Water potential

Water potential:

Water potential is the difference between free energy of water in a system and the free energy of pure water at atmospheric pressure and a defined temperature.  It is represented by Greek letter Psi (Ψ) and is usually measured in bars.

Water always moves from the area of high water potential to the area of low water potential.  The movement of water molecule occurs down the energy gradient. The free energy of a solvent is increased by increasing the temperature and pressure.  The presence of solute particles reduces the free energy of water and thus, decreases the water potential.

Water potential of pure water at atmospheric pressure is zero.  Water potential of the solution is always less than zero or has negative values.  Free energy of water in cell sap or solution is less than that of pure water, i.e., less than zero.

Water potential is equal but opposite in sign to the diffusion pressure deficit (D.P.D.).  If a difference in water potential exists between two regions, spontaneous movement of water will take place and the direction of flow will be energetically downhill, i.e., from the region of higher water potential to the region lower water potential

Components of water potential:

For a solution such as contents of cell, water potential is determined by three sets of internal factors like:

1. Matric potential (Ψm).
2. Solute potential (Ψs).or osmotic potential
3. Pressure potential (Ψp).

Ψ= Ψm+ Ψs+Ψp

Matric potential:

Matrix is the term used for the surface (such as soil particles, cell wall, protoplasm etc.) to which water molecules are adsorbed.  It is the component of water potential which is influenced by the presence of matrix.  It has got a negative value in case of plant cell and tissue.  The equation for water potential may be simplified as follows:

Ψ= Ψs+Ψp

Solute potential or osmotic potential:

Osmotic potential or solute potential is the amount by which the water potential is reduced as a result of the presence of solute.  Solute potentials are always in negative values.  A reference standard is pure water at a water potential equal to zero.  A reduction from this value gives a negative number.

Pressure potential:

Pressure potential is a hydrostatic pressure.  This is a pressure which develops in osmotic system due to osmotic entry or exit of water from it.  A positive pressure develops in a plant cell or system due to entry of water into it.  Positive hydrostatic pressure is  called turgor pressure.  Loss of water produces negative hydrostatic pressure.  It develops in xylem due to loss of water in transpiration.  Pressure potential increases the water potential in the system.

The relationships among the various potentials can be expressed as follows:

Ψ= Ψm+ Ψs+Ψp

Since matric potentials are insignificant and can be ignored, the above relationships can be simplified as:

Ψ= Ψs+Ψp

A flaccid cell has no turgor pressure or pressure potential is zero bars and if the osmotic potential or solute potential is -10 bars, then

Water potential=osmotic pressure+pressure potential

Ψ= Ψs+Ψp=-10 bars

In a fully turgid cell, there would be no water movement.  For example, if a cell has solute potential of -10 bars and pressure potential of 10 bars, the water potential would be zero.

Ψ= Ψs+Ψp=-10 bars+10 bars=0 bars

Fig : Relationship between components of water potential

Role of turgor pressure in plants:

1. The opening and closing of stomata are caused by gain and loss of turgidity by their guard cell. Hence, they are often called “turgor-operated valves.”
2. Turgor pressure (pressure potential or hydrostatic pressure) keeps a check on the excess entry of water into the cell.
3. Due to turgor pressure the cells and their organelles are stretched . This is essential for the proper functioning of the cells.
4. It keeps the leaves fully expanded and properly oriented to light. Flower, young stems and other softer organs are able to maintain their form due to turgidity or turgor pressure.
5. In case of loss of turgidity, the shoots droop down and the leaves show wilting.
6. Turgor changes in the cells of their pulvini cause sleep movements (nyctinasty) of the leaves of many legumes and shock movements, (seismonasty) of sensitive plants, (for example, Mimosa pudica) .

Osmotic Relations of Plant Cells and Water Potential

A typical plant cell has a semi-permeable membrane, a cell wall, a vacuole and an osmotically active solution called cell sap.  A plant cell functions as an osmotic apparatus or osmotic system.

The osmotically active cell sap has an osmotic potential.  It causes the osmotic entry of water which develops a turgor pressure or pressure potential.

Osmotic potential or solute potential is a negative force.  It causes the entry of water in the cell.  As a result a cell protoplast swells and develops the positive force of turgor pressure or pressure potential.  Being a positive force turgor pressure or pressure potential opposes the entry of water into the cell.

Fig: Plant cell as an osmotic system

Osmotic relations of cells according to water potential:

In case of fully turgid cell:

As the cell becomes fully turgid, the value of turgor pressure becomes equal to that of solute potential (Ψs ) so that water potential becomes either zero or equal to that of external hypotonic solution.

There is no net movement of water between the cell and its environment.  The cell is in equilibrium with the water outside and so the water potential becomes zero.  Water potential is equal to osmotic potential + pressure potential.

Ψ=Ψs+Ψp=0

In case of flaccid cell:

A plant cell kept in hypertonic solution loses water by exosmosis.  The volume of the central vacuole decreases.  As a result, the protoplast is reduced in size.  This decreases the turgor pressure or pressure potential and the turgor becomes zero.  A cell at zero turgor has an osmotic potential equal to its water potential.

Ψ=Ψs+Ψp

Ψ=Ψs

In case of plasmolysed cell:

Pressure potential is nil in case of plasmolysed cell . Hence water potential is equal to osmotic potential.

Ψp=nil

Ψ=Ψs

Difference between Matric Potential and Osmotic Potential

 Matric Potential Osmotic Potential 1.  It is reduction in free energy of water due to formation of thin immobile surface layer of water molecules. 1.  It is reduction in free energy of water due to decrease in number of water molecules per molal volume. 2.  Adsorbant or colloidal particles cause matric potential. 2.  Presence of solute particles cause osmotic potential. 3.  It is essential for seed germination, early seedling growth and growth of developing fruits. 3.  It is important in water relation of plant cells.

Difference between water potential and diffusion pressure deficit

 Water Potential Diffusion Pressure Deficit 1  It has a negative value. 1.  It has a positive value. 2.  Water potential is the reduction in free energy of solvent in a system over its pure state. 2.  D.P.D. is the reduction of in diffusion pressure of a solvent in a system over its pure state. 3.  Water is absorbed by a system with lower water potential from another system with higher water potential. 3.  Water is absorbed by a system having higher D.P.D. from another system with lower D.P.D.