Grafts et al. — 76 — Water in Plants 



ductors, on the other hand, tensions of many atmospheres have been pic- 

 tured in tall trees and plants suffering permanent wilting. The contents of 

 such cells are virtually pure water and a dynamic equilibrium exists between 

 the water in such cells and that in the more concentrated vacuolar sap of 

 surrounding parenchyma. Tension in the xylem depends upon the ability 

 of water to exist in a metastable state when no unwet surface is present on 

 which the vapor phase may initiate. The ability of living parenchyma to 

 retain water in competition with forces of such magnitude depends upon 

 the osmotic and imbibitional character of their constituents. 



It follows from the discussion in Chapter VI that if a complete picture 

 of the water relations of the cell is to be embraced by the three terms OP, 

 DPD, and TP, it is necessary to include under OP all forces leading to 

 water absorption by the cell, i.e., leading to a lowering of the DP of water ; 

 where tension exists, its value must be added to the OP. Under TP are 

 included all forces acting in the opposite direction. DPD then represents 

 any difference occurring between these forces, and finds expression as a 

 net force causing water to enter the cell. 



Some prefer to treat problems of plant water relations on a thermodynamic basis, 

 following the general scheme presented on page 51. One such treatment (Edlefsex, 

 1941 ; Edlefsen and Anderson, 1943) deals particularly with the use of soil moisture 

 by plants, and interprets the movement of water on a free energy basis. Water tends 

 to move in a direction such that its free energy is lowered. The partial specific free 

 energy of water in a solution is a function of several components : various force fields 

 (gravitational, electrical, adsorptive) ; dissolved material giving the solution an osmotic 

 value ; and hydrostatic pressure. 



Broyer (1946) has applied these principles directly to the problem of cell water 

 relations in a comprehensive treatment. He aptly describes the free energy concept as 

 applied to movement of water through the plant as follows : "The fundamental principle 

 underlying the movement of materials is that each molecule possesses a total internal 

 energy equal to the sum of its internal kinetic and potential energies .... and the 

 molal (or partial molal) free energy is equal to the product of the mean free energy 

 of the particles and the number of particles in one mole. A system is subject to spon- 

 taneous change if there is any conceivable process whereby the internal energy of the 

 constituent molecules can be effectively reduced. The action, here especially that con- 

 cerned with translation of the particle in space — its escaping tendency — which could 

 be produced by such a conceivable process is determined by the internal free energy 

 of the individual molecules. The free energy of the particles may be modified by any 

 change in condition of the external environment." 



Distinction is made between those partial specific free energies which efifect move- 

 ment of water into the cell, and those producing outward movement. The difference 

 and sign represent the magnitude of the escaping tendency gradient, and the direction 

 of water movement. Thus 



NIF = 2 IF — SEF (2) 



Net influx specific Sum of influx specific Sum of efflux specific 



free energy free energies free energies 



The principal partial specific free energies identified are 



osmotic solute specific free energy, 



hydrostatic specific free energy, 



metabolic specific free energy, 



non-metabolic specific free energy (effect of colloids). 



Each of these may act inwardly or outwardly depending upon the state of the cell and 

 external conditions. 



The relationship between the osmotic specific free energies (F) and osmotic pres- 

 sure (P) is given by Broyer as 



f — P 



P = p - po = = F, (i) 



V° (1.013 X 109) 



