Crafts et al. — 52 — Water in Plants 



Evidently the osmotic pressure of a solution can be equated to the sum 

 of two definite measurable quantities, the diffusion pressure deficit and 

 the turgor pressure of an osmotic system (equation 2). Osmotic pressure 

 is equal to the DPD when turgor pressure equals zero (equation 3) or to 

 the turgor pressure when DPD equals zero (equation 4). The conditions 

 designated by equations (3) and (4), however, represent two different 

 states of the osmotic system. And definitions of osmotic pressure based 

 upon these two distinct states should recognize their mechanical differences. 



The three terms osmotic pressure, diffusion pressure deficit (of water), 

 and turgor pressure seem necessary in a consideration of an aqueous osmot- 

 ic system. Each is a distinct property of the system. The identification of 

 osmotic pressure with diffusion pressure deficit (cf. S. C. Brooks, 1940; 

 C. J. Lyon, 1941) or with turgor pressure (cf. Dutrochet as quoted by 

 Palladin, 1923; and Eyster, 1943) has undoubtedly caused the greatest 

 confusion in discussions of osmotic systems. Wann's (1943) attempt to 

 do away with the concept of osmotic pressure does not eliminate the diffi- 

 culty. 



Because of its relation to turgor in cells, the condition of osmotic equilib- 

 rium termed "state B" in Figure 15 is of much interest to the physiologist. 

 In this state the turgor pressure in the solution is such that movement of 

 water molecules through the membrane is equal in each direction. This 

 is the pressure that is commonly termed the "osmotic pressure" of the cell. 

 Since only at water equihbrium is this turgor pressure equal to the osmotic 

 pressure of the cell, it seems best to term any hydrostatic pressure above 

 the diffusion pressure of the pure solvent, as arbitrarily designated by the 

 base line in Figure 15, the turgor pressure, for at all states but full turgor 

 this is only one component of the value that equals osmotic pressure. The 

 term could serve equally well in describing the properties of a purely physi- 

 cal osmotic system (cf. Meyer, 1945, page 154). The turgor pressure of 

 an osmotic system varies through a range of pressure values which lies 

 above the one atmosphere reference level numerically paralleled by those 

 traversed by the diffusion pressure of the solvent in the region below one 

 atmosphere. And as diffusion pressure deficit of a solution is a measure 

 of the excess diffusion pressure of the pure solvent over that of the solvent 

 in the solution in "state A," turgor pressure measures the excess diffusion 

 pressure of the solute in the osmometer in "state B" over the diffusion 

 pressure of the solute in the solution at atmospheric pressure. In this way 

 the activity of both solute and solvent are expressed in their logical rela- 

 tionship. To limit the term diffusion pressure to the solvent is arbitrary ; 

 the custom has resulted from the unbalanced view of osmosis that grew 

 out of the van't Hoff relation, and from the fact that most membranes used 

 in osmotic pressure studies have been permeable to water, so that osmotic 

 adjustments occurred mainly through water diffusion. 



The "Solvent" and "Solute" Pressure Theories : — Many have re- 

 jected the kinetic view of turgor pressure development in an osmotic sys- 

 tem, chiefly on the basis that the pressure results from the entrance of the 

 solvent into the solution through the semipermeable membrane. Findlay 

 (1919) has pointed out that this criticism rests on a misunderstanding, 

 (See also footnote f, p. 110, of Palladin, 1923). 



When turgor pressure is at its maximum ("state B") and is therefore 

 equal to the osmotic pressure, water is in equilibrium ; hence turgor pressure 

 at this stage cannot be explained on the basis of excess of entry over loss 



