G. E. BLACKMAN 



University of Oxford 



A New Physiological Approach 



to the Selective Action of 



2,4-DLchloropherLOxyacetic Acid 



At the conference in 1955, a preliminary account (1) was given of the 

 first experiments on the uptake of growth substances, particularly 

 2,4-dichlorophenoxyacetic acid (2,4-D) by the aquatic plant Lemna 

 minor. Subsequently the investigations were extended and the results 

 have been described in a more recent paper (4). Since the present study 

 stemmed from this work, it is necessary to recapitulate some of the 



main findings. 



When L. minor is grown under conditions of constant light and 

 temperature and 2,4-D, labeled with carbon-14 in the carboxyl group, 

 is added to the culture solution, there is an initial phase of rapid up- 

 take in the first 30 min., followed by a second phase when the rate 

 decreases progressively to zero after 2 to 3 hrs., and finally by a third 

 phase, extending up to 24 hrs., when there is a net loss of activity 

 from the tissues. This loss of carbon-14 from the plants is not due to 

 decarboxylation and the escape of Ci^Oo but to the egress of 2,4-D 

 into the external solution. This is not a toxic effect, since many of the 

 combinations of concentration and length of exposure investigated 

 either have no effect on, or significantly increase, the growth rate 

 when plants are subsequently placed in culture solution. 



If plants, after exposure to the labeled compound for 30 to 60 

 min., are transferred to culture solution, then within 4.5 hrs. over 90 

 per cent of the labeled 2,4-D originally present in the tissues is found 

 in the external medium, but if this medium contains unlabeled 2,4-D, 

 the rate of loss is retarded. The outward movement is not affected by 

 the external pH, is slowed down but not arrested at 1.25° C, and up 

 to 22.5° C. the Qio is 1.6 to 1.9. Turning to the initial phase of uptake, 



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