Water Potential Calculator

The water potential calculator takes various components into consideration: pressure potential (the physical pressure on water), solute potential (the effect of dissolved substances), gravitational potential (the effect of height differences), and matric potential (the adhesion of water to surfaces).

A researcher studying plant drought response needs to understand if water will move from soil into plant roots. By entering the pressure potential (+0.3 MPa), solute potential (-0.5 MPa), gravitational potential (0 MPa), and matric potential (-0.1 MPa) into the calculator, they can determine the total water potential (-0.3 MPa) and predict water movement direction.

Water Potential Calculation Formula

The Water Potential Formula combines all forces affecting water movement in a system:

Ψtotal = Ψp + Ψs + Ψg + Ψm

Where:

• Ψtotal = Total water potential
• Ψp = Pressure potential
• Ψs = Solute potential
• Ψg = Gravitational potential
• Ψm = Matric potential

All components are measured in megapascals (MPa), with more negative values indicating greater potential for water attraction.

Example calculation: A plant cell has a pressure potential of +0.5 MPa from turgor pressure, a solute potential of -0.7 MPa from dissolved ions, negligible gravitational potential (0 MPa), and a matric potential of -0.1 MPa from cell wall interactions. Using the formula:

Ψtotal = +0.5 + (-0.7) + 0 + (-0.1) = -0.3 MPa

The negative value indicates that this cell would tend to absorb more water if available.

How to Calculate Water Potential

• Enter Pressure Potential (Ψp): Input the pressure component in MPa in the first field. For plant cells, this is typically positive due to turgor pressure (e.g., +0.5 MPa). For soil, it’s often 0 MPa.
• Enter Solute Potential (Ψs): Input the osmotic component in MPa in the second field. This value is almost always negative due to dissolved solutes decreasing water potential (e.g., -0.8 MPa).
• Enter Gravitational Potential (Ψg): Input the gravitational component in MPa. For most small-scale systems, this can be left at the default value of 0 MPa or set to a small negative value for vertical systems.
• Enter Matric Potential (Ψm): Input the matric component in MPa. This is typically negative and represents water binding to surfaces (e.g., -0.2 MPa for soil).
• The calculator will instantly sum all components and display the total water potential along with a breakdown of each component.
• A more negative total value indicates greater water-attracting potential. Water flows from less negative to more negative potentials.

Example using our calculator: A botanist studying water movement from soil to plant roots would:

• Enter 0 MPa for soil pressure potential
• Enter -0.3 MPa for soil solute potential
• Enter 0 MPa for gravitational potential
• Enter -0.4 MPa for soil matric potential
• Click “Calculate” to see the soil’s total water potential: -0.7 MPa

They would then compare this to the root’s water potential (perhaps -1.2 MPa) to predict that water will move from soil into roots since roots have the more negative water potential.

What is Water Potential?

Water potential is a fundamental concept in plant physiology representing the potential energy of water relative to pure water under standard conditions. It determines the direction and rate of water movement—water always flows from areas of higher (less negative) to lower (more negative) water potential.

Example 1: Comparing Soil and Root Water Potential

Soil water potential:

• Pressure potential: 0 MPa
• Solute potential: -0.2 MPa
• Gravitational potential: 0 MPa
• Matric potential: -0.3 MPa
• Total: 0 + (-0.2) + 0 + (-0.3) = -0.5 MPa

Root water potential:

• Pressure potential: +0.1 MPa
• Solute potential: -0.8 MPa
• Gravitational potential: 0 MPa
• Matric potential: -0.1 MPa
• Total: +0.1 + (-0.8) + 0 + (-0.1) = -0.8 MPa

Since root water potential (-0.8 MPa) is more negative than soil water potential (-0.5 MPa), water will move from soil into roots.

Example 2: Leaf Water Potential During Drought

Well-watered leaf:

• Pressure potential: +0.7 MPa
• Solute potential: -0.9 MPa
• Gravitational potential: -0.02 MPa
• Matric potential: -0.1 MPa
• Total: +0.7 + (-0.9) + (-0.02) + (-0.1) = -0.32 MPa

Drought-stressed leaf:

• Pressure potential: +0.1 MPa
• Solute potential: -1.2 MPa
• Gravitational potential: -0.02 MPa
• Matric potential: -0.3 MPa
• Total: +0.1 + (-1.2) + (-0.02) + (-0.3) = -1.42 MPa

The drought-stressed leaf has a much more negative water potential, indicating significant water stress.

Example 3: Osmotic Adjustment in Saline Conditions

Control plant cell:

• Pressure potential: +0.4 MPa
• Solute potential: -0.6 MPa
• Gravitational potential: 0 MPa
• Matric potential: -0.05 MPa
• Total: +0.4 + (-0.6) + 0 + (-0.05) = -0.25 MPa

Salt-exposed plant cell (after osmotic adjustment):

• Pressure potential: +0.5 MPa
• Solute potential: -1.1 MPa
• Gravitational potential: 0 MPa
• Matric potential: -0.05 MPa
• Total: +0.5 + (-1.1) + 0 + (-0.05) = -0.65 MPa

This demonstrates how plants adjust to saline conditions by accumulating solutes, making their water potential more negative to maintain water uptake.

References

• Nobel, P.S. (2009). Physicochemical and Environmental Plant Physiology. Academic Press.
• Taiz, L., & Zeiger, E. (2010). Plant Physiology (5th ed.). Sinauer Associates.
• University of California, Davis. (2023). Water Relations of Plants. Plant Sciences.