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



235 



fluorescent tubes. At the end of 6 to 8 days, matched plants in pairs 

 were transferred to a series of modified specimen tubes, each of which 

 contained an aerating device which prevented splashing but ensured 

 adequate stirring of the same culture solution (pH 5.6) containing 

 1 mg/1 of 2,4-D labeled in the carboxyl group with carbon-14. For 

 the experiment the tubes were kept at 25° C. in a water bath, and 

 the shoots were continuously illuminated at an intensity of 500 foot 

 candles. Replicated samples were withdrawn at 0.5, 1, 2, 4, 8, 16, and 

 32 hrs., the roots washed twice for a few seconds each in a large vol- 

 ume of distilled water, the surplus water removed by blotting be- 

 tween filter papers, the plants divided into root and shoot, and the 

 remains of the seed or the cotyledons separated. The parts were 

 dried at 90° C. and then assayed separately. The method of assay 

 involved liquid combustion and the conversion of the carbon dioxide 

 to barium carbonate which was filtered onto a disk of filter paper. 

 The disk was then secured in a planchette by a brass ring and counted 

 under constant geometry with an end-window Geiger counter (4). 



As examples of the course of uptake exhibited by graminaceous 

 species, acknowledged agronomically to be resistant to 2,4-D, four 

 species have been included in Figure 1, namely Triticiim vulgare, 

 'Eclipse'; Hordeum vulgare, 'Proctor'; Avena sativa, 'Victory'; and 

 Oryza sativa, 'Heenati 3224.' 



0.2 



P = 0.05 



AVENA 



ORYZA 



24 



24 



Fig. 1. The course of uptake of 2,4-D from a solution containing 1 mg/l by roots 

 of intact plants of T. vulgare, H. vulgare, A. sativa, and O. sativa. The signifi- 

 cant differences given in this and subsequent figures refer to the differences bet^veen 

 any two treatments. 



