Applications of kinetics to auxin-induced growth 



continuing uptake is, however, a rapid initial uptake which is consummated 

 within 20-30 minutes. The continuing uptake, by which 2:4-D is actually 

 accumulated in the tissue and which is a metabolism-dependent process, is 

 not, however, related in any simple way to the auxin-dependent growth 

 process. In the first place the total amount of 2:4-D accumulated by the 

 tissue does not control growth rate, since viptake continues long after steady- 

 state growth has been established. Rate of uptake cannot control the growth 

 rate, since rate of 2:4-D uptake increases steadily with increasing external 

 2:4-D concentration and shows no tendency to saturate even at high 

 concentrations {Figure 10). 



The rapid initial uptake of 2:4-D by coleoptile tissue is a complex process 

 and is made up of at least two separable components, the quantitatively 

 more important of which is the diffusion of the auxin into the free space of the 

 coleoptile. As might be expected this diffusion is but little dependent on 

 metabolism, is linearly related to external concentration [Figure 10), and is 

 not inhibited by other auxins. The significance of the remaining minor 

 component of the initial uptake we have not yet assessed. It is clear in any 

 case that neither the active accumulation nor the passive diffusion of 2:4-D 

 into the coleoptile possesses kinetic characteristics similar to or of significant 

 importance in the over-all control of growth rate by auxin. 



SUMMARY 



When auxin is added to a plant tissue such as the Avena coleoptile, the tissue 

 grows. In the presence of different concentrations of auxin, the tissue grows at 

 different rates. Analysis of the auxin concentration-growth rate relation 

 suggests that some sort of saturation phenomenon is involved. The hypothesis 

 may be made that auxin interacts with some receptor entity of the tissue and 

 that growth rate is proportional to the amount of complex thus formed. This 

 hypothesis has been shown to be a useful one because it encompasses and 

 enables quantitative discussion of the interaction of two auxins in the plant, 

 of the inhibition of growth by varied auxin derivatives and of the inhibition 

 of plant growth by high auxin concentrations. The same hypothesis may be 

 applied to predict quantitatively the relative growth-promoting activity of 

 varied auxins. The supposition that auxin interacts with specific reactive 

 sites of the plant tissue to promote growth and the kinetic consequences of 

 this supposition cannot however embrace and render interpretable all 

 interactions of plant and auxin. Neither does it tell us anything concerning 

 the enzymology of auxin action. 



REFERENCES 



Bonner, J. (1933). The action of the plant growth hormone. J. gen. Physiol. 17, 63. 



Bonner, J. (1934). The relation of hydrogen ions to the growth rate of the Avena 

 coleoptile. Protoplasma, 21, 406. 



Bonner, J. (1936). The growth and respiration of the Aveiia coleoptile. J. gen. 

 Physiol. 20, 1. 



Bonner, J. (1949). Limiting factors and growth inhibitors in the growth of the Avena 

 coleoptile. Amer. J. Bot. 36, 323. 



Bonner, J., and Foster, R. J. (1955). The growth-time relationships of the auxin- 

 induced growth in Avena coleoptile sections. J. exp. Bot. 6, 293. 



308 



