D2.3 Water potential

Water Potential (ψ): Water potential (ψ) is a measure of the potential energy of water per unit volume, determining the direction in which water will move. It is measured in kilopascals (kPa).

Just as a ball at the top of a hill has gravitational potential energy and rolls downhill, water molecules have potential energy and move from regions of higher water potential to regions of lower water potential.

Because potential energy is always relative, we need a reference point:

• Pure water at standard atmospheric pressure is assigned a water potential of zero (ψ = 0 kPa).
• Adding solutes or changing pressure will shift water potential away from zero.

Water potential (ψ) is determined by two main components:

${\psi }_w={\psi }_s+{\psi }_p$

Where:

• ${\psi }_w$ = water potential
• ${\psi }_s$ = solute potential (also called osmotic potential)
• ${\psi }_p$ = pressure potential

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Solute Potential (ψₛ)

Solute Potential (ψₛ): The component of water potential due to the presence of dissolved solutes. It is always zero or negative.

• Pure water: ${\psi }_s=0$
• Adding solutes makes ${\psi }_s$ more negative
• The more solutes dissolved, the lower (more negative) the solute potential
• Solutes attract water molecules via hydrogen bonding, restricting their movement and lowering their potential energy

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Pressure Potential (ψₚ)

Pressure Potential (ψₚ): The component of water potential due to physical pressure acting on water.

• Positive pressure potential: occurs in turgid plant cells where the cell wall pushes back on the contents , this raises water potential
• Negative pressure potential: occurs in xylem vessels during transpiration, where water is pulled upward under tension , this lowers water potential
• In a flaccid (limp) cell with no pressure: ${\psi }_p=0$