Chemical Nature of Growth Regulators 171 



Examples of each of the first three types of anti-auxins are given 

 in figure 76, including 2,6-dichlorophenoxyacetic acid which has an 

 obstruction in the ring (A), 2,4-dichloroanisole which lacks the acid 

 group (B), and 2,4-dichlorophenoxyisobutyric acid which has a steric 

 obstruction in its configuration (C). With each of these inhibitors it is 

 evident that the maximinn velocity of growth is unchanged, and hence 

 the observations are consistent with the assumption that these anti- 

 auxins compete with auxins for the same points of attachment to the 

 receptor substance or substances. 



Precisely the same results are obtained with anti-auxins of the 

 fourth type which have good affinity for the receptor but low activity 

 after becoming attached. This has been demonstrated for phenoxyace- 

 tic acid by Ingestad (1953). 



Other Inhibitors of Auxin Action 



Some other giowth inhibitors fail tt) show the unaltered maxi- 

 mum velocity of growth. Ihese include maleic hydrazide, 2,3,5-tri- 

 iodobenzoic acid and coumarin (McRae and Bonner, 1953; McCune 



200' 



150 



LJ 



cr 



< 

 > 



or 



O 



< 

 UJ 

 CL 



100 



IAA,IOmg/l 



/Jux/rr Inhibition 



IAA,30 mg/l 



10 



100 MG/L 



CONC. OF MALEIC HYDRAZIDE 



Fig. 77. The reversal of auxin inhibition of growth by maleic hydrazide in the 

 slit pea test. The optimum auxin concentration for growth was 10 mg./l. indoleacetic 

 acid and the presence of 30 mg./l. resulted in a 145° inhibition. The further addition 

 of 100 mg./l. of maleic hydrazide restored optimal growth (Leopold and Klein, 1952). 



