150 MINERAL SALTS ABSORPTION IN PLANTS 



composition. Gauch and Wadleigh (1944) obtained evidence for 

 the importance of an asmotic factor in kidney beans {Phaseolus 

 vulgaris) by their observation that similar inhibitions of growth are 

 obtained by supplying osmotically equivalent amounts of either 

 sodium chloride, sodium sulphate or calcium chloride in the medium. 

 These solutions seemed to have no effects on growth which could be 

 attributed to individual ions. Magnesium chloride and sulphate 

 solutions, however, caused greater depression of growth than did the 

 other solutions of comparable osmotic pressure indicating that 

 magnesium ions have a specific inhibitory effect independent of 

 osmotic pressure. All species do not react in the same way to excess of 

 a particular salt. Some species of beans and peaches, for example, are 

 more sensitive to solutions containing high concentration of chloride 

 than sulphate, whereas in flax and cotton, the situation is reversed. 



It is evident from simple osmotic considerations that when the 

 concentration of the medium is increased, water absorption is 

 reduced and growth tends to diminish. To some extent, the influence 

 of high external osmotic pressure is offset by an increase in the 

 osmotic pressures of the root cells through increased absorption of 

 salts (Table 16), but this is insufficient to prevent a gradual decline 

 in the difference between the diffusion pressure deficit (DPD) of the 

 cell sap and that of the soil solution. Water absorption and growth 

 presumably stop when the DPD of water in the medium is about 

 equal to the diffusion pressure deficit of that in the vacuolar sap in 

 cells of the root. At this point the cells are nearly flaccid, that is, 

 the wall pressure approaches zero, and thus the osmotic pressures of 

 the cell sap and medium also tend towards equality. The presence of 

 a high DPD in the shoot does not seem to be effective in maintaining 

 an adequate supply of water to the plant in the absence of a difference 

 of DPD between the root cells and external medium. 



There is, however, no apparent correlation between the salt 

 tolerance of different species, and the DPD of the cell sap in roots 

 grown under natural conditions (Table 16). The ability of the root 

 cells to raise the osmotic pressure of the vacuolar sap and maintain 

 it at an adequate level under high salt conditions is of greater 

 importance. Another factor is the transpiration rate of the plant, 

 since water loss must be more than replaced by absorption before 

 growth is possible. Shallow-rooted plants, and those with large 

 shoot-to-root ratios, that is, plants with poor water absorptive 



