CHAPTER 4 



FACTORS AFFECTING SALT 

 ABSORPTION 



Science is based solely on properties and facts ascer- 

 tained by experiment and observation, that is to say on 

 a knowledge that under given conditions certain 

 results must necessarily be obtained. 



W. Pfeffer. 

 The Physiology of Plants (1900). 



A. External Factors 

 1. Temperature 



The rate of salt absorption tends to increase with increasing 

 temperatures until a maximum rate is reached, and then to decrease 

 again at still higher temperatures (Fig. 14a). At temperatures above 

 about 40 °C, absorption is progressively reduced in many plants, 

 presumably because of the gradual inactivation of enzyme systems 

 involved. In addition, the cytoplasm becomes more permeable to 

 passive leakage of salts through it at high temperatures, so that if a 

 concentration gradient exists, net absorption is reduced as the 

 temperature is raised. At temperatures approaching °C, absorption 

 decreases both by diminution in the rate of chemical reactions 

 involved in active transport, and by increased viscosity, and hence 

 higher resistance, of the cell membranes. 



Total salt absorption either by a cell or whole plant can be 

 separated into two components, one with a 2io of about 1 -2 and 

 another with a Q^q of 2-3 or higher. The salt taken up in a short 

 time by a tissue at 1-2 °C is thought to be absorbed mainly by 

 physical processes (diffusion, mass flow, exchange and adsorption), 

 while that absorbed at higher temperatures includes also a com- 

 ponent dependent on respiration. Physical absorption is completed 

 relatively rapidly, whereas the period of metabolic absorption is 

 prolonged (Fig. 14b). 



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