uptake and Fate of C'-Iabeled 2,4-D in Bean Stems 



Table 1. The effect of 2,4-D concentration and exposed 

 area on 2,4-D uptake by bean stem sections. 



279 



posed would be much greater. However, when stems were exposed to 

 2,4-D in liquid, no proliferation took place if they were maintained 

 in liquid (even in the absence of 2,4-D after the initial exposure). If 

 such stems were planted on agar after exposure to 2,4-D in liquid, 

 proliferation did take place. The reason for this difference between 

 liquid and solid culture is not clear and is being investigated further. 

 No clear-cut differences could be established between the effect 

 of 2,4-D on stem sections taken from various internodes of the plants. 



Polarity of Uptake and Transport, and Localization of Radioactivity 



No effect of polarity on the uptake of 2,4-D could be found. The 

 average of eight replicate determinations of ten stem sections each 

 showed a specific activity of 0.303 ± 0.022 cpm/mg fresh weight 

 when the stems were planted with their apical ends down, and 0.307 

 ± 0.031 cpm/mg fresh weight when planted with the basal end 

 down. Figures 2 and 3 show the effects of various lengths of in- 

 cubation with labeled and unlabeled 2,4-D on the distribution of 

 the radioactivity along the stems. To permit a better comparison 

 of the distribution curves shown in Figure 2, the results were factored 

 to adjust all the peaks of radioactivity to the same level (1.0). A 

 clear broadening of the radioactive zone with time is apparent, 

 and the peak of activity appears to shift about 0.5 mm. down the 

 stem in the first 4 hrs. If application of labeled 2,4-D is preceded 

 by application of unlabeled 2,4-D the pattern obtained very closely 

 resembles the one for the equivalent exposure to radioactive 2,4-D 

 (Figure 3). Removal of the 2,4-D, and particularly application of 

 unlabeled 2,4-D after the radioactive material, resulted in a dis- 

 tinct spreading of the peaks. The unlabeled material appears to ac- 

 celerate the loss of the radioactive material from the stems. However, 

 it is of interest that the position of the radioactive peak remains 2 



