Over street and Dean 89 



Salient features of the ion absorption process in plants 



No mechanism proposed thus far for the accumulation of ions by 

 plant cells has received universal acceptance. Moreover, it will not be 

 our purpose here to describe in detail the various mechanisms that have 

 been put forth nor to judge them. On the other hand, we wish to de- 

 scribe a body of uncontested observations concerning the process which 

 has been contributed to by a number of groups during the past 20 years. 

 These groups include those associated with S. C. Brooks, R. Collender, 

 D. R. Hoagland, H. Lundegardh, W. J. V. Osterhout, R. N. Robertson, 

 and F. C. Steward. This information can be outlined as follows. 



1. The ion absorption process requires the expenditure of energy by the 

 plant (isotopic exchanges excepted). No ion accumulation occurs in the 

 absence of respiration and other metabolic activities such as protein syn- 

 thesis, etc. In general, the process is an attribute of tissues capable of 

 growth. When the metabolic activity of the plant is inhibited by reduced 

 oxygen tension, lowered temperature, or poisons, ion accumulation is like- 

 wise inhibited. This is true for the accumulation of both anions and cations. 



2. The ion absorption process is an exchange process. Predominantly cations 

 are absorbed in exchange for H-ions of the plant and are released to the 

 culture medium. Anions are absorbed in exchange for OH" or HC0 3 ~ 

 which are released to the culture medium. The evidence indicates that no 

 ion passes in or out of a healthy plant except by exchange for another ion. 



3. Ion accumulation is to a large extent selective. Due to the exchange 

 character of the process, anions can enter the plant independently of 

 cations and vice versa. Also, ions are not absorbed at the same rates. In 

 general, the cations K + , NH 4 + , Rb + , and Cs + are rapidly accumulated 

 while Ca , Mg + , and Ba ++ are much more slowly taken up. The anions 

 N0 3 , Br - , and Cl~ are usually rapidly absorbed. The anions S0 4 and 

 H 2 P0 4 ~ are moving rather slowly; the anion HC0 3 ~ is apparently not 

 absorbed at all. 



4. With electrolyte solutions, and below concentrations of the order of 

 0.005 N, the rate of accumulation of an ion is dependent on its concen- 

 tration in the culture medium. 



The above-listed facts were essentially all that were firmly established 

 concerning the ion absorption process before the advent of artificially 

 prepared radioactive elements. The absorption process was known to 

 be an exchange reaction which could be written formally as follows. 



