Water potential

Water potential is the potential energy of water relative to pure water (e.g. deionized water) in reference conditions. It quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects including surface tension. Water potential is measured in units of pressure and is commonly represented by the Greek letter $$\Psi$$ (Psi). This concept has proved especially useful in understanding water movement within plants, animals, and soil.

Typically, pure water at standard temperature and pressure (or other suitable reference condition) is defined as having a water potential of 0. The addition of solutes to water lowers its potential (makes it more negative), just as the increase in pressure increases its potential (makes it more positive). If possible, water will move from an area of higher water potential to an area that has a lower water potential.

One very common example is water that contains a dissolved salt, like sea water or the solution within living cells. These solutions typically have negative water potentials, relative to the pure water reference. If there is no restriction on flow, water molecules will proceed from the locus of pure water to the more negative water potential of the solution. This effect can be used to power a osmotic power plant.

Components of water potential
Many different potentials affect the total water potential, and sum of these potentials determines the overall water potential and the direction of water flow:


 * $$\Psi = \Psi_0 + \Psi_\pi + \Psi_p + \Psi_s + \Psi_v + \Psi_m$$

where:
 * $$\Psi_0$$ is the reference correction,
 * $$\Psi_\pi$$ is the solute potential,
 * $$\Psi_p$$ is the pressure potential,
 * $$\Psi_s$$ is the gravimetric component,
 * $$\Psi_v$$ is the potential due to humidity, and
 * $$\Psi_m$$ is the potential due to matrix effects (e.g., fluid cohesion and surface tension.)

Pressure potential
Pressure potential is based on mechanical pressure, and is an important component of the total water potential within plant cells. Pressure potential is increased as water enters a cell. As water passes through the cell wall and cell membrane, it increases the total amount of water present inside the cell, which exerts an outward pressure that is retained by the structural rigidity of the cell wall.

The pressure potential in a living plant cell is usually positive. In plasmolysed cells, pressure potential is almost zero. Negative pressure potentials occur when water is pulled through an open system such as a plant xylem vessel. Withstanding negative pressure potentials (frequently called tension) is an important adaptation of xylem vessels.

Solute potential
Pure water is usually defined as having a solute potential ($$\Psi_\pi$$) of zero, and in this case, solute potential can never be positive. The relationship of solute concentration (in molarity) to solute potential is given by the van 't Hoff equation:


 * $$\Psi_\pi = - miRT$$

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 absolute temperature.

For example, when a solute is dissolved in water, water molecules are less likely to diffuse away via osmosis than when there is no solute. A solution will have a lower and hence more negative water potential than that of pure water. Furthermore, the more solute molecules present, the more negative the solute potential is.

Solute potential has important implication for many living organisms. If a living cell with a lower solute concentration is surrounded by a concentrated solution, the cell will tend to lose water to the more negative water potential of the surrounding environment. This is often the case for marine organisms living in sea water and halophytic plants growing in saline environments. In the case of a plant cell, the flow of water out of the cell may eventually cause the plasma membrane to pull away from the cell wall, leading to plasmolysis.

Matrix potential
When water is in contact with solid particles (e.g., clay or sand particles within soil) adhesive intermolecular forces between the water and the solid can be large and important. The forces between the water molecules and the solid particles in combination with attraction among water molecules promote surface tension and the formation of menisci within the solid matrix. Force is then required to break these menisci. The magnitude of matrix potential depends on the distances between solid particles--the width of the menisci (see also capillary action)--and the chemical composition of the solid matrix. In many cases, matrix potential can be quite large and comparable to the other components of water potential discussed above.

It is worth noting that matrix potentials are very important for plant water relations. Strong (very negative) matrix potentials bind water to soil particles within very dry soils. Plants then create even more negative matrix potentials within tiny pores in the cell walls of their leaves to extract water from the soil and allow physiological activity to continue through dry periods.