THE KINETICS OF AUXINTNDUCED GROWTH 



T. A. Bennet-Clark 

 Botany Department, King's College, University of London 



The treatment proposed by Bonner and Foster in the previous paper has 

 aroused some controversy as it has not been universally admitted that 

 simple enzyme kinetics are applicable to such a complex sequence of 

 reactions as the 'growth process'. This treatment also takes no account of 

 diffusion processes which the work of Housley, Bentley, and Bickle (1954) 

 suggests may be of critical importance. 



If, however, one agrees that there is a roughly hyperbolic relationship 

 between initial extension in auxin and external auxin concentration, one 

 possible explanation is that this extension rate is linearly proportional to the 

 concentration of an auxin-tissue-component complex and that concentration 

 of this complex is determined by a dissociation constant comparable to the 

 Michaelis constant of an enzyme. This aspect of the Californian work taken 

 alone would probably not have evoked any controversy. The major source 

 of disagreement is concerned with the inhibiting action of high concentra- 

 tions of auxin which the Californian workers regard as comparable in its 

 nature to the inhibitions of certain enzymes, such as catalase and acetyl- 

 choline-esterase, by high substrate concentrations. This they explain on the 

 basis of a necessary two-point attachment of enzyme to substrate or auxin to 

 cell complex. 



The failure to agree on this point is entirely due to the inability of different 

 groups of workers to obtain the same experimental results. The only rate of 

 extension growth which can legitimately be used for this enzyme kinetic 

 treatment is the initial rate. If the initial rate is maintained more or less 

 unchanged for a period of several hours experimentation is, of course, made 

 much simpler. 



Bonner and Foster (1955) maintain that their experimental conditions 

 provide a rate of extension which is constant over 12 to 18 hours both at low 

 and at high auxin concentrations. Other workers (Bennet-Clark and 

 Kefford, 1954; Housley, Bentley, and Bickle, 1954; Marinos, 1955; and 

 Turnham (unpublished data)) find that the extension rate remains roughly 

 constant for 12 to 18 hours when auxin concentrations are low and with 

 relatively young coleoptiles, but that at high concentrations there is a rapid 

 decrease in extension rate with time. These workers find that the initial rates 

 at high concentrations have maximal values. In other words, the initial 

 rate-auxin concentration curve is claimed by these workers to be roughly 

 a hyperbola and the reciprocal plot is a straight line of the type shown as a 

 continuous line in Figure 1. The Californian school, on the other hand, 

 consider that the reciprocal plot resembles the dotted line o^ Figure 1, both 

 when initial rates are taken and when one takes extensions over a 15-hour 

 period. 



Unpublished work (Bennet-Clark, Hurst, and Turnham, 1955) has shown 



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