Substituted Benzoic Acids 



253 



Table 2. Percentage inhibition of growth of Avena coleoptiles in- 

 duced by growth regulators in the presence of 2 X 10"^/ 4-ethyl-3- 

 mercaptobenzoic acid (5 X XO'^M with lAA). 



elongation induced by indole-3-acetic acid, 1-naphthaleneacetic acid 

 and 1-naphthaleneacetonitrile, the compounds most active in promot- 

 ing elongation. Greater inhibition occurs in the case of 2,4-dichloro- 

 phenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, and indole-3- 

 butyric acid, which are less active. The greatest inhibitory effect is 

 found with 2,6-dichloro- and 2,6-dimethyl-3-bromo- benzoic acids, 

 which have the lowest activity as auxins. EMBA, at one-tenth of the 

 concentration of 2,6-dichlorobenzoic acid, results in an inhibition of 

 30 per cent of the elongation effect of the latter. 



These results suggest a competitive type of inhibition, and Line- 

 weaver-Burk plots of the reciprocals of elongation and regulator con- 

 centration (5) conform to expectation for the interaction of EMBA 

 and indole-3-acetic acid. The interaction with 2,4-dichlorophenoxy- 

 acetic acid (Figure 2) and 2,6-dimethyl-3-bromobenzoic acid appears to 

 be competitive at high concentrations of the auxin, but at low con- 

 centrations the inhibition tends to be complete. The inhibition by 

 EMBA also appeared to be reversible in an experiment in which in- 



