FACTORS AFFECTING SALT ABSORPTION 67 



partly attributable to their high sugar content. If such roots are 

 starved by keeping them for some time in distilled water, their 

 capacity to take up salt diminishes as sugar content falls. Humphries 

 (1956b) demonstrated a positive correlation between the reducing 

 sugar content of excised pea roots and salt uptake. In the same 

 experiments sucrose content seemed either to be unrelated, or to 

 show a negative correlation with absorption. Phillis and Mason 

 (1940) found that cutting off the carbohydrate supply to roots, by 

 ringing, caused reduced absorption of salt in cotton plants. In 

 intact angiosperms, absorption of salt is sometimes depressed with 

 the onset of flowering, and this is correlated with a fall in the level of 

 carbohydrates in the roots. Eaton and Joham (1944) suggested that 

 much of the decline in mineral intake accompanying heavy fruiting 

 in cotton is attributable to reduced movement of carbohydrate to 

 the roots. 



4. Growth 



Although salts can be absorbed rapidly for a limited time by 

 cells which are not growing (see Chapter 2, p. 17), prolonged 

 absorption is closely bound up with a capacity for growth. Mature 

 cells which have irretrievably lost their ability to grow are totally 

 incapable of accumulating ions, while dormant cells which regain a 

 capacity for absorbing salts actively after a period of washing in 

 aerated water, are also capable of further growth. 



Growth stimulates the process of salt absorption directly and 

 indirectly in several ways. As a result of growth, there is synthesis 

 of new binding sites and carrier molecules as well as additional 

 incorporation of inorganic ions into insoluble cell constituents. 

 This is particularly important in cells which are actively synthesizing 

 protein and undergoing division. The relationship between salt 

 absorption and protein synthesis is discussed in more detail below 

 (Chapter 5, pp. 88-89). Cell expansion results in increased proto- 

 plasmic surface area across which absorption occurs, while the con- 

 comitant absorption of water causes a dilution of the vacuolar sap. 

 These changes tend to stimulate salt uptake, especially after growth 

 has been allowed to occur under conditions of salt deficiency (cf. 

 Chapter 2, p. 14). When growth by enlargement slows down, the 

 internal salt concentration tends to rise and absorption decreases. At 

 the same time, metabolism begins to decline and the cells enter either 



