178 Fundamentals of Auxin Action 



hypothetical receptor. The kinetic studies of auxins and anti-auxins 

 described in the preceding chapter further indicate that the affinity of 

 the auxin for the receptor can be competitively inhibited by anti- 

 auxins (see figure 76). Thus the use of K values and maximum growth 

 velocities have permitted a new level of definition of auxin activities. 



Returning to the concept of two-point auxin attachment, it was 

 found that if the algebraic expressions were modified to fit the assump- 

 tion of two points of attachment between E and S in the given 

 formula, an inhibition of growth by high concentrations of S (auxin) 

 could be predicted. This would result from two auxin molecules be- 

 coming attached to the receptor substance, one at each of the two 

 points of attachment, each preventing the effective functioning of 

 the other. In this way, the inhibition of growth by high auxin con- 

 centrations envisaged earlier by Skoog et al (1942) has found more 

 graphic expression. The two phases of the auxin curve are shown in 

 figure 79. A numerical expression of the inhibitory effects of an auxin 

 can be taken from the slope of the inhibitory section of the growth 

 curve given in figure 78. 



Two reservations should l)e kept in mind concerning the kinetic 



£ 

 E 



u 

 < 



01 



o 



a: 



A A 



c=o 



Clf/\CI CI//\C 



H^C-0\l I H^C-0\ 



p=o \/ c=o 



OH A A_ 



Auxin Inhibition 



10"^ M 



CONC. OF AUXIN 



Fig. 79. The promotion and inhibition of growth by 2,4-D as interpreted by the 

 theory of 2-point attachment. At low concentrations, both attachments are completed 

 and growth is promoted. At high concentrations the auxin competes with itself for 

 the two active sites and inhibition results (adapted from Foster et al, 1952). 



