58 



Fundamentals of Auxin Action 



sensitive to the smallest quantities of auxin. All of these tests except 

 the Avena test and the ferric chloride test respond to differences in 

 auxin concentration in a semi-logarithmic function. Thus the Avena 

 test and the ferric chloride test are most effective in measuring small 

 differences, while the other tests permit measurements over a greater 

 range of auxin concentrations. 



TABLE 4 

 Relative Sensitivities of Some Auxin Tests 



* 2,4-D. 



t Beta-naphthoxy acetic acid. 



In selecting a test, care must be taken to choose one which will 

 yield the type of information desired. For example, if one is seeking 

 to measure auxin activity, in the strictest sense, a test which is specific 

 for auxin activity must be selected, such as the various straight-growth 

 and curvature tests. If a measure of epinastic activity is being sought, 

 such tests as the leaf repression test are best. If the capacity of a given 

 substance to set fruit is being measured, the tomato ovary test is very 

 desirable and the Avena test might be quite misleading. 



In many cases a test is needed to determine whether a given com- 

 pound exhibits auxin activity. In order to establish the existence of 

 such activity, it is important that auxin-free material be used, because 

 non-auxins having competitive effects with auxin will promote growth 

 if added to plant material in the presence of a true auxin. For such 

 determinations, it is desirable to use a slit pea test or one of the 

 straight-growth tests with a 4 hour wash period before the test has 



