New Theory oii Primary Mode of Auxin Action 455 



2. Hu<^"ins, M. L. Hydrogen bonding in high polymers and indusion com- 

 pounds. Jour. Chem.Ed. 34: 480-488. 1957. 



3. Jonsson, Ake. Chemical structure and growth activity of auxins and anti- 

 auxins. Handb. Pfl.-physiol. 14. (In preparation.) 



4. Pybus, M. B., Smith, M. S., Wain, R. L., and Wightman, F. Studies on plant 

 growth-regulating substances. XIII. Chloro- and methyl-substituted phenoxy- 

 acetic and benzoic acids. Ann. Appl. Biol. 47: 173-181. 1959. 



5. , Wain, R. L., and Wightman, F. New plant growth-substances with 



selective herbicidal activity. Nature. 182: 1094, 1095. 1958. 



6. van Overbeek, J. Auxins. Bot. Rev. 25: 269-350. 1959. 



7. Veldstra, H. On form and function of plant growth substances. In: R. L. 

 Wain and F. Wightman (eds.). The Chemistry and Mode of Action of Plant 

 Growth Substances, pp. 117-133. Butterworth Sci. Publ., London. 1956. 



8. , and Booij, H. L. Researches on plant growth regulators. XVII. Struc- 

 ture and activity. On the mechanism of action. III. Biochim. Biophys. Acta. 

 3: 278-312. 1949. 



9. Westeringh, C. van de, and Veldstra, H. Researches on plant growth regulators, 

 XXIII. Structure/activity, VIII. Phosphonic and phosphonous acids. Rec. Trav. 

 Chim. Pays-Bas. 77: 1096-1106. 1958. 

 10. , and Veldstra, H. Researches on plant growth regulators, XXIV. Struc- 

 ture/activity, IX. Tetrazole derivatives. Rec. Trav. Chim. Pays-Bas. 77: II07- 

 1113. 1958. 



DISCUSSION 



Dr. Henderson: We have been studying the mechanism of 2,4-D 

 action at concentrations of 10-^ to IQ-^M. We found that oxygen 

 uptake of oat and pea sections during the time of lAA disappearance 

 was correlated over a six hour period. When one added 2,4-D at 10"^ 

 and 10-5 M, the oat showed a marked decrease in lAA disappearance 

 and the pea showed a more rapid disappearance of auxin. This we 

 called a sparing action of 2,4-D. Recently, a new phenomenon was 

 found which turned out to be the effect of 2,4-D on phototropism in 

 weak blue light. When 2,4-D was sprayed on coleoptiles, phototropic 

 action ceased. When we dipped the coleoptiles in Tween (0.01 per 

 cent) -f 2,4-D the same effect was obtained. In 1957 and 1958 we 

 screened some 30 compounds of six different groups. No single family 

 was necessarily similar in structure and, therefore, there was no func- 

 tional group that satisfied an explanation for this phenomenon. We 

 found that out of some 30 compounds, 15 of these at lO-^M negated 

 phototropism. Only six of these, however, were effective at concentra- 

 tions of 10-5 M or below: lAA, IAN, IBA, NAA, 1-naphthaleneaceto- 

 nitrile, and 2,4-D. Since chemical and physical conditions also must 

 influence this phenomenon, we have experimented with 2,4-D from 

 10-3 to 10-10 M, ^ith the pH from 3 to 9 in buffer solutions. 



My question, based on some of the statements that you have made 

 is. Would this evidence fit into the pattern you have presented? 



Dr. van Overbeek: Offhand, I would certainly think so. You raised 

 two interesting points. One is the antagonism between 2,4-D and 



