176 Fundamentals of Auxin Action 



Veldstra (1953) has suggested a different role for auxin. He found 

 that the degree of fat solubility as influenced by the ring structure 

 and the water solubility as influenced by the side-chain structure could 

 be correlated with auxin activity. The correlation was rather complex, 

 and it was concluded that auxin activity was greatest when the lipo- 

 philic and the hydrophilic properties were balanced. Having also 

 compiled an immense amount of information indicating that the ring 

 and the side-chain should be in different spatial planes for effective 

 auxin activity, he conceived of the auxin action as being something 

 of a physical bonding of some lipoidal material to some more aqueous 

 phase. The identity of the two materials has not been established. 



A third suggestion about the reactions of auxin has sprung from 

 the suggestion of Muir et al (1949) that auxins of the phenoxy acid 

 type may combine with some material (presumably proteinaceous) 

 at the ortho position of the ring. Conceiving, then, of an auxin as 

 reacting with some material in the cell at two positions, (a) at some 

 position in the ring {ortho in the phenoxy acids) and (b) at the acid 

 group of the side-chain, Foster et al (1952) advanced a theory of auxin 

 action by two-point attachment. They supported the theory with a 

 variety of kinetic evidence. 



Kinetic Considerations 



With the increasing importance of kinetic evidence in relation to 

 auxin performance, a brief description of conventional kinetic meth- 

 ods would be in order here. The kinetic methods have been adopted 

 directly from enzyme studies, based on the work of Michaelis and 

 Menton (1913) and Lineweaver and Burk (1934). It is assumed that 

 enzymes attach to a substrate to form an enzyme-substrate complex, 

 and that this complex may further break in such a way as to produce 

 the end product of the reaction and regenerate the enzyme again. 

 Considering the enzyme to be the material with which auxin reacts. 

 the reactions are formulated as follows: 



K k 



E + S , 1 ES > growth + E 



where E is the auxin receptor, S is the auxin, ES is the complex of 

 the two, K represents the attraction of the auxin for the receptor, and 

 k the reactivity in growth. 



For the purposes of this brief discussion, two kinetic values of in- 

 terest may be obtained. One is the value of K and the other is the 

 maximal growth rate attainable, each of which will vary from one 

 auxin to another, all other factors being held constant. The method 



