58 MINERAL SALTS ABSORPTION IN PLANTS 



importance, and potassium, for example, is usually absorbed more 

 rapidly than other alkali cations, although both rubidium and 

 caesium have smaller hydrated ions. Similarly, chloride is absorbed 

 more rapidly than the smaller bromide and iodide ions. 



When two or more ion species with the same electrical sign are 

 present in the external medium, either antagonistic or synergistic 

 effects may be observed. It had often been found that chemically 

 related anions interfere with one another during absorption whereas 

 more diverse anions do not. Chloride uptake is, for example, 

 reduced in the presence of bromide or iodide, but may be unaffected 

 or even stimulated when nitrate or phosphate is present; sulphate 

 and selenate have been shown to compete with one another for 

 uptake in a variety of plants, as also do arsenate and phosphate. 



Effects of such anions as nitrate, phosphate and sulphate in 

 stimulating the absorption of other ions is probably due to enhance- 

 ment of metabolism. Helder (1952) demonstrated that nitrate 

 uptake by maize plants ceases in the absence of phosphate, and he 

 attributed this to cessation of synthetic processes, involving phos- 

 phate, in which nitrate is utilized. Similarly, absorption of potassium 

 and other cations by whole plants, growing in solution culture soon 

 stops when nitrate or other metabolically important anions are 

 withheld from the medium. 



The alkali cations compete with one another to a greater or 

 lesser extent for absorption and the uptake of one is generally 

 reduced when the concentration of another is increased. With 

 certain reservations arising from the fact that absorption of cations 

 is interrelated to that of associated anions, such competition can be 

 treated kinetically in the same way as inhibition of enzyme reactions 

 (Fig. 19). If two ions compete with one another, the straight lines 

 obtained when the reciprocal of the initial rate of absorption of one 

 ion (1/v) is plotted against the reciprocal of concentration (l/[/], 

 at different concentrations of a competitor, cut the ordinate at the 

 same point corresponding to 1/K (Fig. 19 ab cf. 45-6). When non- 

 competitive inhibition occurs, the reciprocal plots tend to give 

 straight lines parallel to one another (Fig. 19d). The results of 

 Fig. 19 indicate that potassium, rubidium and caesium compete 

 with one another for the same carrier sites in barley roots. In the 

 case of sodium and lithium, the data cannot be interpreted un- 

 equivocally because the addition of a competing cation raises the 



