FACTORS AFFECTING SALT ABSORPTION 63 



8. Influence of other Constituents of the Medium 



a. Osmotic effects. There are some reports that sak absorption 

 occurs less rapidly in highly turgid than in flaccid cells and tissues. 

 Jacques (1938), for example, observed that the rate of absorption of 

 potassium ions by mature Va Ionia coenocytes is increased when the 

 pressure exerted by the protoplasm on the inelastic cellulose wall is 

 artificially relieved. The same effect occurs in red beet tissue, when 

 the osmotic pressure of the medium is increased (Sutclifife 1954a). 

 When beet cells are plasmolysed, however, potassium absorption is 

 depressed to a lower level than that observed in turgid cells. This 

 does not seem to be due to decreased surface area of the protoplast, 

 increased thickness, or increased concentration of solutes in the 

 vacuole, because inhibition does not apparently increase with 

 increasing extent of plasmolysis. Absorption of salt by plasmolysed 

 cells can occur when the internal concentration of the sap is higher 

 than would be necessary to prevent absorption in turgid tissue (see 

 pp. 65-66). 



In intact tomato plants Long (1943) showed that water uptake 

 can be reduced to as much as 20 per cent of its original value by 

 addition of sodium chloride or sucrose to the medium without an 

 effect on nitrate absorption. Brouwer (1954) similarly found that 

 increasing the osmotic pressure of the medium caused a considerable 

 reduction in water uptake without much affecting absorption of 

 chloride by intact Viciafaba plants (cf. Chapter 7, p. 1 16). Arisz and 

 Schreuder (1956) observed no effect of external osmotic pressure on 

 the uptake of salt by Vallisneria leaves unless plasmolysis occurred, 

 when there was a considerable decrease in uptake. 



b. Effects through metabolism. There are a number of substances 

 which either stimulate or inhibit salt uptake by an influence upon 

 metabolism. Among those which promote absorption at suitable 

 concentrations are soluble sugars and other respiratory substrates. 

 This effect is most clearly demonstrated in those organisms, for 

 example, bacteria and fungi, which depend on an external supply of 

 carbohydrates for growth. (Pulver and Verzar, 1940; Leibovitz and 

 Kupermintz 1942; Kamen and Spiegelman, 1948) (Fig. 21a). When 

 excised roots, slices of non-green tissues, or photosynthetic organisms 

 in the dark, are depleted of carbohydrate reserves, salt absorption is 

 stimulated by an external supply of sugar. (Hoagland and Broyer, 

 1936; Helder, 1952). Addition of a number of other organic sub- 



