PLANT BIOPHYSICS - Y. I. Posudin - 2004
II. TRANSPORT PROCESSES IN THE SOIL-PLANT-ATMOSPHERE SYSTEM
6. MASS TRANSPORT
6.6. WATER POTENTIAL OF WATER VAPOR IN THE AIR
At The Cell-air interface, directed Water movement constantly occurs due to evaporation and Condensation. The layer of air adjacent to the cell may contain water in both liquid and gaseous phases. The gaseous phase of water is referred to as vapor. Vapor that is in thermodynamic equilibrium with the liquid (i.e., in a state where the number of molecules passing from the liquid to the vapor equals the number of molecules returning to the liquid per unit time) is called saturated vapor. Vapor is characterized by its vapor pressure (partial pressure) e — the pressure that water vapor would exert if it alone occupied a volume equal to the total volume of the gas mixture at the same Temperature — and by saturated vapor pressure E, which is the limiting pressure value corresponding to the equilibrium between vapor and water, i.e., the saturated state of the air at a given temperature. The partial pressure of water vapor determines the Chemical Activity of water in the gaseous phase. Assuming that the vapor behaves as an ideal gas, an expression for the active vapor can be derived:
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When certain substances are dissolved in water, The activity of water decreases, and the saturated vapor pressure drops accordingly. Assuming that for water present in the air as vapor at atmospheric pressure ρ = 0, and using equations (6.4) and (6.7), we obtain the expression for The water potential of the vapor:
The water potential of atmospheric vapor depends on relative humidity and temperature (Table 6.3).
Table 6.3. Dependence of air water potential (MPa) on relative humidity and temperature [Lambers et al., 1998]
Relative humidity |
-ψ, MPa |
||||
10 °С |
15 °С |
20 °С |
25 °С |
30 °С |
|
100 |
0 |
0 |
0 |
0 |
0 |
99,5 |
0,65 |
0,67 |
0,68 |
0,69 |
0,70 |
99 |
1,31 |
1,33 |
1,36 |
1,38 |
1,40 |
98 |
2,64 |
2,68 |
2,73 |
2,77 |
2,81 |
95 |
6,69 |
6,81 |
6,92 |
7,04 |
7,14 |
90 |
13,75 |
13,99 |
14,22 |
14,45 |
14,66 |
80 |
29,13 |
29,63 |
30,11 |
30,61 |
31,06 |
70 |
46,56 |
47,36 |
48,14 |
48,94 |
49,65 |
50 |
90,50 |
92,04 |
93,55 |
95,11 |
96,50 |
30 |
157,2 |
159,9 |
162,5 |
165,2 |
167,6 |
10 |
300,6 |
305,8 |
310,8 |
316,0 |
320,6 |
RT/Ve |
130,6 |
132,8 |
135,0 |
137,3 |
139,2 |
Example. Determine the water potential of the air at: a) a relative humidity of 25 % and air temperature of 0 °С; b) a relative humidity of 75 % and air temperature of 20 °С.
Solution. Substituting the numerical values into equation (6.13):
Control Questions
1. Characterize the main Mechanisms of water transport.
2. What is water potential?
3. What are the main COMPONENTS OF WATER potential?
4. What constitutes the water potential of a cell?
5. What vapor is referred to as saturated?
6. What is water vapor pressure?
7. What is saturated water vapor pressure?
Last update: 07/08/2026
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