Chemical structure and biological activity 



instance the dichloro acid is inactive and behaves similarly to that of di- 

 or^/?osubstituted phenoxyacetic acids, to which acids it bears structural 

 resemblances. This seems to indicate that dior//2osubstitution can annihilate 

 all kinds of root-growth activity in a specific manner. 



In order to corroborate the above classification of the acids, they were 

 investigated in the presence of a supposed true anti-auxin, 3-indolei.yobutyric 

 acid. 



t^OO 



300 



ZOO 



100 



Z-NOAA 

 /-Cl- 



' ^J-diCl- 

 -+ro~^^IiBA 

 — Control 



10'^ 



3x10 



-s 



10 



-5 



3x10'^ 



M 



Figure 2. The interaction between chlorinated naphthoxyacetic acids and 3-indoleisobutyric acid {liBA). 

 Dotted curves controls without HBA; full-drawn curves with addition of I'lBA. 



THE INTERACTION WITH 3-INDOLEUOBUTYRIC ACID 



The results are shown in Figure 2 and can be summarized in the following 

 points. 



1. Up to 3.10""^ M neither of the acids has any effect on the growth 

 increase caused by the uobutyric acid ; on the contrary, its growth-promoting 

 effect and the weak one of the naphthoxy derivatives are obviously additive. 

 This is in agreement with the earlier conclusion (Burstrom, 1954) that these 

 effects are of non-auxinic nature. 



2. With increasing concentrations of the 1- and 1 : 3-chlorinated acids the 

 effect caused by the addition of the isohutyric acid decreases and becomes 

 nil at 3.10-^ M. 



3. For the unchlorinated and 3-chloro acids, the graphs with and without 

 wobutyric acid run parallel within reasonable experimental errors. This 

 means that the activities of these two acids over the whole range of concentra- 

 tions are additive to that of the anti-auxin as nearly as can be expected from 

 two wholly independent kinds of action. The z'j-obutyric acid exerts the same 

 growth-promoting effect in absence of auxin, as when auxin suppresses the 

 elongation by 50 per cent. 



These results are surprising and contrary to what had been expected. They 

 imply that the supposed auxins are not antagonists of the supposed anti- 

 auxin, but that the root auxin and the inactive acid are antagonistic. 



136 



