Extraction and Measurement Techniques 41 



tests. Optimum growth is usually obtained at about 10-^ M indoleace- 

 tic acid. At higher concentrations, the roots are inhibited. A sample 

 concentration curve is shown in figure 20. 



The ability of the roots to respond positively to auxin varies con- 

 siderably between different lots of seed, even of the same strain. The 

 effective use of the pea root test is dependent upon finding seeds of 

 appropriate sensitivity to auxin. 



O 



10 "■ 10 



CONC. lAA 



I0~® lO"^ M 



Fig. 20. Sample results of a pea root test showing the semi-logarithmic propor- 

 tionality of growth to the concentration of auxin, indoleacetic acid (Leopold and 

 Guernsey, 1953). 



The pea root test has the same advantages and disadvantages as 

 those enumerated for the other straight-growth tests, but because of 

 the much greater sensitivity this test is usable as an assay of plant 

 extracts (Leopold and Guernsey, 1953). It has also been used for de- 

 tection of auxins separated by paper chromatography (Audus and 

 Thresh, 1953). 



Root Inhibition Tests 



A great many tests have been established for auxin using inhibi- 

 tions of root growth as the assay method. The first quantitative test 

 of this sort was established by Swanson (1946) and most of the tests 

 described subsequently are near adaptations of his technique, gen- 



