io6 



ELECTROLYTES IN BIOLOGICAL SYSTEMS 



Burk; 2) potassium competitively interferes with the absorption of Rb; j) ex- 

 cept at high Na concentrations the interference of Na with Rb absorption is 

 not competitive. Like K, Cs interferes competitively with Rb absorption; Li 

 interference is not competitive and at low concentrations of Rb, a positive 

 (accelerating) effect of Li on the rate of Rb absorption is observed. 



In an extension of this work to include the absorption of anions (14) the 

 absorption of Br from solutions of KBr exhibited the same response to increas- 

 ing concentrations of the salt. Interference by CI with Br absorption was com- 

 petitive; NO3, on the other hand, did not compete for the halide binding sites. 



e 



o 



- 100 <^ 



60 120 180 



Fig. 2 FiG. 3 



Figs. 2-3. Uptake and loss of labeled strontium by excised barley roots. Black circles and 

 solid lines: roots in solutions of labeled strontium (Sr*). Open symbols and broken lines: roots 

 in unlabeled solutions. Concentration of all salts: i mEq/1. 



PASSIVE EXCHANGE VS. ACTIVE TRANSPORT 



The assumption has been made above that the overall process of active ion 

 absorption by plant roots is essentially irreversible. Hoagland and Broyer (23) 

 found that barley roots do not lose significant amounts of K to distilled water 

 and in the experiments of Broyer and Overstreet (9) only about 10% of pre- 

 viously absorbed K was subject to ready loss through ion exchange. 



We recently obtained different results in a study of the absorption of alkaline 

 earth cations by barley roots (17). The difference, however, is apparent rather 

 than real. Figure 2 shows the time course of the absorption of Sr labeled with 

 gj.89 (Sr*). The concentration of Sr*Cl2 was i mEq/1. When after 60 minutes 

 the roots were exposed to a solution of unlabeled SrCl* of the same concentra- 

 tion (Sr), a large fraction of the Sr* taken up during the first 60 minutes was 

 shown to be readily exchangeable with ambient Sr. The time course of the 

 desorption of e-xchangeable Sr* was similar to that of the initial rapid uptake. 

 This fast process was essentially completed in 30 minutes and further loss of 

 Sr* was negligible. Figure 3 shows that Ca and ]\Ig were as effective in remov- 

 ing the exchangeable Sr* fraction from the roots as was nonradioactive Sr. 



