60 MINERAL SALTS ABSORPTION IN PLANTS 



presence of potassium. Many similar observations have been made 

 upon higher plants growing in soil. 



The physical component of absorption exhibits little discrimina- 

 tion between the alkali cations, selectivity being confined mainly 

 to the metabolic component. In principle, selective absorption can 

 be explained in terms of either separate binding or carrier sites for 

 various ions, differing in number and turnover rate, or a common 

 absorption mechanism which binds and transports certain ions in 

 preference to others. In the first case, the active uptake of a particular 

 ion species should be unaffected by the presence of another, but in 

 the second, uptake of one ion is reduced when a competing ion is 

 present. Both these situations can apparently arise, and it seems 

 probable that potassium and sodium compete in cells for a number 

 of binding sites which have varying degrees of preference. (Epstein 

 and Hagen, 1952; Fried and Noggle, 1958; Bange, 1959). In yeast 

 (Conway, 1955), and in some seaweeds, including Ulva lactuca, a 

 mechanism with a high potassium preference transports ions 

 inwards, whereas a sodium-preferring mechanism causes excretion 

 of this ion from the cells (see p, 154-5). 



A number of observations have been made on the absorption of 

 the alkali earth cations by plant cells. Magnesium is apparently 

 taken up actively at sites which are distinct from those involved in 

 the uptake of calcium, barium and strontium (Collander, 1941; 

 G. T. Scott, 1943; Epstein and Leggett, 1954). 



When alkali cations and alkali earth cations are mixed in the 

 external medium, uptake of the latter is commonly depressed whilst 

 that of the former is often enhanced. Viets (1944) showed that a 

 number of bivalent ions, including calcium, stimulate potassium and 

 bromide absorption by excised barley roots at some concentrations, 

 and are inhibitory at others (Fig. 20a). Overstreet et al. (1952) 

 confirmed these effects of calcium on uptake of potassium and 

 attributed them to a dual influence on the absorption mechanism. 

 They suggested that calcium competes with potassium for its 

 absorption sites, and this results in inhibition of potassium uptake 

 when the ratio of calcium to potassium is high (Fig. 20c). In 

 addition calcium has a stimulatory effect on active transport, possibly 

 by facilitating the breakdown of the potassium-carrier complex. 

 Calcium absorption by barley roots falls rapidly at first as the 

 potassium concentration of the medium is raised, and then decreases 



