The Relationship of Magnesium Ion and Molecular Structure to 2,4-Dichlorophenoxyacetic Acid and Some Related Compounds to the Inhibition of the Respiration of Azotobacter vinelandii.

The independent work of Heath and Clark (1, 2) and Johnson and Colmer (3, 4, 5) introduced a new concept concerning the mode of action of auxin and auxin-like substances. Their evidence suggested interference with metal ion metabolism of the organisms studied. The concept would seem to be more compelling in light of the fact that the two pairs of workers using entirely different approaches, one the measurement of plant growth induction and the other the inhibition of bacterial respiration, arrived at the same conclusion. The argument becomes still more convincing in view of the fact that in one instance the natural auxin indoleacetic acid (IAA) was used and in the other the synthetic auxin-like substance 2,4-dichlorophenoxyacetic acid (2,4-D) was the test compound. Of still further interest are the facts that in both instances a parallel was established between the action of the test substance and known chelating compounds, and, as shown in the latest work of Johnson andl Colmer (5), that the inhibitions produced by 2,4-D, aureomycin, and achromycin were all abolished by low molar ratios of magnesium to the test agents. In view of these new findings it seemed advisable to investigate the relationship between struietural specificity and activity of 2,4-D and related compounds to determine whether or not there was a correlation with the magnesium content of the external environment.