Interference in Carotenogenesis as a Mechanism of Action of the Pyridazinone Herbicide Sandoz 6706

The herbicide Sandoz 6706 (4-chloro-5-(dimethylamino)-2-alpha,alpha,alpha, (trifluoro-m-tolyl)-3(2H)-pyridazinone), when applied as a preplant soil treatment at a concentration of 0.05 mug/g reduced the content of beta-carotene and chlorophylls in 21-day-old wheat seedlings (Triticum aestivum L.) by 55% and 29%, respectively, without affecting the fresh or dry matter of the seedlings. At 0.8 mug/g, the herbicide reduced the content of beta-carotene and chlorophyll by as much as 98%, while the fresh weight of the albino seedlings was reduced by only 24%. The effect of the herbicide on chlorophyll b was much stronger than on chlorophyll a. Time course studies of pigment synthesis in Sandoz 6706-treated seedlings showed that chlorophyll, beta-carotene, cyclic xanthophylls, phytoene, phytofluene, and zeta-carotene were accumulating during the first 7 days after sowing. Later on, there was a sharp decline in the content of chlorophyll and beta-carotene and a gradual reduction in the content of phytofluene, zeta-carotene, and cyclic xanthophylls; the content of phytoene remained essentially unchanged. Coinciding with the drop in content of beta-carotene and chlorophyll, there was a remarkable increase in the content of epoxy phytoene. It is suggested that Sandoz 6706 might act as an inhibitor of the cyclization reaction in the biosynthetic pathway of carotenoids and that other effects, such as the bleaching of chlorophyll, are a consequence of this inhibition.