Phytotoxicity of m-phenoxybenzamides: Inhibition of cell-free phytoene desaturation

Inhibition of carotenoid biosynthesis by herbicidal m-phenoxybenzamide derivatives has been investigated in a cell-free carotenogenic system from Aphanocapsa. Their target is the phytoenedesaturase reaction. Double-reciprocal plots of β-carotene biosynthesis (from ¹⁴C-labeled geranylgeranyl pyrophosphate) showed that 3-(2,5-dimethylphenoxy)-N-ethylbenzamide was a noncompetitive inhibitor of the phytoene-desaturase complex. The K_i value for cell-free inhibition of β-carotene formation was almost identical to the I₅₀ value of intact cells. Furthermore, the influence of certain substituents on herbicidal activity has been investigated. Inhibition increased with the length of the unbranched N-alkyl chains. In addition, substituents at the phenoxy group with higher lipophilicities showed greater inhibitory activities. The presence of a phenoxy or trifluoromethyl moiety at position 3 is essential.