Chemical Nature of Growth Regulators 169 



Anti-auxins can antagonize auxin action in a wide variety of 

 functions. Not only can they inhibit auxin-induced growth, but they 

 can alleviate auxin inhibitions (Aberg, 1950), prevent respiratory re- 

 sponses to auxins (Bonner, 1949), remove the apical dominance effect 

 of auxins, prevent tropic responses, and even protect against the 

 epinastic responses to auxin sprays (Hoffmann, 1953). 



Kinetics of Anti-auxin Action. A method of study of auxin and 

 anti-auxin action which has yielded much valuable evidence of the 

 mechanism of action has been the kinetic method devised by Line- 

 weaver and Burk (1934) for the study of enzyme action. This method 

 of analysis, described in chapter VIII, was adapted to the study of 

 auxins by Foster et al (1952) and of anti-auxins by McRae and Bonner 

 (1952, 1953) and Ingestad (1953). For the purposes of the present dis- 

 cussion the method will be discussed only briefly. 



In studies of simple enzyme systems, if one measures the velocity 

 of a reaction when the enzyme is supplied with various amounts of 

 substrate, one can estimate the maximal rate of the enzymatic action 

 under the conditions of the tests. This maximum velocity is constant 

 under conditions in which only the substrate concentration is varied. 

 If an inhibitor is added, the velocity at most of the substrate concen- 

 trations is reduced, of comse. Now if the inhibitor is acting competi- 

 tively with the substrate, that is, if the inhibitor is combining with the 

 enzyme at the same position as the substrate, the maximum velocity 

 obtainable is unchanged. On the other hand, if the inhibitor is acting 

 non-specifically to lower the effectiveness of the enzyme, the maximum 

 velocity obtainable is changed. In tliis way, one can test to determine 

 whether an inhibitor is combining with the enzyme at the same point 

 as the substrate by matching the maximum velocities obtainable with 

 and without the inhibitors. 



Assuming that auxins combine with some receptor in a manner 

 like the combination of a substrate with an enzyme, one can test 

 whether an anti-auxin competes for the same point or points of attach- 

 ment by this kinetic analysis. Measurements of the velocities of growth 

 with different auxin levels using the Avena straight-growth test have 

 given the results shown in figure 76 A. By the device of Lineweaver 

 and Burk (1934), one plots the inverse of the velocity (growth rate) on 

 the ordinate and the inverse of the substrate concentration (auxin) 

 on the abscissa; the point at which the line obtained intercepts the 

 ordinate is the maximum velocity. Addition of a constant amount of 

 an anti-auxin to each auxin concentration should not alter the point 

 of intercept (maximum velocity) if the anti-auxin specifically com- 

 petes with auxin for the active points of attachment. 



