THE KINETICS OF AUXIN-INDUCED GROWTHt 



J. Bonner and R. J. Foster 

 Kerckhoff Biological Laboratories, California Institute of Technology Pasadena California 



When we supply auxin to a plant tissue we observe a response. This response 

 in its most general terms consists of cell enlargement. Auxin goes into the 

 tissue, some mysterious and unknown events take place, and growth appears. 

 We can, however, find out a little about these obscure events in a very simple 

 way. We can determine how the response of the plant tissue to auxin varies 

 with auxin concentration, and from this we can deduce something about the 

 nature of the reaction or reactions in which auxin participates. In this kind 

 of study we deduce things about the mechanism of auxin action by measure- 

 ments of the growth rate of tissue. We study the inside by measuring the 

 outside. 



If we wish to investigate the dependence of plant growth on auxin concen- 

 tration, it will simplify matters to do it with a tissue which grows little in the 

 absence of added auxin and responds greatly to the addition of the material. 

 We may refer to the growth rate in the absence of added auxin as the endo- 

 genous growth. It is convenient to use a tissue which possesses a low level of 

 endogenous growth but which responds greatly to added auxin. This situa- 

 tion is found with sections of the Avena coleoptile and this paper will refer 

 only to work with such sections. That the growth of excised Avena coleoptile 

 sections is increased by added auxin has been known for over twenty years 

 (Bonner, 1933). In this time, it has become possible to grow sections in a 

 moderately reproducible manner and the several factors in addition to auxin 

 which determine growth rate have become known. The most important of 

 these factors are : 



(a) the pH of the medium, which must be kept constant since growth rate 

 is dependent on pH (Bonner, 1936); 



(b) the concentration of absorbable solutes such as glucose or sucrose in the 

 external medium, since these solutes by their absorption into the tissue con- 

 tribute to the osmotic concentration of the tissue which otherwise decreases by 

 dilution during auxin-induced water uptake (Bonner, 1949; Ordin et al., 

 1955); 



(c) the ratio of external solution to tissue volume, which must be sufficiently 

 large that the removal of auxin from the external solution does not appreciably 

 influence the concentration of the material. 



Suppose that we now place coleoptile sections in solutions containing varied 

 auxin concentrations, bearing in mind the above three points of technique. 

 The data of Figure 1 show that growth rate of the tissue as measured by rate of 

 elongation is constant with time over rather considerable periods. Each 

 concentration of added auxin elicits a growth rate which is constant and 

 therefore characteristic, for our conditions, of that auxin concentration. It is 

 apparent that when the section is placed in an auxin solution the processes 



t This paper was read at the Conference by J. Bonner. 



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