Plant nutrition 



11 



ESSENTIAL NUTRIENTS 



Water 



There is no doubt that water serves as one of the most important 

 plant nutrients. Water is the most abundant ingredient of active 

 plant cells, and comprises 85 to 95 per cent of the fresh weight of 

 actively growing tissue. On the other hand, the water content of 

 dormant structures, such as seeds, may be as low as 5 to 10 per cent. 

 As has been discussed in Chapter 2, water has a number of properties 

 which make it an important physiological substance. In so far as the 

 plant is concerned, water is (1) an important raw material for the 

 synthesis of organic compounds, (2) the mediiun in which solutes are 

 dispersed and through which such solutes move from cell to cell, and 

 (3) the solvent or dispersion mediimi in which vital chemical reactions 

 take place. Moreover, water gives turgor to plant cells. 



Water moves upward in the plant, from the roots to the stems and 

 then to the leaves, where a portion of it is lost to the atmosphere as 

 water vapor. The final process is known as transpiration and accounts 

 for a large percentage of the water taken up by a plant during its life- 

 time. The amount of water transpired by a plant during its lifetime 

 is very large, varying between 200 and 1000 times its dry weight. Such 

 loss cannot be construed as wasteful, however, since transpiration is 

 the end to a means of circulation in a plant. Hence many of the 

 soluble nutrients are translocated within the plant during this process 

 of water movement. 



A number of factors influence the rate of transpiration in plants, 



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