyl)pyridazine-3(2 H )-one], and some new herbicide candidates. All compounds applied reduced the contents of both colored carotenoids and chlorophyll, resulting more or less in the accumulation of colorless carotene, phytoene. A herbicide, 2-(2,4-dichlorobenzoyl)cyclohexane-1,3-dione (I), was found to be the strongest inhibitor of the pigments except for phytoene and was found next to norflurazon in the phytoene-accumulating activity. DTP, as well as norflurazon, was shown to be a potent inhibitor of the carotenoid biosynthetic pathway, producing a concurrent decrease in the pigments. At higher concentrations chlorosis was also induced by our new herbicidal compounds, 3-(2,4-dichlorobenzoyl)-2,4-pentanedione (II), ethyl 2-(2,4-dichlorobenzoyl)-3-oxobutyrate (III), and diethyl 2-(2,4-dichlorobenzoyl)malonate (IV). The pigment patterns produced by application of these compounds were almost the same as those obtained from treatments with I and norflurazon, which suggests that part of the mode of action of these compounds might possibly be the inhibition of steps between phytoene and β-carotene.