234 G. E. Blackman 



the rate is closely linked with the external concentration, is highly 

 sensitive to the pH of the medium, and between 7.5 and 30° C. the Q^o 

 is 2.3 to 2.6. 



At the time these investigations were being completed, papers by 

 other workers on the uptake of 2,4-D by segments of Avena coleoptiles 

 (5) and Chlorella (7) demonstrated that the patterns of uptake are 

 strikingly different. In Avena coleoptiles after an initial high rate of 

 uptake there is a continued steady accumulation Avith no suggestion 

 of a change from a positive to negative rate. Again, when coleoptiles 

 or C/ilorella after treatment with labeled 2,4-D are transferred to cul- 

 ture solution, there is no marked loss to the solution, while the amount 

 lost is enhanced if unlabeled 2,4-D has been added to the external 

 solution. Although these trends were similar, there were also diver- 

 gences. For Chlorella the magnitude of the changes in the initial 

 rate of absorption induced by an alteration in pH indicated that 

 entry was largely in the molecular form, but for Avena the influence 

 of pH was far less. 



The evidence of these investigations clearly established that the 

 external and internal factors governing the course of uptake of 2,4-D 

 were dependent on the tissue under examination, and the questions 

 that came to mind were the extent to which these differences were due 

 to experimental conditions, the nature of the tissue, or the specific 

 physiological differences at cell level. Previous investigations (2) had 

 demonstrated that in terms of the concentration of 2,4-D causing 

 phytotoxic effects L. minor must be regarded as a susceptible plant, 

 and it seemed of higli significance that Avena from an agronomic 

 point of view is a resistant species. To test the hypothesis that re- 

 sistance and susceptibility are linked with the physiological processes 

 underlying absorption, the first approach has been to examine up- 

 take through the roots of intact plants of selected species known in 

 agricultural practice to have a wide range of tolerance to 2,4-D. The 

 next step has been to establish whether the observed specific differ- 

 ences in root ujjtake also held for stem tissue. Lastly, some account 

 is given of the results so far obtained in the analysis of the specific 

 variation in the mechanisms involved. 



UPTAKE OF 2,4-DICHLOROPHENOXYACETIC ACID BY THE 

 ROOTS OF INTACT PLANTS OF DIFFERENT SPECIES 



Seeds of the individual species, after being allowed to germinate 

 in moist sterile sand, were transferred to a modified Steinberg (2) 

 culture solution. The solution was adjusted to pH 5.6, continuously 

 aerated, and maintained at 25° C. The plants were illuminated with 

 an inicnsity of 750 foot candles for 18 hrs. a day by a bank of daylight 



