I08 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



The Ca ion interfered competitively with Sr* absorption, indicating identical 

 binding sites on the carriers for these two ions. On the other hand, Mg at the 

 same concentration interfered to a lesser extent with the absorption of Sr* than 

 did Ca, and its interference was essentially non-competitive. 



The last finding is of interest in connection with the evidence presented in 

 figure 3. It was shown there that in the non-metabolic exchange adsorption 

 Ca and Mg were equally effective in displacing Sr* held by the root exchange 

 surfaces. On the basis of this experiment, Ca and Mg would be expected to be 

 about equally effective in reducing the rate of Sr* absorption by the roots, if 

 this exchange were a rate-limiting step in the sequence of events constituting 

 the absorption process. Figure 4 shows, on the contrary, that Mg was far less 

 effective in interfering with the absorption of Sr* than was Ca. It is concluded 

 that an initial exchange adsorption on the root exchange surfaces is not a rate 

 limiting step in cation absorption. 



The work on ion absorption in terms of the formation of intermediate labile 

 complexes between the ions and carriers, as set forth in this paper, is presently 

 being extended to include phosphate (18) and sulfate (28). In the case of phos- 

 phate, the evidence indicates distinct binding sites for the two ionic species 

 H2P04~ and HP04°°, and interference by hydroxyl ions with both phosphate 

 species. The work on sulfate again brings out the selectivity of the transport 

 mechanism. Selenate competes with sulfate; phosphate, on the other hand, in- 

 creases the rate of sulfate absorption, the more so the wider the ratio of phos- 

 phate to sulfate in the solution. 



RELATION TO OTHER WORK 



The specificities revealed in these studies are concordant with general ex- 

 perience in the field of 'ion antagonisms' in plant nutrition. The hypothesis put 

 forward, of ion-binding by specific groups or sites of 'binding compounds' or 

 'carriers' is in fact at present the only rationale of selectivity and mutual inter- 

 ferences and places the subject of the specificity of ion absorption by roots in 

 the general context of present thinking about metabolites and their analogs 

 (antimetabolites) (2, 32, 47). The particular specificities discovered are not 

 unexpected on the basis of previous experience and especially in the light of the 

 work of Collander (13). CoUander, in long-term solution culture experiments 

 involving numerous species and genera of higher plants, consistently found a 

 far-reaching parallelism between the absorption of K, Rb and Cs; the absorp- 

 tion of Na, on the other hand, bore no consistent relation to the absorption of 

 the other three ions. Compare this with the present conclusion (16) that K, 

 Rb and Cs are bound by identical binding sites, whereas Na does not effectively 

 compete for the binding sites common to those three ions. Among the alkaline 

 earth cations, the absorption of Ca and Sr went hand in hand, in Collander's 

 experiments, but the absorption of ^Nfg was not consistently related to the ab- 



