Water potential and its components
What water potential is, its solute, pressure, matric and gravity parts, how water moves from soil to air, and worked osmosis and plasmolysis sums.
On this page
A tall tree lifts hundreds of litres of water a day from the soil to leaves tens of metres above the ground, with no pump. A slice of potato swells in fresh water and goes limp in salt water. A wilting plant recovers within an hour of irrigation. All of these are explained by one quantity: water potential, written with the Greek letter psi (Ψ).
Water potential is the foundation of plant water relations and appears in almost every plant physiology exam, usually with calculations.
What water potential means
Water potential is the potential energy of water per unit volume, compared with pure, free water at the same temperature and atmospheric pressure, which is given a value of zero. It is measured in pressure units, usually megapascals (MPa); 1 MPa = 10 bar, roughly 9.9 atmospheres.
The key rule is simple: water moves spontaneously from higher (less negative) to lower (more negative) water potential. Anything that lowers the free energy of water, such as dissolved solutes or binding to surfaces, makes Ψ negative; positive pressure makes it higher.
The term was introduced to plant physiology by R. O. Slatyer and S. A. Taylor in 1960, replacing older terms such as diffusion pressure deficit and suction pressure that still appear in some older textbooks.
The components
- Solute (osmotic) potential, Ψs: dissolved solutes lower the free energy of water, so Ψs is always zero or negative. For dilute solutions it follows the van 't Hoff equation.
- Pressure potential, Ψp: hydrostatic pressure. In a living, turgid cell it is positive (turgor pressure); in the xylem of a transpiring plant it is usually negative (tension).
- Matric potential, Ψm: the lowering of water's energy by adsorption to surfaces and capillary forces in small pores, as in soils, cell walls and dry seeds. It dominates in soils and in imbibing seeds and is often folded into Ψs or Ψp for cells.
- Gravitational potential, Ψg: about 0.01 MPa per metre of height. It is negligible in cells and short crops but matters in tall trees.
where i is the ionisation constant (1 for sucrose, about 1.8 for sodium chloride in practice, up to 2 in ideal dilute solutions), C the molar concentration (mol/L), R the gas constant (0.008314 L·MPa/(mol·K)) and T the absolute temperature (K).
Water in cells: turgor, wilting and plasmolysis
For a plant cell, Ψ = Ψs + Ψp.
- In pure water, a cell takes up water until turgor pressure balances its solute potential and Ψ reaches zero: the cell is fully turgid.
- As it loses water, turgor falls; at zero turgor, Ψ = Ψs and the cell is flaccid. Leaves wilt as many cells approach this point.
- In a strong solution, the protoplast shrinks away from the cell wall: plasmolysis. Incipient plasmolysis, when the protoplast just begins to separate, is used to estimate a tissue's solute potential.
Which way will water move?
A cell has a solute potential of −0.8 MPa and a turgor pressure of 0.5 MPa. It is placed in a 0.1 M sucrose solution at 25 °C.
- Cell: Ψ = −0.8 + 0.5 = −0.3 MPa.
- Solution: Ψs = −iCRT = −1 × 0.1 × 0.008314 × 298 ≈ −0.25 MPa (and Ψp = 0 in an open beaker).
- The solution (−0.25) has a higher water potential than the cell (−0.3), so water moves into the cell until the cell's turgor rises by about 0.05 MPa and the two are equal.
In a 0.4 M sucrose solution (Ψ ≈ −0.99 MPa), water would leave the cell; turgor would fall to zero, and the cell would plasmolyse once its Ψs (which becomes slightly more negative as it loses water) could no longer match the solution.
From soil to air: the continuum
Water moves through the soil–plant–atmosphere continuum (SPAC), always down a gradient of water potential.
- Soil: matric potential dominates. Field capacity is often taken as about −0.01 to −0.033 MPa and the permanent wilting point as about −1.5 MPa.
- Roots take up water because their Ψ is lower than the soil's.
- Xylem: water is pulled up under tension, as explained by the cohesion–tension theory of H. H. Dixon and J. Joly (1894, published 1895). Evaporation from leaf cell walls creates tension, and the cohesion of water molecules transmits it down unbroken water columns to the roots.
