FACTORS AFFECTING SALT ABSORPTION 57 



(b) diffusion is a limiting factor in transport of ions to the 

 binding sites, or 



(c) a fraction of the sah absorbed is taken up passively by mass 

 flow. 



Where diffusion or mass flow are involved, absorption tends to 

 become directly proportional to concentration, and, when the 

 carrier system is saturated, independent of it. Kostytschew (1926) 

 maintained that plants absorb about the same amounts of salt 

 from dilute as from more concentrated solutions, and Olsen (1950) 

 demonstrated that the rate of uptake of nitrate and phosphate by 

 rye plants is unaffected by external concentration over a wide range 

 (Fig. 18b). Olsen concluded that the rate at which individual ions 

 are absorbed is determined by the ratio of their concentration to 

 those of other ions in the medium, rather than by concentration 

 itself. 



Knaus and Porter (1954) noticed that the relationship between 

 concentration and absorption of salt by Chlorella cells varies for 

 different ions. For several cations, uptake is directly proportional 

 to concentration, whereas absorption of sulphate and phosphate 

 varies directly as the logarithm of external concentration when the 

 medium is dilute, and becomes independent of concentration in 

 stronger solutions. Steward and Millar (1954) reported an interesting 

 difference between rapidly growing and slowly growing carrot cells 

 in tissue culture. In the former, caesium absorption was found to 

 increase linearly, and in the latter logarithmically, with increasing 

 concentration. 



7. Interaction between Ions 



Ions in a solution affect the absorption of one another in a 

 variety of ways, and the more ion species there are present, the more 

 complex the situation becomes. From single salt solutions, cations 

 tend to be absorbed more rapidly in the presence of a readily 

 absorbable anion than of one which is taken up more slowly, and 

 vice versa. The radius of the hydrated ion is one of the factors 

 influencing the rate at which individual ionic species are absorbed, 

 and in general, for this reason univalent ions are absorbed more 

 rapidly than are bivalent and multivalent ions. Inherent preferences 

 exhibited by the absorption system are, however, of paramount 



i 



