48 



P. Larsen and T. Aasheim 



Bioassay 



One of the findings that indicated the aldehyde nature of the 

 neutral auxin in Pisimi extracts in earlier work was the fact that it 

 could be converted to lAA by treatment with soil (21) . The activity 

 of a soil-treated preparation was many times higher than that of an 

 untreated, neutral fraction. Using the Avena coleoptile ciuvature test, 

 we therefore determined the auxin activity of synthetic lAAld and 

 IAN both directly and after treatment with soil. The soil treatment 

 was carried out as described by Larsen (21) : The auxin was transferred 

 to a 1-mm. thick agar platelet (1 sq. cm.) by the ether-dropping 

 method. The 1-mm. platelet was covered with an agar platelet of the 

 same area, but only 0.5 mm. thick. A suitable amotint of soil was 

 placed on top of the cover platelet. The soil was always taken from 

 one of the vials in which the day's test plants were grown. This means 

 that the physical and biological conditions in the soil samples were 

 standardized. After the lapse of 90 min. the cover platelet with the 

 soil was removed and the lower 1-mm. platelet cut into test blocks, 2 

 X 2 X 1 mm., which were later applied to twice-decapitated test 

 plants. 



The curves marked lAAld in Figure 1 represent the results of 

 tests with this compound. The solid line is the concentration-activity 

 curve obtained directly, without soil treatment, and the broken line, 

 the corresponding curve for soil-treated material. It will be noted that 

 the activity of the preparation was increased by several hundred per 

 cent by the soil treatment. In this respect the synthetic lAAld re- 



UJ 



q: 

 o 



UJ 



o 



(r 



3 



o 



12 



CONCENTRATION 



Fig. 1. EfTctt of soil ircaimciits on auxin activity of Visum extracts and of iiulolc- 

 3-acelaldchyde. Solid lines represent activity obtained directly and the broken 

 lines activity after treatment with soil. 



