Water potential is the tendency of water to move from one place to another. It is typically measured in units of atmospheric pressure: pascals or pounds force per square inch or bars or dynes per square centimeter.

Pure water has a defined water potential of zero. It is possible for the water potential to be positive or negative depending on the size of $\backslash Psi\_p$ or $\backslash Psi\_\backslash pi$

Relation to free energy

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Water potential ($\backslash Psi$) is related to Gibbs free energy by the following equation, where $V\_w$ is the molar volume of water:

$\backslash Psi\; =\; \backslash frac\{G\}\{V\_w\}$

Relation to water content

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A water retention curve depicts matric water potential ($\backslash Psi\_m$) as it relates to water content(θ). Different wetting and drying curves may be distinguished due to hysteresis.

Simple Systems

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Multiple different potentials affect the total water potential. In a simple system, the primary two components are the pressure potential ($\backslash Psi\_p$) and the solute potential ($\backslash Psi\_\backslash pi$ sometimes also $\backslash Psi\_s$). In this simple system, the water potential is given by the following formula:

$\backslash Psi\; =\; \backslash Psi\_p\; +\; \backslash Psi\_\backslash pi$

Pressure potential

See also: osmotic pressure Pressure potential (sometimes called turgor pressure) is increased as water enters a plant cell. As water passes through the cell wall and cell membrane, it increases the total amount of water present inside the cell, which exerts a pressure on the cell wall that is retained by the structural rigidity of the cell wall.

Solute potential

Pure water has a solute potential ($\backslash Psi\_\backslash pi$) of zero. As solute is added, the value for solute potential becomes negative. The relationship of solute concentration (in molality) to solute potential is given by the Van't Hoff Equation:

$\backslash Psi\_\backslash pi\; =\; -\; miRT$

where $m$ is the concentration in molarity of the solute, $i$ is the Van 't Hoff factor, the ionization constant of the solute (1 for glucose, 2 for NaCl, etc.) $R$ is the ideal gas constant, and $T$ is the temperature.

For example, when a solute is dissolved in water, the water molecules are less likely to diffuse away via osmosis than when there is no solute. Assuming atmospheric pressure, a solution will have a lower and hence more negative water potential than pure water. The more concentrated a solution is, the more negative its water potential will be. Because water will spontaneously attain the lowest energy level possible, water will move from a higher potential to a lower potential. Thus, a cell with a lower solute concentration than the surrounding environment will have a higher water potential than the surrounding environment, and will lose water to the surrounding environment. In the case of a plant cell, this will eventually cause the cytoplasm to pull away from the cell wall, leading to plasmolysis.

Complex Systems

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There are other contributors to water potential, and their contribution is given by the following equation:

$\backslash Psi\; =\; \backslash Psi\_0\; +\; \backslash Psi\_\backslash pi\; +\; \backslash Psi\_p\; +\; \backslash Psi\_g\; +\; \backslash Psi\_v\; +\; \backslash Psi\_m$

where $\backslash Psi\_g$ is the gravimetric component, $\backslash Psi\_v$ is the potential due to humidity, and $\backslash Psi\_m$ is the potential due to matrix effects (eg, fluid cohesion and surface tension.)

HydrostaticsSoil physics

Wasserpotenzial