FURTHER STUDIES ON THE MODE OF ACTION OF 2,4-DICHLOROPHENOXYACETIC ACID ON AZOTOBACTER VINELANDII AS RELATED TO MAGNESIUM AND PHOSPHATE

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Abstract
Previous work has shown a direct relationship between the concentration of Mg in the external environment and the inhibition of respiration of A. vinelandii by 2,4-D. By means of selective washing with either KC1 or phosphate buffer, cells of A. vinelandii were prepared for respiratory studies in the conventional Warburg apparatus. Mannitol-grown cells of A. vinelandii are unadapted to succinate. The adaptation to succinate is Mg dependent. When Mg is added to either KC1- or PO4-washed cells there is a marked increase in O2 uptake as compared to washed cells without added Mg. Respira- tion is severely impaired at an equimolar concentration of 2,4-D and Mg, and at a molar ratio of 2 to 1, 2,4-D to Mg, respiration is almost completely inhibited. The toxicity of 2,4-D for cells respiring on acetate was less evident than in the case of succinate oxidation, but the presence of the herbicide had its usual inhibitory action, and, again, the inhibition was greater in the presence of phosphate. Ca added to PO4-washed cells had a stimulating effect on their oxidation of acetate. At both an equimolar concentration and a molar ratio of 2 to 1, 2,4-D to Ca, 2,4-D inhibited this Ca-stimulated respiration. These results support hypothesis that the mode of action of 2,4-D for the cell is intimately related to its chelation of essential ions.