54 P. Lay sen and T. Aasheim 



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DISCUSSION 



Dr. Thimann: How do you account for the disappearance of 

 activity in the various fractions? What happens in the soil? 



Dr. Larsen: As regards lAAld, I imagine that it diffuses from the 

 agar platelet into the soil where we know it becomes oxidized to lA.A.. 

 lAA will diffuse from the soil to the agar platelet, thereby increasing 

 the auxin activity of the agar. But if we leave the soil in contact with 

 the agar for a time considerably longer than 90 mintites, the auxin 

 activity decreases again, indicating that lAA is being inactivated by 

 the soil. The inactivation must be assumed to take place also during 

 the time when the auxin activity in the agar is increasing, but we 

 have a steady state (lAAld-^ IAA-> inactive products) for some time. 

 Plotted against time, the auxin activity shows a broad optimiun 

 around 90 minutes after application of the soil. 



As regards IAN, there are two possibilities. (1) If we assume that 

 IAN is converted to lAA by the soil, this conversion Avill not be mani- 

 fested as an increase in activity, because these two substances are 

 equally active in our test. On the contrary, as soon as some IA.\ has 

 been formed, it will be subject to inactivation, thus loAsering the total 

 auxin activity of the agar-soil system. (2) IAN may be inactivated 

 ^\•ithout a preceding conversion to lAA. In both cases the auxin 

 activity will be steadily decreasing. 



Dr. Bcnnet-Clark: Why did you use soil in preference to one of the 

 conventional oxidizing agents such as alkaline iodine solutions or 

 hydrogen pcroxitle? 



Dr. Larsen: Because the soil treatment was a simple procedinc. and 

 because we wanted to show that the synthetic lAAld reactetl in the 

 same way as the material in plant extracts which we had studied in 

 the past. But of course it will be important to accomplish the oxida- 

 tion also by other means, such as aldehyde dehydrogenase or inorganic 

 oxidizing agents. Gray has shown that his synthetic I.\Ald can be 



