Theories of Auxin Action 



177 



of obtaining the values is based on an elaboration of the assumptions 

 made in the equation given. The derivation methods are described 

 clearly by Foster et al (1952) and by Bonner (1953). By measuring 

 growth rates over a range of auxin concentrations, and plotting the 

 results by the double inverse method of Lineweaver and Burk (1934), 

 the maximal growth rate may be estimated as the intercept of the 

 plotted line with the abscissa. The method is illustrated in figure 78. 

 The K value may be estimated by the slope of the plotted line. An 

 auxin with a weak attraction for the receptor should have a relatively 

 steep slope of the plotted line. Also a weak auxin should yield a low 

 maximum velocity of growth, i.e. the intercept of the plotted line with 

 the abscissa should be high, the growth rates being plotted on an 

 inverse scale. 



Maximal growth rates and K values have been measured for several 

 auxins by Foster et al (1952), who found that auxins which produced 

 the greatest maximal growth rates also had the highest affinity for the 



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or 



o 

 a: 



CD 



o 



LU 

 (J) 

 QC 

 LU 

 > 



2i" 



Auxin 

 Promotion 



max 



20 



40 



60 



80 



100 



INVERSE OF AUXIN CONC. (l/g) 



Fig. 78. The promotion and inhibition of ;3^owth by indoleacetic acid plotted by 

 the double inverse method. Growth rate of Avena coleoptile sections as inverse of 

 mm. per 12 hrs., auxin concentration as inverse of mg. per 1. (from Foster et al, 

 1952). 



