Metabolism and mode of action 



specific activity of the then available labelled material did not exceed 

 0-78 mc/mmol, it was not feasible, with the existing technique of assay, to 

 estimate accvirately uptake over short periods. Most of the measurements 

 were made after 1 1-24 hours. As with Lemna minor, the amount absorbed in 

 21-5 hours is accelerated as the pH is reduced from 6 to 4, but once again the 

 rate is not directly proportional to the calculated concentration of undis- 

 sociated molecules in the solution. In Avena coleoptiles cultured in a 

 buffered sucrose solution at concentrations up to 0-25 p. p.m., the gains in 

 extension growth after 22 hours match the amounts absorbed, but at higher 

 concentrations (up to 2 p. p.m.) this linearity no longer holds. When 

 observations are made at 1 1 and 22 hours the rate of absorption by Triticiim 

 coleoptiles does not fall off appreciably with time. From this aspect it 

 would seem that this tissue resembles artichoke discs more than Lemna minor, 

 since Hanson and Bonner (1955) have reported that the uptake of 2:4- 

 dichlorophenoxyacetic acid continues for 18 hours and that it is only when 

 the discs are transferred to a solution containing unlabelled material that any 

 loss takes place, and then it is only on a small scale. It may well be that such 

 differences are not so much a question of a varying specific reaction but 

 rather the physiological level of the growth regulator. In coleoptiles and 

 artichoke discs the concentrations were such as to accelerate either extension 

 growth or water uptake and it is possible that loss to the external solution 

 only takes place at inhibitory concentrations. If this proves to be the case 

 then a new approach to the interpretation of the toxic action of 2:4-dichloro- 

 and other phenoxyacetic acids may be provided. In this connection it is 

 significant that a concentration which induces in Lemna minor a loss to 

 the external solution after 1-2 hours does not cause any detectable reduction 

 in the growth rate for at least four days, by which time the plants have 

 tripled in weight. 



REFERENCES 



AsANA, R. D., Verma, G., and Mani, V. S. (1950). Some observations on the 

 influence of 2 : 4-dichlorophenoxyacetic acid (2 : 4-D) on the growth and develop- 

 ment of two varieties of wheat. Physiol. Plant. 3, 334. 



Blackman, G. E., and Robertson-Cunninghame, R. C. (1953). The influence of pH 

 on the phytotoxicity of 2 : 4-dichlorophenoxyacetic acid to Lemna minor. New 

 Phytol. 52, 71. 



Blackman, G.E., and Robertson-Cunninghame, R. C. (1954). Interactions in 

 the physiological effects of growth substances on plant development. J. exp. 

 Bot. 5, 184. 



Broyer, T. C. (1951). Mineral Nutrition of Plants, University of Wisconsin Press, 



p. 187. 

 Freiberg, S. R., and Clark, H. E. (1952). Effect of 2: 4-dichlorophenoxyacetic acid 



upon the nitrogen metabolism and water relations of soybean plants grown at 



different nitrogen levels. Bot. Gaz. 113, 322. 

 Hanson, J. B., and Bonner, J. (1955). The nature of the lag period in auxin-induced 



water uptake. Amer. J. Bot. 42, 411. 

 Klingman, G. C, and Ahlgren, G. H. (1951). Effects of 2:4-D on dry weight, 



reducing sugars, total sugars, polysaccharides, nitrogen and allyl sulfide in wild 



garlic. Bot. Gaz. 113, 119. 

 Nance, J. F. (1949). Inhibiuon of salt accumulation in excised wheat roots by 2:4- 



dichlorophenoxyacetic acid. Science, 109, 1 74. 



258 



