A plant can pull water up to the top of a tree. No pump does this work. The reason is a number called water potential. Scientists write it with the Greek letter Ψ, which is pronounced "psi".
This guide explains what that symbol means. It also shows how the number is built from smaller parts. You will see why water always moves toward the lower value, and how a real plant uses all of this to stay alive.
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| Leaves in solutions of different water potential change differently — the most visible result of a number you cannot see. Image: Pj28400, CC0, via Wikimedia Commons |
What Water Potential Means
Water potential is a way of saying how much energy water has to move around. It is measured for each unit of volume, and it is always compared with pure water under standard conditions. Pure water is the reference point, so it sits at zero.
Why use the letter Ψ? It is only a short label. Scientists use it so the number does not get confused with a word. The letter itself hides no special meaning.
The label is useful because it packs several different effects into one number. Those effects include osmosis, gravity, mechanical pressure, and the pull of water toward solid surfaces. Experts use the single value to work out water movement in plants, animals, and soil.
Several effects can act at the same time, and they do not all push the same way. Adding solutes pushes the value down. Raising the pressure pushes it up.
The direction is easy to predict from this. If nothing blocks the flow, water moves from a higher water potential to a lower one. Salt water has a negative value, so pure water will move toward it. The flow stops when the two values match, or when pressure or height balances the difference.
One Correction Worth Making
The first version of this article said the side with more water molecules has the higher water potential. That is wrong. Water potential is about free energy per unit volume, not about counting molecules. A single drop of pure water on a dry sponge has plenty of molecules, yet its potential is strongly negative.
Measuring Water Potential
Water potential is given as energy per unit volume. In practice it is quoted in pascals (Pa), kilopascals (kPa), or megapascals (MPa). Some older work also uses bars, and one bar is close to 100 kPa.
Most real values are negative, and they get more negative as things dry out. Soil at field capacity sits near −33 kPa. A plant hits its permanent wilting point at −1,500 kPa. Dry air reaches about −100 MPa. That huge gap is why a tree can move water upward without using energy of its own. The air outside is pulling far harder than the roots are.
The Parts of Water Potential
Several things can change the total. When you add them all up, you get the value that decides both how much water there is and which way it flows:
Ψ = Ψ₀ + Ψπ + Ψₚ + Ψₛ + Ψᵥ + Ψₘ
| Symbol | Part | What it stands for | Usual sign |
| Ψ₀ | Reference correction | Sets the zero the others are measured against | Set by convention |
| Ψπ | Solute (osmotic) potential | Dissolved substances lowering the energy of the water | Negative |
| Ψₚ | Pressure potential | Mechanical pressure, including turgor and xylem tension | Positive in cells, negative in xylem |
| Ψₛ | Gravity part | The effect of height | Negative, and it grows as you go up |
| Ψᵥ | Humidity part | Water vapor in the air | Strongly negative |
| Ψₘ | Matrix potential | Water sticking to solid surfaces and capillary action | Negative |
Different jobs use different subsets of these terms. Soil science usually sets its reference as pure water at the soil surface.
Solute or Osmotic Potential
Pure water is defined as having an osmotic potential of zero. Because of this, solute potential can never be positive. Van ’t Hoff’s equation links the solute concentration to that potential:
Ψπ = −MiRT
Here M is the concentration in molarity. The letter i is the van ’t Hoff factor, which compares the real effect to the ideal one. R is the gas constant, and T is the temperature in kelvin. This is where temperature enters water potential. It is not a separate part of its own.
Pressure Potential and Turgor
Pressure potential comes from mechanical pressure. It matters a lot inside a plant cell. As water enters, the pressure potential rises. The extra water pushes outward, and the rigid cell wall pushes back. That opposing pressure is what keeps the plant firm.
This firmness has a name: turgor. Without turgor, a plant loses its shape and droops. Inside a healthy plant cell the pressure potential is positive. In a plasmolyzed cell it is nearly zero.
