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

The herbicidal actioIn of 2,4-dichlorophenoxyacetic acid (2,4-D) has been the subject of many investigations in the years sinice this compound was first demonstrated to be a plant toxicant. Almost as many possible mechanismiis for this toxicityhave been suggested. These includle depletioni of respiratory substrate, abnormal cellular proliferation, the production of toxic substances in response to 2,4-D application, changes in nitrogen mletabolismii or ioIn uptake and nietablolismi. interference with the level of endogenous auxin, andl abnormlal plhosphatase activity (4, 7, 9). While any of these imight be responsible for the death of plants, contrary evidence mnay be adduced for each of thenm, and it has becomle commnon to think of the herbicidal action of 2,4-D as a complex of many responses with nIo oIne response aloine producing the toxic effect. A possible explaniationi for the phytotoxic effect of 2,4-D imiay lie in its ability to uncouple phosphorylationi fromi oxidationi. The imiportance of oxidative phosphorylatioin in growtlh and maintenance of life is well recognize(d (1) and severe disruption of cellular metabolism as a result of uncoupling can easily result in the death of cells. In 1952 Brody (3) demonstrate(l that 2,4-D was an uncoupling agent for phosphorylationi associatedl with pyruvate oxidation in rat liver iiiitochoindria. andl later Switzer (5) showed a simiilar effect in soybeannmitochoindria using succinate. pyruvate, and a-ketoglutarate as substrates. In intact Chlorella cells 2,4-D inhibits the incorporation of inorganic phosphate into nucleotides while simiultanieously stimulatilng either endogenous or citrate oxidation (8). This uncoupling effect of 2.4-D has Inot been investigated as intensively as seems appropriate to its potential as an explanation for the weed-killing attributes of 2,4-D, and many questions regarding the miagnitude of this effect and its relation to growth of intact cells remaiin unainswvered. The work reported here will provide a confirmation of the previous reports of uncoupling (lue to 2,4-D and some additional information on aspects of this effect which have not previously been reportedl.