Grafts et al. 



— 50 



Water in Plants 



equals numerically the osmotic pressure of S. This is the pressure that the 

 physicist has termed osmotic pressure and the physiologist has termed 

 turgor pressure at full turgor. 



2.) If pressure is lowered in W by moving Pi to the left, S remaining 

 at atmospheric pressure, the diffusion pressure of the water molecules in 

 W may be lowered to equal that of the water in S. The hydrostatic pres- 

 sure (subatmospheric) in W now equals the difference originally existing 

 between the diffusion pressure of water molecules in W and those in the 

 solution S — namely, the diffusion pressure deficit of the water in the system. 

 As Haldane has shown, this diffusion pressure deficit of the water in the 

 system numerically equals the osmotic pressure of the solution (turgor 

 pressure at water equilibrium). 



PPP = OP 



Fig. is. — Relations among osmotic pressure, diffusion pressure deficit, and 

 turgor pressure for an ideal osmometer. 



To illustrate the numerical relationships between these values, plant 

 physiologists have long used a simple diagram (see Figure 19, page 73). 

 As presented by Meyer and Anderson ( 1939, p. 149) , this diagram has a 

 curved sloping line to designate osmotic pressure, a feature necessary to 

 account for the changes in concentration resulting from increase in volume 

 as the cell becomes more turgid. For years this diagram in varied forms 

 has aided the student in understanding the osmotic relations of the vacuo- 

 lated plant cell. It is introduced here in idealized form to aid in an under- 

 standing of the fundamental mechanics of osmosis and osmotic pressure 

 as they occur in physical systems. 



If we visualize an ideal osmometer having a perfect differentially per- 

 meable membrane and rigid enough to resist volume changes, Figure 15 

 illustrates the relations ainong osmotic pressure (OP), diffusion pressure 

 deficit of water (DPD), and turgor pressure (TP) for the system. The 

 expression diffusion pressure deficit has been given a number of names 

 by physiologists including suction force, suction tension, suction pressure, 

 turgor deficit, water-absorptive power, net osmotic pressure, osmotic dif- 

 fusion pressure, and functional pressure. It is of great importance in plant 

 physiology, as it is a measure of the force with which a cell may absorb 

 water. For convenience it has been contracted to DPD. 



