Response of Oxidation & Coupled Phosphorylation in Plant Mitochondria to 2,4-Dichlorophenoxyacetic Acid

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Abstract
The synthetic auxin 2,4-dichloro-phenoxyacetic acid severely uncoupled phosphorylation associated with the oxidation of malate, citrate, succinate and DPNH by mitochondria isolated from a variety of plants. This uncoupling effect is large enough that it seems adequate to account for the acute toxic responses elicited from plants by applying 2,4-D in the weed-killing range of concentrations. Concentrations of 2,4-D which profoundly uncouple phosphorylation in isolated mitochondria also completely inhibit growth of disks prepared from the same tissues. Indoleacetic acid in the same concentration range is without an uncoupling effect on these mitochondria. A comparison of the 2,4-D effects with uncoupling due to 2,4-dinitrophenol showed that 2,4-D is about l/20th as effective as DNP, although part of this difference is due to the differences in the concentrations of undissociated molecules of the 2 compounds. With both compounds the uncoupling effect increases as the pH is lowered, supporting the suggestion that the reactive sites for their action are located behind a diffusion barrier whose permeability to molecules is greater than to ions. In addition to a decrease in phosphorylation, 2,4-D also causes an inhibition of malate and citrate oxidation, but not with succinate of DPNH. This is probably related to an effect on the DPN-requiring dehydrogenases, since 2,4-D inhibits the oxidation of malate or reduction of oxalacetate by purified malic dehydrogenase.