Question

Briefly describe water potential. What are the factors affecting it?

Answer 1

The term water potential was first used by Slatyer and Taylor (1960). The free energy per mole of any particular chemical species in a multicomponent system is defined as the chemical potential of that species. The chemical potential of water is referred to as the water potential (ψ_w). Since the ψ of pure water is zero (0), the presence of solute particles reduces the free energy of water, thus decreases the water potential (negative value). Therefore,ψ of solution is always less than zero or its highest value is zero.

For solutions water potential is determined by three internal factors, i.e.,ψ_w = ψ_m +  ψ_s + ψ_p (where ψ_m is matric potential which is used for the surface such as soil particles or cell wall to which water molecules are absorbed, ψ_sis solute potential, also called osmotic potential, the amount by which water potential is reduced and ψ_p is pressure potential such as TP and WP). Since in plant system ψ_m is disregarded the equation may be simplified as : ψ_w = ψ_s + ψ_p     

Answer 2

Water potential quantifies the tendency of water to move from one part to the other during various cellular processes such as diffusion, osmosis, etc. It is denoted by the Greek letter Psi or Ψ and is expressed in Pascals (Pa). The water potential of pure water is always taken as zero at standard temperature and pressure.

Water potential (Ψw) is expressed as the sum of solute potential (Ψs) and pressure potential (Ψp).

Ψw = Ψs + Ψp

When some solute is dissolved in water, the water potential of pure water decreases. This is termed as solute potential (Ψs), which is always negative. For a solution at atmospheric pressure, Ψw = Ψs.

The water potential of pure water or a solution increases on the application of pressure values more than atmospheric pressure. It is termed as pressure potential. It is denoted by Ψp and has a positive value, although a negative pressure potential is present in the xylem. This pressure potential plays a major role in the ascent of water through the stem