In addition to these three principal components, the water potential is affected 

 by temperature and gravitational forces. The water potential decreases as the tempera- 

 ture decreases because of a loss of heat (free energy) from the individual water mole- 

 cules. Effects of gravity may be important to considerations of water movement over a 

 considerable vertical distance, such as water columns in tall trees, but gravity can 

 usually be neglected in studies of soils and most plants. Water potential is usually 

 determined at the ambient temperature and then corrected back to 25°C. (or the tempera- 

 ture of calibration under isothermal conditions) at normal atmospheric pressure, and 

 over a very small vertical extent. The effect of temperature is eliminated by this 

 adjustment. As previously stated, the influence of gravitational forces is usually 

 negligible . 



Under most conditions in the soil-plant system, the most important variables af- 

 fecting water potential (other than temperature influences which must be corrected) are 

 the osmotic, matric, and pressure potentials. Since the pressure component does not 

 affect the soil water potential appreciably, an equation expressing the sum of the 

 components affecting the total soil water potential can be written as: 



where : 



ip is the total water potential; 

 subscript it is the osmotic component; and, 

 subscript m is the matric component. 



In the plant, an equivalent expression for the total plant water potential is: 



ib = ib + ib + il) (4) 

 r n m T p 



where 



ib is the pressure potential. 



It is important to understand that equations (3) and (4) express water potential (a 

 negative quantity) as the algebraic sum of osmotic potential (negative) matric potential 

 (negative), and pressure potential (positive). Usually the matric potential is not 

 considered a major component in the plant water potential, and there is a tendency to 

 combine it with the osmotic potential. The distinction between these two components 

 can become somewhat arbitrary, but there are recent convincing arguments that they are 

 not strictly additive quantities (Salisbury and Ross 1969). More detailed discussion 

 of the characteristics of the individual components of the water potential are beyond 

 the scope of this paper, but they are presented elsewhere (Boyer 1967, Wiebe 1966, 

 Wilson 1967, and Slatyer 1967). 



When water potential gradients are established, there will be a tendency for water 

 to diffuse from the region of higher free energy to a region of lower free energy. Thus, 

 equilibrium will be favored even though true equilibrium may never be reached under 

 natural conditions. The thermodynamic principles that explain the theory of water 

 potential serve to adequately predict the direction, but not the rate, of either dif- 

 fusion or energy transfer. Transfer rates are strongly dependent upon the nature and 

 sources of resistances imposed on the diffusion pathway, but a thorough treatment of 

 these principles is beyond the scope of this paper. The interested reader is referred 

 to Slatyer (1967), Taylor and Cary (1965) , ' Biggar and Taylor (1960), Letey (1968), and 

 Briggs (1967). 



4 



