Chemical Nature of Growth Regulators 153 



It is apparent that accurate distinctions between types of agents 

 which increase growth will not always be easy. Thus the stimulation 

 of growth in a plant or plant part by small amounts of a compound 

 may be due to auxin activity, synergistic activity or epinastic activity 

 of the compound. It should be remembered that one compound will 

 not necessarily show only one type of growth effect. An auxin may 

 have epinastic activity, and compovmds which are auxin synergists in 

 one circumstance may be growth inhibitors in another. 



MOLECULAR STRUCTURE AND AUXIN ACTIVITY 



The exhibition of auxin activity by compoiuids of diverse molec- 

 ular structures has led to the fascinating study of the relationship of 

 molecular structure to this activity. The field has been slightly con- 

 fused by the neglect in some studies of structural requirements to use 

 tests which are specific for auxin activity. Even with the limitations 

 of the data available some remarkable generalizations can be drawn. 

 To date such generalizations have been highly productive in the 

 development of an understanding of the mechanism of auxin action. 



The basic molecular requirements for auxin activity recognized 

 by Koepfli et al (1938) are as follows: 



(a) An unsaturated ring 



(b) An acid side-chain 



(c) A particular spatial arrangement between the ring and 

 the side-chain. 



These workers did not stipidate what spatial arrangement was essen- 

 tial, but they realized that spatial arrangement played an important 

 role in auxin activity. 



The Structure of the Ring 



Analysis of the structure of the ring as it pertains to the auxin 

 activity of indoleacetic acid (I) shows that the nitrogen in the indole 

 ring is not essential, for substitutions of an oxygen (II) or a carbon 

 (III) for the nitrogen will still permit auxin activity (Thimann, 1935). 



CHjCOOH f \ nCHjCOOH f \ nCH,COOH 



III 



This is not surprising since many other rings can serve as well as the 

 indole ring. The ring may be small, as in the case of a phenyl ring 



