Physiological Approach to Selective Action of 2,4-D 



247 



touch on this problem that the increased rate of extension growth of 

 Avena coleoptiles is dependent on the external concentration of 2,4-D 

 but that at each concentration the response is linear with time over 

 several hours. If the rate of extension growth is dependent on the 

 total internal concentration of 2,4-D, then the rate of extension growth 

 should be curvilinear, not linear, since 2,4-D is steadily accumulating 



in the tissues. 



Dr. Andreae: In connection with Dr. Bennet-Clark's interesting ob- 

 servations on lAA uptake and secretion, I should like to report our 

 findings with lAA-treated tissues. Pea root tips accumulate lAA to ten 

 times the external concentration. lAA, once inside the root tips, is 

 metabolized and, according to our quantitative data, is not lost again 

 to the external solution. 



Dr. Bonner: Prof. Blackman has shown us some most interesting 

 results. We can conclude that the 2,4-D is actively accumulated by 

 many kinds of cells and that from some kinds of cells it leaks out again. 

 He has also shown us, beyond any doubt, that the uptake of one auxin 

 can be inhibited by another. I think everyone that has investigated 

 this subject would agree that the various auxins and anti-auxins are 

 competitive with one another in their uptake into the cell. Now, I 

 feel quite confident that someone is thinking that the way auxins 

 interact with one another in controlling growth or the way anti- 

 auxins inhibit the growth-promoting activities of the auxins, is merely 

 by preventing the uptake of the active material into the cell. Well, I 

 wish to state that this isn't so. The effect of anti-auxins in inhibiting 

 growth is due to some different phenomenon than the effect of anti- 

 auxins on inhibiting the uptake of active auxins. That this is so can 

 be shown by simple quantitative relationships. The concentration of 

 lAA which is required to half-inhibit the growth promotion caused 

 by 2,4-D in Avena coleoptile section, is somewhat lower than the 

 2,4-D concentration used. The concentration of lAA required to in- 

 hibit by half the rate of 2,4-D uptake by the Avena coleoptile section is 

 of the order of one thousand times the concentration of the 2,4-D 

 used. The 2,4-D uptake is less sensitive to the presence of lAA than is 

 the promotion of growth of the section by 2,4-D. So apparently there 

 are two entirely different processes. 



