92 Fawcett, Wain, and Wightman 



is due solely to its conversion to indole-3-acetic acid in the test tis- 

 sues or whether it is due partly to the production of the acetic acid 

 and partly to the fact that the butyric acid molecule itself possesses 

 growth-promoting activity. I favor the view that indole-3-butyric 

 acid is active per se, because I find it difficult to explain the high ac- 

 tivity of this acid in the wheat and pea tests in terms of the small 

 amount of lAA produced in metabolism experiments. 



Dr. Thimann: It would mean 100 per cent conversion. 



Dr. Wightman: Yes, it would, because if you examine the activity 

 of indole-3-acetic and indole-3-butyric acids over a wide range of con- 

 centrations, you find that the butyric acid is more active than the 

 acetic acid at lower concentrations. Unless you assume that the bu- 

 tyric acid penetrates the tissue more readily and is then converted al- 

 most 100 per cent to the acetic acid, it is difficult to explain the high 

 activity of the butyric acid at low concentrations, solely in terms of its 

 conversion to lAA. 



Dr. Thimann: But still you do have two peaks in the bioassay of 

 the chromatogram of metabolized indole-3-butyric acid. 



Dr. Wightman: That is correct. The first peak is due to the small 

 amount of indole-3-acetic acid produced from the metabolism of the 

 butyric acid and the second peak is due to the residual, unchanged 

 butyric acid. Now, as we have already discussed, the activity shown 

 by the residual butyric acid may be due either to the compound it- 

 self or the result of its conversion to lAA in the tissue used in the bio- 

 assay. 



Dr. Andreae: We tried solutions and could never find indole-3- 

 acetylaspartic acid. Do you analyze solutions only, not the tissues? 



Dr. Wightman: No, we do analyze both the residual solutions and 

 the treated tissue. In our extraction procedure we first remove the 

 segments of tissue from the residual solution and immediately grind 

 them uj) with acid-washed sand. The macerated tissue is then added 

 back to the residual solution and after acidification the homogenate 

 is extracted several times with ethyl acetate. One point I would like 

 to make is that our pea metabolism experiments differed from yours 

 in that we used much less tissue. According to your papers, you used 

 100 g. of pea stem tissue per treatment, whereas we used only approxi- 

 mately 1 g. of tissue per treatment. The reason why we extracted both 

 the treated tissue and the residual solution was because we found in 

 earlier metabolism experiments with phenoxy compounds that we 

 obtained metabolites in the solution, and since we were anxious to 

 obtain as much of these metabolites as possible, we extracted not only 

 the tissue but also the residual solution. This, of course, resulted in 



