48 Fundamentals of Auxin Action 



The range of sensitivity of these tests is from 0.2 to 100 mg./liter 

 of indoleacetic acid. Sample curves for each of these techniques are 

 found in figure 24. The range of sensitivity is too limited to be of 

 use in assaying most plant extracts, but the method is very useful for 

 in vitro studies such as the enzymatic inactivation of indoleacetic acid 

 in solutions. The tests are fairly specific for indoleacetic acid, although 

 some color is obtained with indole, skatole, and indolebutyric acid. 



>- 

 O 



z 

 < 



CD 

 (H 

 O 

 C/) 

 CD 

 < 



1.0 



0.8 



0.6- 



0.4 



0.2 



•' Sulfuric 

 Acid 



A^* 



10 



20 



30 



40 MG/L 



CONC. OF lAA 



Fig. 24. Results of ferric chloride color tests for indoleacetic acid showing the 

 relative effectiveness of perchloric acid and sulfuric acid as reaction solvents (Gordon 

 and Weber, 1951). 



The perchloric acid technique of Gordon and Weber is more specific 

 for indoleacetic acid, is slightly more sensitive, and the color developed 

 is more stable than in the Tang and Bonner technique. 



A color test for 2,4-D using chromotropic acid has been sug- 

 gested by Freed (1948), and this test has been used as a quantitative 

 assay by Marquardt and Luce (1951) and by Stewart et al (1952). The 

 method involves taking the 2,4-D up in carbon tetrachloride, evaporat- 

 ing to dryness, and then adding chromotropic acid which develops 

 a purple color with 2,4-D. The test is sensitive to concentrations of 



