Concept of Water Potential

CHAPTER: 6

Concept of Water Potential

Water Potential:

•      The water content in the soil, plants and atmosphere is usually described as water potential (Ψw).

•      By definition, the potential of free pure water at atmospheric pressure and at a temperature of 25°C corresponds to 0 (zero) Mpa or bar.

•      More precisely, the water potential represents all the water pressure in a given system and it is the sum of osmotic potential (Ψs), matrix potential (Ψm), hydrostatic pressure or the turgor potential (Ψt) and the gravitational potential (Ψg).

•      Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure, or matrix effects such as capillary action (which is caused by surface tension).

•      Water potential has proved especially useful in understanding water movement within plants, animals, and soil. Water potential is typically expressed in potential energy per unit volume and very often is represented by the Greek letter Psi (Ψ ) .

Ø  This  water potential depends upon the given five factors;

1) concentration of the substances      2) Pressure

3) Temperature

4) chemical potentiality of the substance in the solution

5) Matrix System

Ø  Water potential can be expressed in equation as,

             Water Potential (Ψw) = Ψs + Ψm + Ψt+ Ψg 

                        where, osmotic/solute potential (Ψs),

                                      matrix potential (Ψm) ,

                                      turgor potential (Ψt) or pressure potential                  

gravitational potential (Ψg)  (significant in tall plant.)

Importance of water potential:

•      It is an importance force which determines the water status in plant cell or organ.

•      The osmotic movement of water into or out of the system is due to Water Potential (Ψw)

•      The cells suffering from water deficit or water system are therefore able to avoid injury by obtaining water from other cell.

•      The air dried seed and spores are able to penetrate and avoid temperature (low & high) injury and drought due to very low water potential.

•      Plants use water potential to transport water to the leaves so that photosynthesis can take place.

•      Water potential is a measure of the potential energy in water as well as the difference between the potential in a given water sample and pure water.

Thermodynamical Concept of Water Potential:

•      According to thermodynamics laws every component of a system possesses free energy capable of doing work under constant temperature condition.

•      Osmotic movement of water involves certain work done and in fact the main driving forces behind this movement is the difference between free energies of water on two sides of the semi-permeable membrane.

•      For non electrolytes, free energy/mol is known as chemical potential(Ψ). With reference to water this is called as water potential (Ψw).  

•      Like other substances the absolute value of water potential cannot be determined and measured ,instead this value for pure water is arbitrarily fixed as zero at one atmosphere and a particular temperature.

•      Water potential is lowered by the addition of solutes. So, due to the zero value of water potential for pure water, all other water potential values will be negative.

•      The movement of water will takes place in osmotic or other system from a region to higher water potential (less -ve) to a region of lower water potential(More -ve).

       1. Explain the relationship between osmotic potential & pressure potential with water potential.

       Water potential mainly depends on concentration, pressure, and gravity. If the symbols of water potential, the effects          of solutes, pressure, & gravity are denoted by Ψw, Ψs, Ψp & Ψg  , then water potential can be expressed as;

       Water Potential (Ψw) = Ψs + Ψp+ Ψg 

·                     In plants of small height (less than 5 meters), Ψg is negligible. So the equation becomes as;

                                                Ψw = Ψs + Ψp

·                     Pure water is usually defined as having osmotic potential (Ψs) of zero. As the solute is added solute potential or osmotic potential (Ψs) decreases. So, in this case solute potential can never be positive.

·                     The pressure potential (turgor potential) on the other hand in living plant cell is usually positive. In plasmolysed cells & open system , Ψp = 0. Negative pressure potential occurs when water is pulled through an open system such as a plant xylem vessels.

Ø     So, in the living cells,

·                     If , Ψs = -ve  & Ψp = +ve (or, when pressure potential is less negative than the osmotic potential) then,  Ψw =  -ve.

·                     If Ψs =  Ψp i.e., for e.g. Ψs = -1 &  Ψp = +1 ( or, when pressure potential equals to osmotic potential) then, Ψw =   0 (zero).

·                     If the value of pressure potential exceeds the value of osmotic potential then, Ψw =  +ve. ( But this is not practically feasible because  it is considered that the value of water potential for pure water is zero).

2.   Give the different parameters involved in the determination of water potential. OR,

What are the factors involved in affecting the water potential?

·                     Basically, there are three parameters involved in the determination of water potential (Ψw). They are:

1.                   Solute concentration           2. Pressure             3. Garvity

·                     Sometimes matrix potential of the system also affects the water potential.

a.                   Solute concentration:

 In pure water the value of water potential is maximum i.e., it is zero. Addition of solutes reduces the free energy of water. The term Ψs is used for denoting the concentration of the solute and its effect on the water potential. It is termed solute potential or the osmotic potential.

b.                   Pressure:

During osmosis the entry of water results in the development of hydrostatic or turgor pressure which is here called as pressure potential (Ψp). If the pressure potential is positive it will add to the water potential but if it is negative it reduces the value of water potential.

c.                    Gravity:

The term Ψg termed gravity potential denotes the effect of gravity on the water potential of a water column in a vertically growing plant. It’s magnitude depends on the height of the plant from the ground level as well as on the density of water and the acceleration due to gravity. In plants of small height (less than 5 meters) the Ψg is negligible.

Ø     Water potential is decreased by factors which reduce the relative water vapor viz., by addition of solutes, negative pressure or tensions, reduction in temperature and by matrix forces.

Ø     Water potential is increased by factors which increase the negative vapor pressure, mechanical pressure and increase temperature.