EMANUEL EPSTEIN 109 



sorption of the other two ions. Again, our findings (17) are in iiarmony with 

 CoUander's. Collander did not study the absorption of anions in those experi- 

 ments. Our results (14) indicate competition among ("1, Br, and I, and no com- 

 petition on the part of NOg for the haUde binding sites. The paper by Aberg 

 (i) should be consulted for previous evidence concerning ion specificity in the 

 case of the monovalent anions. 



ABSORPTION vs. FUNCTION OF THE ELEMENTS 



The mechanism of absorption has been discussed here without any reference 

 to the essentiality or metabolic function of the several ions. In each of the groups 

 of ions which were found to be competitive with one another in the absorption 

 process, there is at least one ion known to be essential for the growth of plants. 

 This immediately points to one mechanism for the toxicity of the other ions 

 within that group of competitive ions. By competing with the essential element 

 in the absorption mechanism, they will tend to reduce the absorption of the es- 

 sential element. The degree of exclusion of the essential element depends on the 

 concentration of the two ions in the substrate, and their relative affinities for 

 the binding sites common to the two ions. In our experiments we have found that 

 the affinities of the the various ions within each group for their common binding 

 sites are of the same order of magnitude, so that the concentrations of the ions, 

 or rather their ratios, become the dominant factor. 



Conversely, if it is desired to exclude a non-essential element because of its 

 toxicity, it can be 'diluted' by high concentrations of its essential analog. The 

 papers by Hurd-Karrer (24, 25) and Aberg (i) are instructive in this con- 

 nection. 



Since the absorption mechanism fails to discriminate among certain competi- 

 tive ions, though some of these may be essential and others toxic, the plant has 

 no defense mechanism against entry of certain toxic elements if these should 

 be present in significant concentrations in the substrate. Had plants been ex- 

 posed to considerable concentrations of the toxic elements during their evolu- 

 tionary history, biochemical adaptations would have had to be elicited for sur- 

 vival. The fact that such adaptations are apparently non-existent, for such 

 ion pairs as K-Rb, and Ca-Sr, suggests that plants have not had to cope with 

 significant concentrations of Rb and Sr during the period in which their present 

 biochemical make-up evolved. 



REFERENCES 



1. Aberg, B. On the mechanism of the toxic action of chlorates and some related substances 

 upon young wheat plants. Ann. Roy. Agr. Coll. Sweden 15: 37-107, 1948. 



2. Adrien, a. Selective toxicity. New York: Wiley, 1951. 



3. Arnon, D. I. Microelements in culture-solution experiments with higher plants. Am. J. 

 Botany 25: 322-325, 1938. 



4. Arnon, D. I. and P. R. Stout. Molybdenum as an essential element for higher plants. 

 Plant Physiol. 14: 599-602, 1939. 



