90 Mineral Nutrition of Plants 



For cations, 



R • H + K + > R • K + H + 



and for anions, 



R' • OH + Or > R' • CI + OH _ 



where R and R' symbolize the plant root. It was not known whether 

 the combinations R-K and R'Cl represented a chemical binding or 

 some physical entrapment. With the introduction of the use of radio- 

 active isotopes, certain important advances have resulted in our com- 

 prehension of the fixation of ions by plants. These advances have largely 

 resulted from the study of isotopic exchange reactions. 



The significance of isotopic exchange reactions 

 in the study of the absorption process 



The study of isotopic exchange reactions has been employed exten- 

 sively by chemists to determine the nature of chemical binding. The 

 most general expression for an exchange reaction may be expressed by 

 the following equation, where A* represents the tagged or radioactive 

 isotope and A represents the untagged or stable isotope: 



A*Y + AX^=±AY+ A*X 



Starting with A*Y, measurements of the rate of exchange can be 

 accomplished by noting the appearance of A*X on the right-hand side 

 of the equation. This rate is indicative of the type of chemical combi- 

 nation between A* and Y. In general, if A* and Y are held together 

 by electron pair bonds, the exchange reaction does not proceed unless 

 some side reaction is possible that involves a breakage or elimination 

 of the covalent bond. If the bond between A* and Y is electrovalent 

 in character, the reaction ordinarily proceeds very rapidly, provided the 

 ions entering into the reaction are physically accessible for exchange. 



Isotopic exchange reactions were used for the first time to study the 

 nature of the combination of inorganic elements with plant roots by 

 Hevesy in 192^ (18). Hevesy studied the rate of the reaction 



R • Pb* + Pb ++ > R • Pb + Pb* ++ 



where R symbolizes the plant root. Hevesy found that with Vicia faba 

 plants, complete isotopic equilibrium was attained in 24 hours between 



