Physiological Approach to Selective Action of 2,4-D 



239 



optimal, the trends for uptake are similar. For Triticum at 0.5 and 

 3 mg/1 there is again a steady accumulation; at 45 mg/1 between 3 and 

 6 hrs., 2,4-D is lost from the tissues, but subsequently uptake recom- 

 mences. From the fresh-weight data it is to be observed that after be- 

 ing subjected for 6 hrs. to the highest concentration the tissues start 

 to lose water, and though the negative rate of uptake between 3 and 

 6 hrs. could reasonably be ascribed to the onset of these conditions, it 

 is somewhat surprising that uptake should proceed at an appreciable 

 rate when water is being lost from the tissues. 



Comparison of Figure 3 with Figure 4 reveals that just as in the 

 case of uptake by the roots the pattern of uptake for Gossypium and 

 Hclianthiis segments is strikingly divergent from that of Avena and 

 Triticum. There are the three characteristic phases: an initial high 

 rate of uptake, a period when the rate falls to zero at crt. 12 hrs., and 

 a third phase when there is a loss from the tissues. Though the 

 changes are most evident at 45 mg/1, statistical analysis shows that 

 for both species the losses between 12 and 24 hrs. at the lower concen- 

 trations are significant. It should also be noted that at each concen- 

 tration there is a progressive gain in fresh weight over the 24 hrs. 

 For Gossypium the concentration range is from sub- to supraoptimal. 



Supporting evidence that other susceptible species can be charac- 

 terized by the pattern of uptake is provided in Figure 5. For both 



cr 



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Fig. 5. The course of uptake of 2,4-D by etiolated stem segments of (A) B. alba 

 and (B) P. sativum, treated with concentrations of 0.5, 3, and 45 mg/1. 



