156 Fundamentals of Auxin Action 



activity has been obtained with 2,3,6-trichlorobenzoic acid (IX). In 

 the benzoic acids, the 77ieta position appears to be the active site of 

 reaction, and the substitutions in the other positions serve to activate 

 the 7neta position (Thimann, 1952). It is interesting to note that as 

 the acid side-chain shortens, the active site on the ring retreats to a 

 position more remote from the side-chain. 



The importance of the ortho positions in other auxins such as 

 indoleacetic and naphthaleneacetic is obscure, but nevertheless the im- 

 portant point made by the Muir theory is that the ring of an auxin 

 must have an active position at which a reaction with another mate- 

 rial is assumed to take place. This contribution has provided a corner- 

 stone for some of the most provocative theories of auxin action, de- 

 scribed in chapter VIII. 



Structure of the Side-Chain 



Changes in the acid side-chain can have a profound influence on 

 auxin activity. For example, side-chains with two carbons or two 

 carbons plus an oxygen appear to be optimal for auxin activity. 

 Longer side-chains such as the propionic (X) and butyric acid (XI) 

 derivatives are generally less active (Koepfli et nl, 1937). 



CH,CH,COOH f n nCH,CH,CH,COOH 



When the side-chain of the phenoxy acid series contains an even 

 number of carbons, auxin activity is obtained, whereas with odd num- 

 bers of carbons auxin activity is almost absent (Synerholm and Zim- 

 merman, 1947). This is evidently because these acids, like fatty acids 

 in general, are broken down by removal of 2-carbon fragments. In 

 figure 70 the auxin activities of phenoxy acids with side-chains of 

 various lengths are compared with the rates of destruction of the side- 

 chain, indicated by the appearance of phenol in plant tissues supplied 

 with them. It is evident that odd-numbered carbon side-chains give 

 rise to large amounts of phenol, denoting that they have been metab- 

 olized to the one-carbon formic derivative which is unstable and 

 decomposes to phenol (Fawcett et al, 1952). Even-numbered carbon 

 side-chains do not break down in this way. Presumably they are 

 metabolized down by 2-carbon fragments to the acetic acid derivative 

 which is stable as well as being the most active auxin. 



