52 MINERAL SALTS ABSORPTION IN PLANTS 



interesting effect on the differential absorption of anions and cations. 

 When carbon dioxide is fixed, organic acids are synthesized in the 

 cells and hydrogen ions apparently exchange for other cations from 

 the external medium causing salts of organic acids to accumulate. 

 In this way, absorption of cations in excess of anions, may occur. 

 (Jacobson and Ordin, 1954). In some tissues placed in alkaline 

 media, the apparent excess absorption of cations over anions is 

 balanced by uptake of bicarbonate ions, which are converted to 

 organic acid anions within the tissue (Hurd and Sutchffe, 1957; 

 Hurd 1958: see Fig. 17 a, b and also Fig. 33 p. 92). 



Carbon dioxide and bicarbonate ions at high concentrations aveh 

 inhibitory effects on salt absorption as on other physiological 

 processes. Steward and Preston (1941) found that at various pH 

 values of the medium, the effect of a simultaneous increase of carbon 

 dioxide and bicarbonate concentrations was to inhibit bromide 

 absorption by potato slices. Chang and Loomis (1945) similarly 

 observed that absorption of salt by wheat {Triticum aestivum), 

 maize and rice plants, was reduced to a greater extent by bubbling 

 carbon dioxide through the nutrient solution for 10 minutes out of 

 every hour than it was by similar treatment with nitrogen. These 

 observations may receive a partial explanation in the fact that 

 cytochrome oxidase activity in mitochondrial preparations and 

 whole roots is inhibited by bicarbonate ions at high concentrations 

 (Miller and Evans, 1956). 



5. Hydrogen Ion Concentration 



The pH value of the medium affects salt absorption in several 

 ways. Bicarbonate ions, for example, accumulate in alkaline 

 solutions and this may lead to enhanced cation absorption by 

 increasing the effective concentration of salt (Fig. 17 a, b). If, how- 

 ever, bicarbonate or hydroxyl ions compete with other anions, for 

 example chloride, or nitrate, absorption of these may be reduced at 

 high pH values. In agreement with this contention, Hoagland and 

 Davis (1923) found that Nitella cells absorb nitrate and chloride 

 more rapidly from an acid than from an alkaline medium. Olsen 

 (1953) showed that inhibition of anion absorption at alkaline 

 pH values in Elodea canadensis was greater at low than at high salt 

 concentrations, presumably because bicarbonate ions compete more 

 effectively when the ratio of bicarbonate to other anions is high. 



