900 ADVENTURES IN RADIOISOTOPE RESEARCH 



In an interpretation of the figures obtained from a solution free of 

 phosphorus it should be kept in mind that not only inactive P inter- 

 changes with active P, but also other anions present in the nutrient 

 solution, though these can be expected to be less effective in removing 

 active P from the roots than is inactive P (cf. p. 892). Furthermore a mig- 

 ration of anions simultaneously with an equivalent amount of cations 

 from the roots into the nutrient solution must also be envisaged. 



Another series of experiments gave instead of 2.2 the average value 

 of 3.0 for the ratios listed in Table 11. 



A possible explanation of the fact that an increase in the phosphate 

 concentration of the nutrient solution leads to an increased migration 

 of 32p fj-ojn the roots is the following. An increased non-labelled phosphate 

 concentration of the nutrient solution leads to an increased non-labelled 

 phosphate concentration of the liquid phase of the roots. The non- 

 labelled phosphate present in the liquid phase of the roots gets into 

 a kinetic, respectively enzymic, exchange-equilibrium with a part of the 

 labelled free phosphate and the labelled phosphate incorporated in 

 organic compounds, respectively, present in the solid phase of the roots. 

 The replacement of the labelled phosphate of the roots by non-labelled 

 phosphate is promoted by an increase in the non-labelled phosphate 

 concentration of the liquid phase which, in turn, leads to an enhanced 

 exodus of labelled phosphate from the roots into the nutrient solution. 



In distilled water, we would expect the roots to lose less radio-phosphate 

 than in a phosphorus free nutrient solution. As seen from Table 12, the 

 loss of radio-phosphate in distilled water differs from our expectance. 



Table 12. — Ratio of Removal of 

 Labelled Phosphate fkom the Roots 



OF Wheat Seedlings in -Distilled 

 Water and in Phosphate-Free 



Nutrient Solution Respectively 



According to Table 12, the roots lose more ^sp in distilled water than in 

 a phosphorus-free nutrient solution. In another series of experiments, the 

 average value of the above-mentioned ratio was found to be 3.4. The 

 exudation of nucleotides containing radio-phosphorus is, as discussed on 

 p. 903, much smaller than the loss of phosphate and, correspondingly, a 

 precipitation of some of the nucleotides together with the precipitation 



