Inhibition of carotenogenesis by substituted diphenyl ethers of the m-phenoxybenzamide type

Diphenyl ethers exhibit different modes of action according to their chemical constitution. Diphenyl ethers of the m-phenoxybenzamide type, which were found to be effective on carotenogenesis resulting in an accumulation of colorless carotenoid precursors, mostly phytoene, indicative of inhibition of desaturation, are discussed. As seen with other carotenoid biosynthesis inhibitors, a concurrent loss of chlorophyll was observed as a secondary effect caused by the absence of protective carotenoids. In contrast to peroxidative p-nitrodiphenyl ethers like oxyfluorfen (2-chloro-4-trifluoromethylphenyl-3′-ethoxy-4′-nitrophenyl ether), the m-phenoxybenzamides assayed showed the same phytotoxic mode of action in the dark as observed when using heterotrophic Scenedesmus cultures. As expected, chlorophylls were not affected. The decrease of carotenoids was not due to their degradation but to inhibited carotenogenesis. Examination of carotenoid fractions show that the m-phenoxybenzamides, e.g., 3-(2,5-dimethylphenoxy)-N-ethylbenzamide, used here act similarly to 2-phenylpyridazinones like norflurazon [4-chloro-5-methylamino-2-(2-trifluoromethylphenyl)-pyridazin-3(2H)one]. All these inhibitors strongly decrease the α- and β-carotene content, while xanthophyll content is not lowered as much.