Physiological Approach to Selective Action of 2,f-D 243 



Avena and Gossypium tissues have been described in some detail be- 

 cause they provide clear-cut evidence of specific differences in the 

 physiological processes involved. Other experiments, which can only 

 be reported briefly, aim at a further analysis of differences between 

 Avena and Pisiim. 



Comparative experiments have been undertaken to examine the 

 influence of anaerobic conditions on uptake. If nitrogen free from 

 oxygen rather than air is bubbled through a solution containing either 

 segments of Avena coleoptiles or Pisum stems, then the diminution in 

 the rate of uptake of 2,4-D which is observed after 30 min. is greater 

 for Pisu7n. 



A further approach has been to compare the rates of loss of 2,4-D 

 from segments after they have first been allowed to take up labeled 

 auxin before transference to culture solution. In this instance the 

 specific differences are striking; segments of Pisum stem lose more 

 than Avena coleoptiles. 



In conclusion, the purpose of this paper has been to draw attention 

 to the potential value of comparative studies of the pattern of auxin 

 uptake in elucidating specific differences in the mechanisms of action 

 at all levels. At the present stage neither time nor knowledge permits 

 of a detailed analysis of the implication of these findings. The factors 

 which may contribute to the progressive change from a positive to a 

 negative rate of absorption have been considered both in the previous 

 paper (4) and in a further paper (3) in which it has been shown that 

 the pattern of uptake of 2,3,5-triiodobenzoic acid (TIBA) by L. minor 

 has many features in common with that of 2,4-D. 



It is generally accepted that, apart from the mode of action at cell 

 level, the selectivity of a herbicidal application is dependent on dif- 

 ferential retention, the amounts entering either through the shoot or 

 the root, coupled with the subsequent distribution and accumula- 

 tion in the different parts of the plant. Weintraub and others (8) have 

 followed the level of penetration into leaves, the degree of breakdown, 

 and the extent of the movement of 2,4-D and related compounds in 

 a range of susceptible and resistant species. They concluded that the 

 major difference between the two categories was in the amount trans- 

 ported from the treated leaf to the shoot and that the separation was 

 particularly clear-cut between susceptible and resistant inbred lines 

 of Zea mays. At first sight the present findings that 2,4-D after ab- 

 sorption can be readily released from the stem tissues of susceptible, 

 but not resistant, species are compatible with the view that a restric- 

 tion of movement after penetration into the shoot may in part confer 

 resistance. On the other hand, the studies of root uptake have shown 

 that the amount of radioactive material transported to the shoot at 



