EMANUEL EPSTEIN 



107 



The initial rapid uptake and its reversal are interpreted as due to non- 

 metabolic cation exchange in which the root acts as a solid exchanger, like clays 

 or synthetic exchange resins. The slower linear phase of uptake is considered 

 to represent absorption proper, in the sense of active transport. Evidence for 

 the non-metabolic exchange character of the fast process is as follows: i) The 

 time-course of this phase is that typical of exchange reactions, and an equilib- 

 rium is quickly approached. 2) The Sr* taken up by this mechanism is ex- 

 changeable with Sr, Ca, Mg, Na, K, and H ions, and the relative effectiveness 

 of the divalent and monovalent ions in displacing Sr* from the roots suggests 

 the lyotropic series which is common in exchange reactions on solid exchangers, 

 j) Whereas the relatively slow, steady-state phase of absorption is abolished 



Fig. 4. Double reciprocal plot 

 of interference by Ca and Mg with 

 Sr absorption by excised barley 

 roots. Sr concentration (S), 0.25 to 

 1. 00 mEq/1. Rate of Sr absorp- 

 tion, V, in /iEq./gm fresh weight/3 

 hr. 



^S. 



by anaerobic conditions, the same is not true of the fast, initial exchange re- 

 action. 



In order to determine the rate of the metabolic absorption of Sr* unobscured 

 by the additional exchange increment, we let the roots absorb Sr* under the 

 given experimental conditions, and then 'stripped off' the non-metabolically 

 held, exchangeable Sr* by means of a 30-minute exposure to non-radioactive 

 Sr. In this manner, we measured only the non-exchangeable fraction, i.e. the 

 fraction that had been absorbed as distinguished from the exchangeably ad- 

 sorbed Sr* fraction. Measured in this way, absorption was found to be a linear 

 function of time and a single measurement, at the 180-minute point, gave an 

 adequate determination of the rate, v, of Sr* absorption. 



Figure 4 shows a double-retiprocal plot of Sr* absorption as a function of the 

 concentration of Sr* (S), in the absence and presence of Ca and Mg as inter- 

 fering ions. Straight lines were obtained as in the case of the monovalent ions. 



