Transpiration, Cohesion-Tension, and Stomatal Regulation

Transpiration is the loss of water vapor from the aerial parts of a plant, mainly through the stomata, and according to the cohesion-tension theory it pulls a continuous column of water up through the xylem, from roots to leaves, while guard cells regulate the size of the stomata. In brief: water evaporates from the wet surfaces of mesophyll cells, creating a negative pressure (tension) that pulls on the water in the xylem, and cohesion between water molecules and adhesion to the vessel walls keep the column unbroken. A large tree can transpire hundreds of liters of water a day. Stomata open when guard cells take up water and become turgid, and close when they lose water.

Transpiration is unavoidable because stomata must open for CO₂ to enter, so the plant needs to balance gas exchange against water loss. This guide explains how transpiration works, the cohesion-tension mechanism, the factors that affect the rate, how guard cells regulate stomata, how transpiration is measured, works through a potometer calculation and corrects common mistakes.

What is transpiration?

More than 95 percent of the water that a plant absorbs is lost as vapor, and only a small fraction is used in photosynthesis or remains in the plant tissue. Most of the loss takes place through stomata (stomatal transpiration), with a small part through the cuticle. A single maize plant can transpire about 200 liters of water during a growing season, and a large oak tree several hundred liters per day.

Transpiration is not just a waste. It provides the pull that moves water and minerals from the roots to the shoot, supplies water for photosynthesis, cools the leaf by evaporation and keeps cells turgid. The path of water is: soil → root hair → cortex → xylem of the root → xylem of the stem → leaf veins → mesophyll cell walls → air spaces → stomata → atmosphere.

How does the cohesion-tension theory explain water movement?

1. Evaporation: water evaporates from the thin film on the walls of mesophyll cells into the air spaces of the leaf. As water leaves, the water menisci in the tiny spaces between cellulose fibers curve inward, creating a surface tension that pulls water from deeper in the wall.
2. Tension in the xylem: this pull lowers the pressure (water potential) in the leaf, which draws water from the xylem. The water in the xylem is therefore under negative pressure, i.e. tension. In tall trees, the pressure can be −1 to −3 MPa (about −10 to −30 atmospheres).
3. Cohesion: water molecules are attracted to each other by hydrogen bonds, so the pull is transmitted down the whole column, like a chain, to the roots. Pure water in a thin tube can withstand a very high tension without breaking.
4. Adhesion: water molecules are also attracted to the lignified walls of the xylem, which helps to hold the column against gravity.
5. Uptake in roots: as water is pulled up, water is drawn into the root from the soil, down the water potential gradient (from soil to root to leaf to air).

The process is passive and powered by the sun’s energy, which drives evaporation. No ATP is needed for the movement through xylem. The theory was tested by measuring the diameter of tree trunks, which shrinks slightly during the day, when tension is the highest. Root pressure, from active uptake of ions, can contribute a small push at night, and explains guttation (drops of water at leaf edges), but it cannot explain the transport in tall trees.

What factors affect the rate of transpiration?

• Light: light causes stomata to open, so more light increases transpiration.
• Temperature: higher temperature increases evaporation and also the capacity of air to hold water vapor, so the water vapor gradient is steeper.
• Humidity: dry air has a lower water potential, so the gradient is steeper and transpiration is faster. In humid air, it is slower.
• Wind: removes the layer of humid air around the leaf, which steepens the gradient and increases transpiration.
• Water supply: if the soil is dry, the guard cells lose turgor and stomata close.
• Leaf structure: number and size of stomata, thickness of the cuticle, leaf area and hairs.

How do guard cells regulate stomata?

Each stoma is surrounded by two guard cells. They have unevenly thickened walls: the inner wall (next to the pore) is thicker and less elastic than the outer wall, and cellulose microfibrils run around the cell. When water enters, the cells swell and become turgid, and because of their structure they bend outward, which opens the pore. When they lose water, they become flaccid, and the pore closes.

The mechanism of opening is:

1. In light (especially blue light), proton pumps in the guard cell membrane use ATP to pump H⁺ out of the cell.
2. This makes the inside more negative, and K⁺ ions enter through channels. Chloride ions and malate (made from starch) also accumulate.
3. The solute concentration in the guard cells rises and their water potential falls, so water enters by osmosis. The cells become turgid and the stoma opens.

Stomata close in the dark, when CO₂ concentration inside the leaf is high, or when the plant is short of water. The hormone abscisic acid (ABA), produced in roots and leaves during water stress, causes K⁺ and anions to leave the guard cells. Water follows by osmosis, the cells go flaccid and the stomata close. Most plants have more stomata on the underside of the leaves, where it is cooler and less water is lost. Typical densities are 100 to 300 per square millimeter.

Factors and their effects

Worked example: potometer measurements

A potometer measures the uptake of water by a cut shoot, which is used as an estimate of transpiration. A shoot is attached to a capillary tube with an internal diameter of 1.0 mm. A bubble moves 45 mm in 5 minutes in still air and 90 mm in 5 minutes in front of a fan. The shoot has a total leaf surface area of 100 cm² (0.01 m²).

1. Cross-sectional area of the tube: A = πr² = 3.14 × (0.5 mm)² = 0.785 mm².
2. Volume of water taken up in still air: 0.785 mm² × 45 mm = 35.3 mm³ (0.0353 cm³ or mL) in 5 minutes.
3. Rate in still air: 35.3 ÷ 5 = 7.1 mm³/min.
4. Rate with the fan: 0.785 × 90 = 70.7 mm³ in 5 minutes, i.e. 14.1 mm³/min, twice as fast.
5. Rate per unit area (still air): 7.1 mm³/min ÷ 100 cm² = 0.071 mm³ per cm² per minute.

The fan doubled the uptake, which shows the effect of wind. Potometers measure water uptake, not exactly water loss, because a small amount of water is used for photosynthesis and for keeping the cells turgid, but over a short time the difference is small. Controlling the other variables (temperature, light, and humidity) and repeating measurements makes the experiment more reliable. Cutting the shoot under water avoids bubbles in the xylem.

How do plants reduce water loss?

Xerophytes, plants of dry places, have features that reduce transpiration. Marram grass rolls its leaves to trap humid air; cacti have spines instead of leaves, a thick cuticle and stomata that open at night (CAM photosynthesis); and many desert plants have hairs, sunken stomata and deep or wide-ranging roots. Some plants drop their leaves in the dry season. Hydrophytes, living in water, have the stomata on the upper leaf surface, because their lower surface is in water.

How was the mechanism discovered?

Stephen Hales measured transpiration in the early eighteenth century by weighing potted plants. Henry Dixon and John Joly proposed the cohesion-tension theory in 1894, and Dixon showed that water in thin tubes can withstand tension. Per Scholander measured the tension in xylem with a pressure chamber in the 1960s, and confirmed that it can be several MPa negative. The role of ABA in stomatal closure was shown in the 1960s and 1970s.

Which mistakes are common?

Mistake 1: Saying that water is pushed up from the roots

It is mostly pulled up from the leaves by transpiration. Root pressure is a minor effect.

Mistake 2: Saying that transpiration needs energy from the plant

The energy comes from the sun, which drives evaporation. The plant does not use ATP for the movement of water in the xylem.

Mistake 3: Confusing the effects of humidity

High humidity reduces transpiration, because there is less of a gradient between leaf and air.

Mistake 4: Forgetting the role of guard cells

Stomata are opened and closed by changes in turgor in the guard cells. They are not muscles.