Relationship between osmotic potential & pressure potential with water potential

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).

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.     Gravity

·        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.