156 



MINERAL SALTS ABSORPTION IN PLANTS 



grow equally well as halophytes or glycophytes, but plants 

 accustomed to growing in soil with a low salt concentration grow 

 more slowly when subsequently transferred to a high-salt medium, 

 and vice versa. 



One of the characteristic features of halophytes is the high 

 osmotic pressure of the cell sap, which is due mainly to high sodium 

 and chloride content (Fig. 48). As the concentration of sodium 

 chloride in the external medium increases, so does that in the cell 



E 

 o 



q" 



a. 



Q 



NaCl content of soil 



Fig. 48. Diagrammatic representation of the relationship between sodium 

 chloride content of the soil and the diffusion pressure deficit (DPD) of the 



cell sap in halophytes. Total DPD of cell sap = ; DPD due to sodium 



chloride= DPD of soil solution= (redrawn from Adriani, 1958, 



after Walter). 



sap. Hill (1908) showed that the DPD of the sap in root hairs of 

 young plants of Salicornia was equivalent to a 5-6 per cent solution 

 of sodium chloride when these plants were grown in .soil of which 

 the soil water contained about 3 per cent of chlorides. The root 

 hairs of Salicornia seedlings were found to have a corresponding 

 lower DPD after they had been bathed with weaker salt solutions. 

 Conversely when seedlings which had acquired a low DPD were 

 transferred to successively stronger solutions the root hairs were 

 found to regain a higher DPD. Very high osmotic pressures can be 

 attained in the cell sap of extreme halophytes. Ruhland (1912) 



