Chemical Nature of Growth Regulators 159 



\oCH,SO,H /\r\cH,NO, f\ n CH,C=N 



XVII XVIll XIX 



The degree of acidity of the caiboxyl group itself varies from one 

 compound to another, and the auxin activity has been found to vary 

 with the degree of acidity expressed as the dissociation constant. Van 

 Overbeek et al (1951) have shown that the relative activity of a given 

 auxin in growth of pea sections can be related to the relative number 

 of undissociated molecules available at a given pH, whereas Lunde- 

 gardh (1949) has found quite the opposite for wheat roots: growth 

 was related to the number of dissociated ions. While the issue is not 

 settled as to whether the carboxyl group is more effective in the 

 undissociated or the dissociated state, the fact that strong auxins have 

 low dissociation constants suggests that the degree of acidity of the 

 side-chain has an influence upon activity. 



The requirement of an acid group on the side-chain for auxin 

 activity indicates that the carboxyl is probably another site for re- 

 action of auxins. 



Spatial Configuration 



Since Koepfli et al (1938) first suggested that the spatial configura- 

 tion of the auxin molecule was important to its activity, the require- 

 ments in this regard have become much better known, though the 

 reasons for them still remain obscure. Veldstra (1944) examined a 

 large variety of auxin materials, and concluded that the configuration 

 must allow the acid side-chain to be out of the plane of the ring 

 nucleus. He proposed that the ring must have surface activity by 

 which it becomes absorbed on some lipoidal materials in the cell, and 

 the side-chain must project out of that plane. The acidic group on the 

 end of the side-chain would function by attachment to a non-lipoid 

 material. The classical example illustrating Veldstra's theory of con- 

 figuration conferring activity is cinnamic acid (XX), in which the cis 

 form fulfills Veldstra's requirement and is an auxin, whereas in the 

 i)a7is form the side-chain cannot exist in any other plane than that of 

 the ring (figure 71) and it is not an auxin. Its anti-auxin characteris- 

 tics are discussed later in this chapter. 



In another instance, tetrahydronaphthylideneacetic acid (XXI) 

 has a cis and a trans form of which the cis form is active and the trans 



