FACTORS AFFECTING SALT ABSORPTION 55 



Honert (1933) studied the effect of pH on phosphate absorption by 

 sugar cane plants and found the results to be consistent with the 

 view that only univalent ions are absorbed to an appreciable extent 

 (Fig. 17d). Hagen and Hopkins (1955) concluded that the effects 

 of Hp on uptake of phosphate by excised barley roots can be 

 explained in terms of the differential rates of absorption and relative 

 concentrations of uni- and bivalent ions in solution. Arnon et al. 

 (1942) decided that the deleterious effects of alkaline pH values on 

 growth of some plants may be attributed to their inability to absorb 

 sufficient phosphate under these conditions. 



Increase in the hydrogen ion concentration of the medium 

 generally causes a decrease in the rate of absorption of cations, 

 probably as a result of competition between the similarly charged 

 ions for binding and carrier sites (cf. p. 58). Osterhout (1936) believed 

 that cation accumulation is only possible when the pH of the 

 medium is more alkaline than that of the cell sap (see p. 76). Hoag- 

 land and Broyer (1942) reported, however, that excised barley roots 

 will absorb potassium and other cations when the external medium 

 is more acid than the apparent pH value of the cell sap, although 

 they do so at a reduced rate. At extremely acid pH values, net 

 uptake of salt may be depressed by damage to the cell membranes 

 which results in increased passive leakage of salts from vacuoles. 



6. Externa! Concentration 



The early work of Stiles and Kidd (1919) indicated that the 

 relationship between external concentration and absorption in tissue 

 slices resembles the Freundlich adsorption isotherm. More recent 

 investigators (Helder, 1952; Epstein and Hagen, 1952) have put 

 greater emphasis on the vahdity of the Langmuir equation (cf. 

 Chapter 3, p. 36). Data on the effect of external concentration on 

 the absorption of rubidium by excised barley roots show that both 

 relationships are applicable as a first approximation (Fig. 18a). 



Uptake and external concentration are often, but not always, 

 related to one another in the manner just described. Departures 

 from the hyperbolic relationship may be expected if: 



(a) the adsorption sites or carrier molecules are saturated with 

 salt, so that uptake depends on the rate of synthesis of 

 unoccupied binding sites, 