- Leaves: Ψ is lowest in the plant, commonly −0.5 to −2 MPa by day, and much lower in drought-adapted species.
- Air: even humid air has an extremely low water potential, which drives transpiration.
The water potential of air
Air's water potential depends on relative humidity (RH):
Ψ = (RT ⁄ Vw) × ln(RH), where Vw is the molar volume of water (18 × 10−6 m3/mol).
At 20 °C, RT ⁄ Vw = 8.314 × 293 ÷ (18 × 10−6) Pa ≈ 135 MPa.
- At 50 per cent RH: Ψ = 135 × ln 0.5 = 135 × (−0.693) ≈ −94 MPa.
- At 98 per cent RH: Ψ = 135 × ln 0.98 ≈ −2.7 MPa.
Even at 98 per cent RH the air is drier, in energy terms, than most leaves, which is why leaves lose water whenever stomata are open.
Measuring water potential
- Pressure chamber (Scholander bomb, 1965): a cut leaf or shoot is sealed in a chamber with its cut end outside, and pressure is raised until sap appears at the cut surface; that pressure equals the negative of the xylem tension. It is the standard field method, used for irrigation scheduling in vineyards and orchards.
- Thermocouple psychrometers measure the humidity in equilibrium with a tissue sample.
- Tissue weight-change and Chardakov dye methods find the solution in which tissue neither gains nor loses water.
- Plasmolysis methods estimate solute potential.
- In soils: tensiometers (wet range) and soil water potential sensors.
Key points
- Water potential (Ψ) is the energy of water relative to pure water (Ψ = 0); water moves from higher to lower Ψ.
- Ψ = Ψs (solute, negative) + Ψp (pressure, positive in turgid cells, negative in xylem) + Ψm (matric) + Ψg (gravity, 0.01 MPa/m).
- Ψs = −iCRT; 0.1 M sucrose at 25 °C ≈ −0.25 MPa.
- Turgid, flaccid and plasmolysed cells; incipient plasmolysis estimates Ψs.
- Water moves soil → root → stem → leaf → air under the cohesion–tension mechanism; the largest drop is from leaf to air.
- Pressure chambers, psychrometers and tensiometers measure water potential.
What the exams ask
- School-leaving and entrance papers: osmosis, turgor, plasmolysis, and the direction of water movement between cells.
- Degree and research-fellowship entrances: components of water potential, van 't Hoff calculations, water potential of air, SPAC, cohesion–tension theory, and measurement methods.
- Professional and research examinations: plant water status in irrigation scheduling, drought physiology, and osmotic adjustment.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is water potential negative in plants?
Pure water is defined as zero, and solutes, surface binding and tension all lower water's free energy below that. Only positive pressure, such as turgor, raises it, and in living plant tissue the net result is almost always negative.
What is osmotic adjustment?
The accumulation of solutes, such as sugars, proline and potassium, in cells under drought or salinity. It lowers Ψs, so cells keep turgor and keep taking up water at lower soil water potentials.
Why does salty soil make plants wilt even when it is wet?
Salts lower the soil's osmotic potential, so roots must reach a lower water potential to take up water. If they cannot, the plant suffers water stress in wet soil, known as physiological drought.
Sources
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer Associates, 2015. Chapters 3 and 4.
- Slatyer, R. O. and Taylor, S. A. "Terminology in plant–soil–water relations." Nature 187, 922–924 (1960).
- Dixon, H. H. and Joly, J. "On the ascent of sap." Philosophical Transactions of the Royal Society B 186, 563–576 (1895).
- Scholander, P. F., Hammel, H. T., Bradstreet, E. D. and Hemmingsen, E. A. "Sap pressure in vascular plants." Science 148, 339–346 (1965).
- Kramer, P. J. and Boyer, J. S. Water Relations of Plants and Soils. Academic Press, 1995.
More in plant physiology and botany
Discussion
Questions, corrections and notes from readers on this article. Every comment is checked before it appears: keep it on the topic, and do not share phone numbers, links or contact details.