Changes in composition of chlorophylls,carotenoids, and prenylquinones in green seedlings of Hordeum and Raphanus induced by the herbicide San 6706—an effect possibly antagonistic to phytochrome action

The substituted pyridazinone herbicide San 6706 (4-chloro-5-(dimethylamino)-2-(α,α,α-trifluoro-m-tolyl)-3-(2H)-pyridazinone) inhibits accumulation of chlorophylls and carotenoids to about the same degree in Hordeum and Raphanus seedlings under continuous illumination. Stronger inhibition of pigment accumulation in general is correlated with a stronger inhibition of the prenylquinones plastoquinone-9,α-tocopherol, and α-tocoquinone; but the amounts of inhibition are much lower for the prenylquinones. Such an inhibition pattern, which is observed in the two plants of different ages and when different herbicide concentrations are applied, points to a site of action which regulates the biosynthesis of these prenyllipids together. There is a different degree in the change of the relative proportions (percentages of herbicide-treated plants as related to the respective control values) of the single carotenoids induced by the herbicide. In this sense there was the highest increase for zeaxanthin and lowest for β-carotene both in Hordeum and Raphanus. The order of relative change of the carotenoids analyzed is about the same as in etiolated barley seedlings of equal age illuminated with white light—but with an opposite sign. The relative proportions of the benzoquinones might be changed in an analogous way. It is suggested, that with respect to carotenoid synthesis and perhaps also benzoquinone synthesis San 6706 acts on the same reaction chain like phytochrome but in an antagonistic way, possibly at the cytoplasmic ribosomes.