Water is a key molecule for living beings. Many physiological processes in both animal and plant cells take place in water solutions or water interfaces. Water has several special properties such as: the hydrogen bond (key to water's properties), liquid at room temperature, nearly constant volume for general purposes, specific heat (almost one calorie to raise 1 g of water 1 ºC), high latent heats of vaporization and fusion, low viscosity, adhesive and cohesive forces because of its polar nature and high dielectric constant (high capacity solvent). Water molecules have potential energy that can be converted into kinetic energy which are important components of its free energy. The free energy per unit quantity (mol) of a compound is known as chemical potential. The chemical potential of any substance is expressed in energy units such as joules/kg or joules/mol. Since 1962, for water in particular, the chemical potential units are expressed in the base of the partial molar volume of water. Thus, the units for water potential are pressure units.
The water potential is one of the most important properties that can be measured in the soil-plant-air system. It is the chemical potential of water in a system or part of a system expressed in units of pressure, and compared to the chemical potential of pure water at atmospheric pressure and at the same temperature. The chemical potential of the reference water is considered zero.
The water potential can be expressed as the following expression:
Wp = (qw-qw*)/Vw
Where:
Wp = water potential.
qw = Chemical potential in the system under consideration.
qw* = Chemical potential of pure water at atmospheric
pressure and at the same
temperature
as the system under consideration.
Vw = Partial molar volume of water (18 cm3/ml).
Water potential is indicated as force per unit of area and the unit measurement is commonly the bar or Megapascal (MPa): 1 bar = 0.1 MPa = 106 dynes per cm-2 = 0.99 atmospheres and = 102 Joules per kg-1.
Water potential in soils and plants is less than 0 bars. The more negative is the value, the lower the water potential.
Another way to express the total water potential can be based on its main components:
Wp = p + m + s + g
Where:
Wp = water potential.
p = pressure potential. This component is numerically equal to the hydrostatic pressure. Its value has a positive sign because as the pressure increases the pressure potential increases. (In the xylem tissue, pressure potential could be negative due to the tension).
m = matrix potential. This represents suction, which is produced as a result of the interaction between water phase and solid surface. Its sign is negative.
s = osmotic potential. This is produced by the presence of solutes which reduces the water potential and has a negative value.
g = gravitational potential. This is very important in water from soil-water systems, but it is insignificant in plants.
In plant tissue, it is possible to find all the components of water potential. The pressure bomb is a device which can help to determine its value.
In the atmosphere there is a very low water potential. This produces a gradient in the soil-plant-atmosphere system. Such a gradient is the main cause of water movement from the soil throughout the plant and to the atmosphere.
A better understanding of the water potential in the soil-plant-atmosphere system is shown below.
Component....................Water Potential *
Atmosphere................... <-30.0 MPa
Boundary layer.............. <-10.0 MPa
Stomata........................... ~-2.0 MPa
Substomatal cavity........... ~-1.2 to -2.0 MPa
Leaf mesophyll................ ~-1.2 MPa
Xylem............................. ~-0.7 MPa
Root surface................... ~-0.3 MPa
Soil................................ 0 to -0.1 MPa
* Adapted from Mexal, J. 1997. The use of the pressure bomb. Note class. Soil620.