Chemical Nature of Growth Regulators 161 



Such a strong influence of optical isomerism on auxin activity has 

 suggested that a third site of reaction of auxins is at the a carbon on 

 the side-chain (Wain, 1951). If a hydrogen atom were necessary for 

 substitution on one side of the carboxyl, then the influence of the 

 optical isomerism could be explained. However, several compounds 

 which have no hydrogen in an a position on the side-chain still have 

 auxin activity (a-methylene phenylacetic acid XIII, the benzoic acid 

 auxins IX). No other theory has been proposed which accounts for 

 the optical isomerism effect. 



What can be concluded from these findings relating to molecular 

 structure? Certainly the three requisites to auxin activity recognized by 

 Koepfli et al (1938) have been confirmed many times: that an un- 

 saturated ring is necessary, that an acid side-chain is necessary, and 

 that some spatial configuration is necessary. 



There seems to be a need for the unsaturated ring to be reactive 

 in the 2, 4 or 6 positions in the case of the phenoxyacetic acid series, 

 and in the 3 or 5 positions in the benzoic acid series (Thimann, 1952). 

 Substitution of chlorine into soyne of these active positions seems to 

 activate the others, whereas substitution into all of them essentially 

 removes activity. All of this plus the requirement for an unsaturated 

 bond in the ring implies that some rather specific reaction or attach- 

 ment of the ring must occur as a part of auxin activity. Particularly 

 nice evidence supporting the concept of two points of attachment in 

 auxin action has been reported by Foster et al (1952). Their evidence 

 is discussed as a mechanism of action in the next chapter. 



The acid side-chain, which is assumed to be the second point of 

 attachment, must apparently be of such size and shape as to fit some 

 spatial pattern in relation to the point of attachment of the ring. If 

 the side-chain is too long, too short, or of the wrong configuration to 

 permit easy fit to this pattern, auxin activity is apparently reduced or 

 lost. 



The molecular requirements for auxin activity have evoked some 

 highly provocative theories about the mechanism of auxin action, and 

 these are discussed in the next chapter. 



The selection of certain auxins for specific uses in agriculture can 

 be explained in many instances on the basis of knowledge of con- 

 figuration characteristics. Thus, where high activity and high mobility 

 are desired, as in herbicide work, chlorinated phenoxyacetic acids are 

 unexcelled. Where moderate activity and low mobility are desired as 

 for parthenocarpic fruit-set, weakly substituted phenoxyacetic and 

 phenoxy-a-propionic acids are superior. 



