CHAPTER 9 



SALT TOLERANCE 



Enough is as good as a feast . . . too much of a good 

 thing is good for nothing. 



Theodore Hook. 



A. Salt Tolerance of Glycophytes 



The growth of most plants is retarded when the sah content of 

 the soil exceeds a rather low value, and to these salt-sensitive plants 

 the term "glycophytes" or "glyptophytes" is applied. Halophytes 

 (Gr. halas = salt), on the other hand, grow habitually on soils or in 

 solutions containing a high concentration of salts, and are seldom, 

 if ever found elsewhere. The salt sensitivity of crops is of some 

 commercial importance as well as of academic interest since yields 

 are reduced when the salt content of soils exceeds an optimum 

 value. The increasing irrigation of crops in the field, as well as in 

 hot-houses with tap and river water which is rich in salts, is now- 

 adays rendering the problem more acute. It is said that crops on 

 25-30 per cent of the arable land in the United States of America 

 suffer to some extent from salt excess. One of the important stages 

 in the reclamation of land from the sea, as practised, for example, in 

 the Netherlands, is the leaching of salt from the soil until its concen- 

 tration is sufficiently low, to allow the growth of satisfactory crops. 

 Deleterious effects of high-salt soils on the growth of plants may 

 be attributed to two separate factors: 



(i) reduced water absorption because of the high external 

 osmotic pressure, and 



(ii) specific effects of certain ions on metabolism when they are 

 present in the medium at a high concentration. 



It is not easy to distinguish between these possibilities, but attempts 

 have been made to do so by comparing growth of plants, placed 

 with their roots, in iso-osmotic aqueous solutions of different 



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11 M.S.A.P. 



