Transpiration: Why Plants Lose Almost All Their Water

A plant drinks water from the soil all day. It uses only a tiny part of it. Almost all of the rest leaves the plant as vapor through its leaves. That loss is called transpiration.

This post explains what transpiration is, how water climbs a tree without a pump, what controls the rate, and how some plants have solved the problem.

Droplets on a leaf after a hot day. Much of this water left the plant as vapor, cooled the night air, and came back as liquid — Image: Doggo19292, CC BY 2.5, via Wikimedia Commons

What Transpiration Actually Is

Transpiration has two parts. First, water moves through the inside of the plant. Second, some of that water turns into vapor and leaves. Both together are transpiration.

The name tells you this. It joins two Latin words. Trans means across. Spirare means to breathe. So the name means breathing across, and the vapor really is water that the plant has breathed out.

The plant spends no energy on this. It burns no sugar and makes no ATP. The heat of the sun does the work instead. This is the one large piece of plant life that runs on physics alone.

Transpiration also cools the plant, in the same way sweat cools you. Water soaking up heat at the leaf surface keeps leaves from getting too hot in full sun.

Where almost all the water goes

Roots take in water and minerals by osmosis. A growing plant builds new tissue with only a small share of that water. Between 97 and 99.5 percent is lost again through transpiration and guttation. That is why a thirsty field needs so much irrigation.

The amounts look strange until you see them written out:

Plant or process	Amount of water lost
One acre of corn in one growing day	3,000–4,000 US gallons (11,000–15,000 L)
One large oak in one year	about 40,000 US gallons (150,000 L)
Growing a crop	200–1,000 kg of water for every 1 kg of dry plant matter
A leaf over one growing season	many times the leaf’s own weight

Where Water Leaves the Plant

Roots are covered in tiny hairs. Water crosses into them by osmosis, and from there it climbs a pipe network. That network is the xylem. The xylem carries water up to the leaves.

In a leaf, the water has one main exit. The leaf surface is dotted with pores called stomata. Each stoma is a hole in the leaf skin with a pair of guard cells around it. The two guard cells can swell and shrink like a pair of hands. Swollen, they curve apart and open the pore. Flaccid, they meet and shut it.

How guard cells work. Water enters them, they swell, and the pore between them opens — Image: June Kwak and Pascal Mäser, public domain, via Wikimedia Commons

One open stoma on a tomato leaf, seen down a microscope — Image: Photohound, public domain, via Wikimedia Commons

A stoma, its guard cells, and the accessory cells around them are together called the stomatal complex. A leaf can hold tens of thousands of stomata, and a single square millimeter of leaf can carry a few hundred.

One Correction Worth Making

The first version of this post said stomata are the only way water leaves a plant. That is not true. Water also evaporates through the waxy layer that covers the whole leaf. This is called cuticular transpiration. Young stems lose water too, through little breathing holes called lenticels.

Stomata still handle most of the loss, which is why the plant keeps such fine control over them. Leaves are built with two different sides. The top side carries the thick cuticle, and the underside carries most of the stomata and all of the leaf hairs.

A leaf in cross section. The two sides are built differently, and only the lower one carries most of the stomata — Image: Mnolf, CC BY-SA 3.0, via Wikimedia Commons

Some plants push water out of their leaf tips instead. The water collects as clear drops, and this is called guttation. It happens at night or early morning, from pores at the leaf edge called hydathodes. Unlike dew, which comes from the air, guttation water comes from inside the plant.

How Water Climbs Against Gravity

Water climbs to the top of a tall tree with no pump at all. Two sticking properties of water do the work.

The first is cohesion. Water molecules hold on to each other, so a chain of them can be pulled without the chain breaking. The second is adhesion. Water molecules also stick to the inside walls of the xylem, which helps hold the column in place. Both come from the way water molecules form hydrogen bonds with each other.

A cut celery stalk. The dots are xylem vessels, and they are the pipes a plant uses to carry water upward — Image: fir0002, GFDL 1.2, via Wikimedia Commons

The cohesion-tension theory

Here is the part that surprises most people. Evaporation does not do the pulling. It creates the tension, and the tension does the pulling.

Inside a leaf, water soaks the cell walls. When a molecule evaporates from a wet wall, the curve of water left behind pulls gently on the molecule next to it. That one pulls on the next. The pull travels all the way down the xylem to the roots.

The leaf then has a lower water potential than the soil. Because of that difference, water flows inward on its own. The root is not pushing and the plant is not pumping. The gap between wet soil and dry air is doing all of the work.

Xylem vessels are long tubes. Water has to travel a long way inside them, so they need wall strength and a low friction lining — Image: Zaoui58, CC BY-SA 3.0, via Wikimedia Commons

What Controls the Rate

A plant does not transpire at a fixed speed. The rate changes through the day, and the plant changes it on purpose by opening and closing stomata. The main controls are:

Factor	What happens	Why
Light	Rate goes up during the day	Stomata open to take in carbon dioxide
Humidity	Rate goes down as the air gets damp	Damp air slows evaporation
Wind	Rate goes up, up to a limit	Moving air removes the humid layer at the leaf
Temperature	Rate goes up as it gets warmer	More heat means faster evaporation
Soil moisture	Rate falls as the soil dries	Less water reaches the roots

Two of these work in opposite ways, and that is worth understanding. Temperature speeds evaporation, but it also lowers the humidity of the air, and drier air speeds evaporation again. So a hot dry day gives the highest losses of the year.

