152 MINERAL SALTS ABSORPTION IN PLANTS 



another with an opposing influence (ion antagonism). It has often 

 been reported, for example, that sodium and calcium ions counteract 

 one another in this way, a solution containing both these ions at 

 high concentration having a smaller inhibitory influence than 

 solutions containing either of the two separately (Osterhout, 1912). 



There is some evidence that salt tolerance is related to the ability 

 of plants to absorb potassium in competition with other cations. 

 Bernstein and Ayers (1953) showed that in several varieties of carrot 

 salinity, achieved by addition of calcium and sodium chlorides to the 

 soil, increased the rate of absorption of calcium and depressed that 

 of potassium. Those varieties which tended to absorb most calcium 

 and least potassium at a given level of salinity gave relatively poorer 

 yields than the others. It is not surprising, therefore, that, in some 

 plants, salt tolerance can be improved by increasing the amount of 

 potassium supplied relative to sodium and calcium. 



Some species, e.g. beets, cotton and tomatoes, which are fairly 

 tolerant of high concentrations of salts when grown in nutrient 

 solutions, give poor yields in soils of comparable salinity. This is 

 attributed to an effect of high salt content, particularly sodium ion 

 concentration, on the physical condition of the soil. The soil 

 becomes hard and compacted with consequent adverse effects of 

 moisture conditions, aeration and resistance to root penetration. 

 Salt-sensitive species, on the other hand, are commonly affected by 

 salt concentrations which are without evident effect on soil texture 

 and they tend to suffer less when grown in soil than in solution 

 culture, presumably because soil exerts some buffering action, for 

 example, by release of calcium, to moderate cation unbalance in 

 the supplied solution. 



B. Submerged Halophytes 



There are a number of plants which grow permanently submerged 

 in salt water. These include many algae, mainly belonging to the 

 Phaeophyceae and Rhodophyceae, and a few angiosperms including 

 Zostera maritima. The salt content of the bathing medium ranges 

 from that of brackish water which may be only a little more con- 

 centrated than fresh water, to that of some inland seas and lakes, 

 e.g. the Dead Sea (Israel); Wadi Natrum (Egypt) and Salt Lake 

 (U.S.A.) in which salt may reach a concentration of 10 per cent or 