Pressure can also go negative. This happens when water is pulled through an open pipe such as a xylem vessel. The pull is called tension. Withstanding it is a key job of the xylem, and it can be measured with a pressure bomb.
Turgor pressure is the force that pushes the membrane against the cell wall from the inside. It is also called hydrostatic pressure. You find it in plants, fungi, and bacteria, and in protists that have a cell wall. Animal cells do not have it, because without a wall they would burst under the pressure.
Matrix Potential
Water touching a solid particle feels a strong pull. Clay and sand grains in soil are full of them. That pull, together with the attraction between water molecules themselves, makes surface tension. It holds water as thin films between the grains.
The size of this effect is often larger than the other parts. It pulls the energy of water down close to every particle surface. Water moves slowly this way, but it still matters. It feeds plant roots, and engineers rely on it too.
Water Always Moves Down Its Own Gradient
Osmosis is the everyday version of this rule. Osmotic pressure is the least pressure you must apply to a solution to stop pure solvent from flowing inward through a membrane. The highest pressure that could build without such a membrane is the potential osmotic pressure. It is the gap between the pushing pressure of the solution and of the pure solvent.
Osmosis needs two solutions with different solute amounts, separated by a membrane that lets some things through but not others. Solvent passes through more easily on one side, and it keeps going until the two sides balance. That balance means the water potentials now match.
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| Osmosis is water diffusing down its own concentration gradient. Image: OpenStax, CC BY 4.0, via Wikimedia Commons |
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| The same physics, visible in a kitchen. Image: Symoum Syfullah Priyo, CC BY 4.0, via Wikimedia Commons |
Tonicity: Hypertonic, Hypotonic and Isotonic
These three words describe the same comparison, seen from one solution. What they really report is which way the net water flow goes. Mixing them up is the most common error with water potential.
A hypertonic solution has a lower water potential than the cell next to it. Water leaves the cell, the vacuole shrinks, and the membrane pulls away from the wall. This is called plasmolysis. The cell keeps almost no pressure potential afterward. Salty soil is the extreme case. Its osmotic potential can be low enough that the cells of young seedlings collapse.
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| In a hypertonic solution the cell loses water and shrinks. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons |
A hypotonic solution has a higher water potential than the cell. Water moves in, the vacuole swells, and the cell presses against its wall. This is how a plant cell becomes firm.
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| In a hypotonic solution the cell takes up water and swells. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons |
An isotonic solution has the same water potential as the cell. Nothing moves overall. The cell neither swells nor shrinks.
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| In an isotonic solution there is no net movement of water. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons |
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| Real plasmolysis under the microscope: the vacuole has shrunk. Image: Mnolf, CC BY-SA 3.0, via Wikimedia Commons |
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| At high magnification the membrane separates from the wall. Image: Nicholas.H.Hale, CC BY-SA 4.0, via Wikimedia Commons |
Water Potential in a Living Plant
Soil moisture decides how well a young plant grows. Water is needed for photosynthesis, for moving nutrients around, and for holding up the cells. Too little water means wilting and slow growth. Too much is also harmful. It fills the air gaps in the soil, so roots run short of oxygen and slow down.
Field Capacity and the Permanent Wilting Point
Two numbers mark the range that plants care about. At 0 kPa the soil is saturated. Every pore holds water, and gravity drains the large pores. At −33 kPa, which is about one third of a bar, the soil has reached field capacity. Air now sits in the large pores and water in the small ones. This is the best condition for growing plants and for microbes. Sand reaches it sooner, at −10 kPa.
At −1,500 kPa the soil has hit its permanent wilting point. Roots can no longer pull water out by osmosis. Soil keeps drying below that, down to a hygroscopic stage where only a thin film clings to the grains. Air is far more negative still. Dry air sits near −100 MPa, depending on heat and humidity.