Light usually opens stomata, because the plant needs to take in carbon dioxide to make sugars. There is one big exception, and it is covered below.

How scientists measure it

A potometer is the simple classroom tool. It counts the water droplets pulled up a tube by a leafy shoot. A lysimeter weighs a block of soil and measures how much water the soil loses. A porometer measures how fast a leaf is losing vapor. Bigger tools, such as photosynthesis systems and sap flow sensors, measure whole plants or whole trees.

When water and heat both leave a field together, the total is called evapotranspiration. Isotope studies show that transpiration is the larger share of it. This matters for forecasting and for river water supply, because plants decide how much water a landscape returns to the sky instead of to the stream.

Plants That Have Solved the Problem

Plants in dry places have to lose far less water, or lose it at times when the air is already damp. There are two main strategies.

The first is to shrink the loss. Plants called xerophytes live in deserts and dry scrub. They use small leaves, thick waxy cuticles, sunken stomata set in little pits, and dense hairs on the leaf surface. The hairs trap a layer of humid air, so the air right at the surface is damper than the air above it, and evaporation slows down.

Leaf hairs under an electron microscope. They hold still air at the leaf surface, which slows water loss — Image: Dartmouth Electron Microscope Facility, public domain, via Wikimedia Commons

Many cacti go further and drop their leaves altogether. A green, fleshy stem does the photosynthesis instead, and a round or upright shape keeps the surface area small for the amount of tissue inside.

Leaf shapes of dry-country plants. Small, thick, and waxy beats large and thin — Image: Rickproser, CC BY-SA 3.0, via Wikimedia Commons

A cactus that has lost its leaves. All the photosynthesis happens in the stem, and the spines shade it — Image: Kalebzhan, CC0, via Wikimedia Commons

The second strategy is to change the timing. Many succulents, including cacti and pineapples, use CAM photosynthesis. CAM stands for crassulacean acid metabolism. In these plants the stomata open at night and close in the day. At night the air is cooler and much damper, so far less water is lost. The carbon dioxide taken in at night is stored as an acid and used in daylight, when the pores are shut.

Eucalyptus leaves hang edge-on to the sun, so they lose less water at the hottest part of the day — Image: JonRichfield, CC BY-SA 4.0, via Wikimedia Commons

When the System Breaks

The whole setup has a weakness. The water in the xylem is under tension, and a stretched column can snap. If the plant cannot replace the water fast enough, the tension gets too strong and the column breaks. The break is a bubble of water vapor inside the pipe. Air can then enter and the xylem stops working.

This is called cavitation. It happens in real plants in hot dry weather, in freezing weather, and when roots rot or are damaged. Some plants can refill the pipe afterwards, but refilling costs energy and takes time. That is part of why stomata close during a heat wave, even when the light is good for growth. Closing them slows transpiration and protects the pipes.

Key Takeaways

• Transpiration is water moving through a plant and evaporating from its surfaces, mostly its leaves.
• It is passive. The plant spends no energy on it.
• Between 97 and 99.5 percent of the water a plant takes up is lost this way.
• Stomata are the main exit. Guard cells open and close them to set the rate.
• Water also escapes through the leaf cuticle and through lenticels on stems.
• Evaporation creates tension in the xylem, and that tension is what pulls water up. The plant has no pump.
• Light, humidity, wind, temperature, and soil moisture all change the rate.
• Desert plants cut the loss with hairs, sunken stomata, small leaves, or green stems. CAM plants simply do it at night instead.

Frequently Asked Questions

Is transpiration the same as evaporation?

No. Evaporation is water turning into vapor anywhere. Transpiration is that evaporation happening inside a living plant, after the water has climbed from the roots. A lake evaporates. A tree transpires.

Why is transpiration bad for a plant?

It costs nothing to run, but the water has to be replaced. A plant has to grow in a place with enough water to refill what it loses. That is the whole reason desert plants are shaped the way they are.

Why is transpiration not a waste after all?

For the plant it is a necessary cost. For the whole landscape it does a lot of good. It shades and cools the ground, so it helps cities and forests stay cooler than the bare ground and roofs around them. That is the opposite of the urban heat island effect, which happens when plants are replaced with hard surfaces.

Do plants close their stomata at night?

Most do, because there is no need for carbon dioxide once the sun is down. Some plants open their stomata at night instead, and use the carbon dioxide the next morning. That is the CAM method.

What is guttation, and is it dew?

Guttation is water pushed out of the leaf tip by root pressure, usually at night. Dew is vapor from the air condensing on a cold surface. They look the same but come from opposite directions.

What is cavitation, and can a plant recover from it?

Cavitation is the breaking of the water column in a xylem vessel, usually by a bubble of vapor. The vessel stops carrying water. Many plants can push the bubble back out, but it costs them energy and growth.

Sources: Wikipedia articles on transpiration, stoma, the cohesion–tension theory, xylem, hydathode, cavitation, xerophyte, CAM photosynthesis and evapotranspiration. Images: Wikimedia Commons, with authors and licenses noted in each caption.

Water Potential (Ψ): What It Is and How to Calculate It

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.

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.

Osmosis is water diffusing down its own concentration gradient. Image: OpenStax, CC BY 4.0, via Wikimedia Commons

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.

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.

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.

In an isotonic solution there is no net movement of water. Image: Blausen.com staff, CC BY 3.0, via Wikimedia Commons

Real plasmolysis under the microscope: the vacuole has shrunk. Image: Mnolf, CC BY-SA 3.0, via Wikimedia Commons

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.

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.

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