The kinetics of auxin-induced growth 



is reduced to approximately 1 to 1. For or/Aochlorophenoxyacetic acid one 

 such receptor entity in six bears a two-point attached auxin molecule; a 

 reflection of the fact that singly attached (through the carboxyl group) 

 or^Aochlorophenoxyacetic acid is actually more stable than the doubly 

 attached form. 



We are now in a position to test the hypothesis that the maximum growth 

 rate elicited by a particular auxin is determined by the extent to which the 

 substance occupies the auxin-receptive sites of the plant with doubly attached 

 auxin molecules. Table 3 contains the appropriate comparison. This table 

 contains firstly the ratios of £'.S'i_2 to ^totai calculated as indicated above for 

 each auxin and for the case of that concentration of each auxin which elicits 

 maximum growth rate. It contains in addition the absolute rate of growth of 

 coleoptile sections in the optimal concentration of each auxin. It is evident 



Table 3 



Comparison of maximum growth rates elicited by various auxins at optimal concentration 



with calculated ratios of ESi oj EtotaX 



that the relations between the absolute maximum growth rates elicited by 

 each avixin are in good quantitative agreement with the ratios o^ ES^^ to 

 •^totai calculated for each auxin. Table 3 includes data on the native auxin, 

 lAA. Measurements of the individual single-point interaction affinities of 

 lAA carried out as outlined above, have shown that with this auxin, given at 

 its optimal concentration, the tendency to two-point attachment is so great 

 that essentially all receptor sites are present as ES^^. The maximum growth 

 rate elicited by lAA is larger than that elicited by 2:4-D by an amount which 

 is precisely that expected on the basis of the kinetic calculations. 



The above considerations have been extensive and involved. They have 

 been carried through because they provide an elaborate test of the applic- 

 ability of kinetic considerations to auxin-induced growth. The results of the 

 test support the view that the auxin-coleoptile system does in fact behave 

 quantitatively as expected of a system in which auxin interacts reversibly to 

 form a variety of related auxin-coleoptile complexes which are in equi- 

 librium with one another. The outcome of the investigation is also in agree- 

 ment with the view that each auxin receptor site which becomes two-point 

 attached to an auxin molecule contributes the same amount to the growth of 

 the plant, regardless of the nature of the auxin which achieves two-point 

 attachment. 



We have thus far considered aspects of auxin-coleoptile interaction which 

 may be treated in kinetic terms in a straightforward manner and with 



305 



