Differential Effect of Selected Phenoxy-acid Compounds on Oxidative Phosphorylation of Mitochondria fromVicia fabaL.

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Abstract
The effect of six phenoxy-acid herbicides, 4-chloro-2-methylphenoxyacetic acid (MCPA), 4-(4-chloro-2-thylphenoxy)butyric acid (MCPB), 2, 4-dichlorophenoxyacetic acid (2, 4-D), 4-(2, 4-dichlorophenoxy)butyric acid (2, 4-DB), 2, 4, 5-trichlorophenoxyacetic acid (2, 4, 5-T), and 4-(2, 4, 5-trichlorophenoxy)butyric acid (2, 4, 5-TB) on oxidative phosphorylation of mitochondria isolated from young hypocotyls of Vicia faba L. has been investigated. When NADH was used as substrate all the test herbicides were found to stimulate state 4 respiration with the loss of phosphorylation and respiratory control in varying degrees. When malate and succinate were used separately as substrates, treatment with 2, 4-DB, 2, 4, 5-T, and 2, 4, 5-TB at low concentration resulted in a marked stimulation of state 4 respiration; this effect was not obtained with MCPA, MCPB, or 2, 4-D. At higher concentrations all herbicides strongly inhibited respiration. These compounds released oligomycin inhibition during NADH oxidation in varying degrees, stimulated mitochondrial adenosine-triphosphatase activity, and induced swelling of isolated mitochondria. In many respects and in differing degrees they resemble 2, 4-dinitrophenol (DNP) in their action as uncouplers. Phenoxy-butyric acids were found to be more toxic in vitro as uncouplers than their corresponding phenoxyacetic acids. Phenoxyacetic acids were very active as uncouplers in vivo while phenoxybutyric acids had negligible effect. It is concluded that in vivo, non-activity of phenoxybutyric acids is due to their restricted entry into plants and that if available at the site of action they would be inherently toxic.

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