uptake and Fate of C^'>-laheled 2,4-D in Bean Stems 285 



fectiveness of 2,4-D. Furthermore, the material appeared to be slightly 

 inhibitory both in the bean stem sections and in the Avena internode 

 test (23). In bean stems no evidence of callus formation at the apical 

 cut surface could be found in the sections which had been exposed 

 to this preparation, while the control stems always show some swelling 

 at this point. In the Avena test a concentration of 3 X lO'^M of this 

 material was slightly inhibitory to growth, while lower concentrations 

 were totally inactive. The maximal activity of 2,4-D in the Avena 

 assay is found between lO*' and 10-^M. It appears likely, therefore, 

 that this material is a detoxification product of 2,4-D and does not 

 have the biological properties of the auxin itself. This conclusion is 

 also supported by the findings of Fang et ah (10) that 3-(/7-chloro- 

 phenyl)-l,l-dimethylurea (CMU) is converted to a similar inactive 

 product in beans and by those of Hay and Thimann (13) on the fate 

 of 2,4-D in vivo. The findings of Andreae and Good (1) that 2,4-D is 

 not detoxified by the formation of the aspartyl derivative nearly as 

 readily as are other herbicides, seem to be contradictory to these find- 

 ings. However, in the absence of any reliable information on the na- 

 ture of this material, and in view of the large differences of experi- 

 mental conditions employed, the two findings may not be incom- 

 patible. 



SUMMARY 



Using an in vitro incubation system it was shown that exposure 

 to 10-'*M 2,4-D for 4 hrs. is sufficient to cause the initiation of ex- 

 tensive callus formation in bean stem sections. The uptake of 2,4-D 

 during this period is dependent on the concentration of 2,4-D sup- 

 plied and on the area exposed. Further, no clear-cut difference be- 

 tween the various internodes of young bean plants could be demon- 

 strated, nor could a difference in uptake from the apical or basal 

 end of the stems be shown. 



After apical application of l-Ci^-2,4-D to the stems, the peak of 

 radioactivity was a few mm. below the point of application and 

 traveled down with time. The width of the peak increased markedly 

 at longer exposures. Apical application of unlabeled 2,4-D accelerated 

 the disappearance of radioactivity from stems which had previously 

 been exposed to Ci^-2,4-D, although the location of the peak of radio- 

 activity remained essentially unchanged. Decarboxylation and release 

 of C"Oo appear to be a minor path in the metabolism of 2,4-D by 

 the bean stems. Most of the radioactivity was recovered in the form 

 of a complex of unknown structure which yielded 2,4-D upon acid 

 hydrolysis. 



