Applications of kinetics to auxin-induced growth 



which resuh in growth are quickly initiated and are then maintained in a 

 steady-state condition for several hours. Our further analysis will deal only 

 with the steady-state conditions. 



Let us now proceed with the question of how section growth rate depends 

 on auxin concentration. It has been known for some time (Bonner, 1933) 

 that as the external auxin concentration is increased growth rate of the 



7/4/1 concn. 



i 



4 



Time 



Figure 1. Elongation o/Avena coleoptile sections in different concentrations of indoleacetic acid {lAA) 

 as a function of time. After Bonner and Foster (1955). 



section increases to a maximum and then drops off. We will first consider the 

 low range of auxin concentrations, those below that which elicits maximum 

 growth rate, often referred to as the physiological range of concentration. In 

 Figure 2, the growth rates characteristic of each of a series of auxin concentra- 

 tions are plotted as a function of auxin concentration for each of three auxins. 



l^max. 



A'max. 



ao/ 01 



0-5 

 Auxin concenfnafion 



10 



mg/l. 



Figure 2. Growth rate of Avena coleoptile sections as a function of auxin concentration. The three 

 curves are, from top to bottom, for indoleacetic acid (lAA), 2A-dichlorophenoxyacetic acid (2:4-D), and 

 naphthalene acetic acid {NAA). After Foster, McRae, and Botmer (1952). 



It is evident that growth rate tends to increase towards a limiting value for 

 each auxin and that the curves are hyperbolae such as are characteristic of 

 saturation phenomena. They suggest that auxin interacts with something in 

 the section, that growth rate is proportional to the amount of such interaction 

 products, and that maximum growth rate is achieved when this something is 



296 



