124 L. J. Aiidus and J. K. Bakltsh 



heightened lAA-oxidase content but this cannot alone account for 

 sensitivity reduction since the responses of these roots to 2,4-D, which 

 is not destroyed by lAA-oxidase, is similarly lowered. 



Roots grown in low 2,4-D concentrations (3 X ^0"^ g/'^^) show 

 similar reduction in over-all sensitivity of segments to both lAA and 

 2,4-D. Here auxin destruction can play no part, since these roots have 

 an lAA-oxidase content not significantly different from that of nor- 

 mal roots. Analysis of the growth pattern of individual epidermal 

 cells reveals several striking sensitivity changes, particularly in the 

 youngest, immature cells of the segment. These 2,4-D-adapted cells 

 are stimulated to extend as much as normal cells in sucrose alone by 

 lAA concentrations which halve the growth of normal cells. 



Roots adapted in solutions of 2,3,5-triiodobenzoic acid show 

 growth behavior to lAA treatment very similar to that of lAA-grown 

 roots and also a much enhanced lAA-oxidase content. On the other 

 hand, 2,4-dichloroanisole-grown roots, which also have a much height- 

 ened lAA-oxidase content, exhibit a somewhat different pattern of 

 sensitivity changes. 



It is concluded that these changes in growth response sensitivity 

 are predetermined in the meristem and their effects persist during 

 subsequent extension. They are complex in nature and in all cases 

 due, at least in part, to real changes in the reactivity of the growth 

 centers to the auxin molecules. 



LITERATURE CITED 



1. Audus, L. J., and Das, N. The interactions of auxins and anti-auxins in the 

 stimulation of root growth. Jour. Exper. Bot. 6: 328-347. 1955. 



2. , and Shipton, M. E. 2,4-Dicliioroanisole — auxin interactions in root 



growth. Physiol. IMant. 5: 430-455. 1952. 



3. , and Thresh, R. The effects of synthetic growth-regulator treatments on 



the levels of free endogenous grow th substances in plants. Ann. Bot. II. 20: 

 439-459. 1956. 



4. Burstrom, H. On the adaptation of roots to /3-indolyl-acetic acid. Physiol. 

 Plant. 10: 187-197. 1957. 



5. Cohn, M., and Monod, J. Specific inhibition and induction of enzyme biosyn- 

 thesis. Symp. 3: Soc. Gen. Microbiol. Cambridge University Press, pp. 132-149. 

 1953. 



G. Galston, A. W. Some metalK)lic consequences of tlic administration of indole- 

 acetic acid to plant cells. Iti: R. L. Wain and F. ^Vightman (eds.), The Chem- 

 istry and Mode of Action of Plant Growth Substances, pp. 219-233. Butterworth 

 Sci. Publ., London. 1955. 



7. , and Dalbcrg, L. Y. The adaptive formation and physiological significance 



of indoleacetic acid oxidase. Amer. Jour. Bot. 41: 373-380. 1951. 



8. McRae, D. H., and Bonner, J. Chemical structure and antiauxin activity. 

 Physiol. Plant. 0: 48.5-510. 1953. 



DISCUSSION 



Dr. Galston: I am very happy to learn of Dr. .Audus' confirma- 

 tion of our report on the enhanced activity of indoleacetic acid 



