Extraction and Measurement Techniques 43 



2,4-D concentrations ranging from 10-^ to 10-^ M. This is approxi- 

 mately 0.002 to 0.2 mg./liter. The procedure of Moewus for cress roots 

 is much more sensitive, having an effective range from lO-^^^tolQ-'^M 

 of indoleacetic acid. 



It should be pointed out that this test is not specific for auxins, 

 and other compounds in a test solution which may inhibit root growth 

 can influence the results obtained. However, if the experiment is car- 

 ried out with sufficient controls, this defect should not be serious. It 

 is a very valuable technique for field herbicide studies and it is so 

 accinate quantitatively that it has provided an excellent technique for 

 studying inhibition efl^ects on root growth. 



A j)articularly eflective modification of the test has been developed 

 by Audus (1951). This involves placing cress seedlings on the rim of 

 a (vlinder of filter paper in a glass tube containing the solution for 

 testing. Consistently straight roots can be obtained, and measurements 

 can be made at intervals through the glass without disturbing the 

 test plants. 



A concentration curve for a r(jot inhibition test is given in figure 

 21. 



Leaf Repression Test 



A method of comparing growth regulator activity by using a leaf 

 repression technique was described by Thompson et al (1946), but it 

 remained for Rice (1948) and Brown and Weintraub (1950) to estab- 

 lish this method as a quantitative assay. The test has generally been 

 used as a means of comparing activity of chemicals such as potential 

 herbicides, but it has also been used as an assay for 2,4-D penetration 

 into plants (Rice, 1948). 



The basis for such a test lies in the capacity of auxins to inhibit 

 leaf enlargement. It should be pointed out, of course, that inhibition 

 of leaf enlargement is a characteristic not confined to auxins alone and 

 consequently it should be remembered in describing results of such a 

 test that auxin activity in the strict sense is not necessarily being 

 measured. 



The technique of Brown and Weintraub will be described here 

 because of its slightly greater simplicity and quantitative accuracy. 



PROCEDURE FOR THE LEAF REPRESSION TEST 

 Plant material 



Seedlings of garden bean are grown in the greenhouse until the first trifoliate 

 leaf is beginning to enlarge and the intcrnode below it is 3 to 7 mm. long. Black 