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| Past the permanent wilting point, roots can no longer pull water from the soil. Image: Pewebe, CC BY-SA 3.0, via Wikimedia Commons |
The Soil–Plant–Atmosphere Chain
For water to rise on its own, the values must fall in the right order at every step. Roots must sit lower than the soil. The stem must sit between the two, below the roots but above the leaves. Only then does water flow by itself, from soil to root to stem to leaf to air.
The engine at the top is transpiration. This is water moving through a plant and evaporating from the parts above the soil, such as leaves, stems, and flowers. The plant spends no energy on it. Transpiration also cools the plant, changes osmotic pressure inside cells, and pulls mineral nutrients along in a bulk flow.
Transpiration cannot run forever without a limit. When roots take in less water than the leaves lose, the plant closes its stomata, the tiny pores on the leaf. That saves water. It also slows nutrient uptake and blocks carbon dioxide from entering, which cuts photosynthesis and slows growth. The plant is stuck between two bad choices. Stay open and it dries out. Stay shut and it starves.
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| Stomata on the leaf underside are where the plant controls its water loss. Image: Zephyris, CC BY-SA 3.0, via Wikimedia Commons |
Key Takeaways
- Water potential (Ψ) is the energy of water per unit volume, measured against pure water.
- The total is the sum of the solute, pressure, gravity, humidity, and matrix parts.
- Water always moves from a higher water potential to a lower one, and stops when they match.
- Solute potential is never positive, and follows Ψπ = −MiRT.
- Pressure potential is positive in a firm cell, near zero in a plasmolyzed one, and negative in xylem under tension.
- Soil at −33 kPa is at field capacity. At −1,500 kPa it has reached the permanent wilting point.
- For passive flow, roots must be lower than the soil, and leaves lower than the stem.
Frequently Asked Questions
Why is water potential written with a Greek letter, and what does it mean?
The symbol Ψ, "psi", is just a short label. Scientists use it the same way they use letters for other quantities. It hides no extra meaning. What matters is the number it stands for: the energy of water per unit volume, compared with pure water.
Is water potential the same thing as osmotic pressure?
No, and their signs point in opposite directions. Water potential is the total, and it includes solute, pressure, gravity, humidity, and matrix terms. Osmotic pressure is much narrower. It is only the pressure needed to stop solvent from flowing inward through a membrane. In a dilute solution the two match in size, but one is negative and the other is quoted as positive.
If water potential is just a number, why measure it?
Because it turns a complicated situation into one comparable value. Once every part of the soil, plant, and air has a Ψ, you can work out the direction of flow by comparison alone. You no longer need to track solutes, pressures, and heights one by one.
How is soil water potential measured in the field?
Four tools cover different ranges. Tensiometers read from 0 to about −85 kPa. Electrical resistance gypsum blocks cover −90 to −1,500 kPa. Neutron probes cover 0 to −1,500 kPa. Time-domain reflectometry reaches down to −10,000 kPa. With no equipment at all, a simple scale can still estimate how much water is there.
Why does too much water also damage a plant?
Because it pushes the air out of the soil pores. Roots need oxygen for respiration. Young plants need the right amount of moisture to develop well. Too little gives wilting and slow growth. Too much waterlogs the soil, and the lack of oxygen slows root respiration and the growth that depends on it.
Does a plant use energy to push water upward?
No. Transpiration is passive and costs the plant nothing. The plant does spend energy on growth, on opening and closing stomata, and on moving solutes. Those solutes do change water potential. But the bulk movement of water is driven by the gradient set up in the leaves. At the far end of that gradient, the air is far more negative than the soil.
Sources: Wikipedia articles on water potential, osmotic pressure, turgor, osmosis, transpiration, van’t Hoff’s equation and the soil–plant–atmosphere continuum. Images: Wikimedia Commons, with authors and licenses noted in each caption.
Water Potential, Osmosis, Solute Potential, Turgor Pressure, Plant Water Relations, Transpiration, Plant Water Uptake
