242 



G. E. Blackman 



The next step was to investigate the competitive influence of POA 

 without pretreatment. Table 1 gives the resuks of an experiment in 

 which the uptake of 2,4-D (3 mg/1) in the presence and absence of 

 POA (150 mg/1) was recorded for segments of Avena and Gossypium. 

 It is once more apparent that the effect exerted by POA is dependent 

 on the species: The induced depression in the rate of entry of 2,4-D 

 is again much greater for Avena. 



A possible explanation for the disparate reactions of the t\\o spe- 

 cies is that Avena coleoptiles absorb far more POA than Gossypium 

 tissues and that in consequence there is a greater competition between 

 POA and 2,4-D for internal sites. Therefore, experiments were under- 

 taken to ascertain the rates of absorption of labeled POA. The data 

 of Figure 7A demonstrate that in fact the accumulation of POA by 

 Avena coleoptiles is somewhat greater. 



If the action of POA in inhibiting the uptake of 2,4-D by Avena 

 coleoptiles is largely confined to competition for internal sites then 

 it would be expected that as the ratio of the external concentrations 

 of 2,4-D to POA is raised, so the balance will shift in favor of the 

 uptake of 2,4-D. That this is so is evident in Figure 7B. When the ex- 

 ternal concentration of POA is kept constant at 150 mg/1 and the 

 concentration of 2,4-D raised from 3 to 27 mg/1, the inhibitory influ- 

 ence of POA on 2,4-D uptake vanishes. 



Experiments on the competitive uptake of POA and 2,4-D by 



- 0.5 



Fig. 7. (A) The uptake of phciiowacelic atid by segments of A. saliva coleoptile and 

 G. hirsutiim stem from tontcntiations of 1 and 3 mg/1. (B) The cllcct of POA 

 (150 mg/1) on the uptake of 2,1-D (3, 9, and 27 mg/1) by segments of A. satwa 

 colco|)|iic. 



