Physiological Approach to Selective Action of 2/f-D 237 



It might be advanced that since the concentration of 2,4-D in the 

 external solution was only 1 mg/1, the difference between susceptible 

 and resistant species would not be maintained if the concentration 

 were increased. This aspect will be more fully explored in the next 

 section. At present it is sufficient to add that when the concentration 

 is raised by fivefold, the pattern of uptake for Hordeum is in no way 

 altered. 



UPTAKE OF 2,4-DICHLOROPHENOXYACETIC ACID BY 

 STEM SEGMENTS OF DIFFERENT SPECIES 



In the studies of uptake by stem tissues the procedures of ob- 

 taining suitable segments followed closely those employed in the 

 standard extension growth tests, i.e., Avena coleoptile, wheat cylinder, 

 and pea internode, using the varieties noted above. The only de- 

 parture was that the first leaf was not removed from the segments of 

 Avena coleoptiles. For other species, such as Gossypium and Hclian- 

 thus, the plants were also raised in sterile sand in the dark at 24.5° C. 

 and segments (10 mm.) excised from just below the cotyledons 6 days 

 after the plants were sown. 



Batches of segments selected at random were placed individually 

 in a series of glass tubes immersed in a water bath maintained at 25° 

 C. These tubes contained the same culture solution (adjusted to pH 

 5.1) as that used in the root studies. To ensure an adequate oxygen 

 supply and effective stirring, the solutions were vigorously aerated 

 with water-saturated air. At each sampling occasion the segments 

 were washed twice for a few seconds each in distilled water, blotted, 

 weighed on a torsion balance, dried at 90° C, reweighed, and the car- 

 bon- 14 assayed by the methods already described. 



It seemed of interest to investigate to what extent any specific 

 differences in the course of uptake are dependent on the concentra- 

 tion of 2,4-D, more particularly when in terms of extension growth 

 the concentrations are suboptimal, optimal, and supraoptimal. On 

 the basis of unpublished work by S. Novoa in this department, it was 

 apparent that for several of these species the optimal concentration 

 lay within the range of 2 to 9 mg/1, and to allow for effective compari- 

 son, the concentrations of labeled 2,4-D were standardized at 0.5, 3, 

 and 45 mg/1. 



The results for Avena and Triticum are given in Figure 3. At each 

 concentration the Avena segments continue to accumulate 2,4-D 

 throughout the 24 hrs. It will be observed that irrespective of con- 

 centration the tissues increase in fresh weight and that though the 

 lowest and highest concentrations are clearly suboptimal and supra- 



