The kinetics of auxin-induced growth 



nature of the substrate concentration growth rate relation suggests that the 

 coleoptile possesses spots which can interact with auxin. We do not find out 

 by our treatment where the spots are or what they do. It is necessary to try 

 to find out what the spots are by other types of experiments. We have 

 approached this problem with the aid of C^^ carboxyl-labelled 2:4-D of high 

 specific activity (10 millicuries/millimole). One might first ask whether the 

 process whose kinetics we study when we study growth rate is simply the 



Uptake of 2 -A-D {0-5 mgll.) by Avena 

 coleoptile sections with time. 



Figure 9. Time course of C^*-labelled 

 2'A-D uptake by the Avena coleoptile 

 section. Auxin supplied at a concentration 

 of 0-5 mgll. After Bonner and Johnson 



(1955). 



m JO SO 



Incubation time 



120 

 min 



Uptake of 1 A-D by Avena coleoptile 

 sections as a function of 2 A-D con- 

 centration. 



/Absorbed 

 (pens Tirs) 



Figure 10. Concentration dependence 

 of 2 A-D uptake by Avena coleoptile 

 sections. The upper curve refers to the 

 amount of 2 A-D taken up by the 

 continuing metabolically powered accumu- 

 lation. The lower curve refers to that 

 taken up in the initial 20-30 minutes. 

 After Bonner and Johnson (1955) . 



S:¥-/) concn. 



penetration of auxin into the tissue. The kinetics of penetration of 2:4-D 

 into coleoptile tissue have therefore been investigated (Bonner and Johnson, 

 1955). 



The data o{ Figure 9 show that when coleoptile tissue is placed in 2:4-D of 

 low concentration (2-5x10-6 M), the 2:4-D is taken up at a rate which 

 is constant with time over a prolonged period. Superimposed on this 



307 



