Future Scientists

Revision note 3.6

Transpiration

Transpiration is the loss of water vapour from a plant. It links evaporation inside leaves with diffusion through stomata and water uptake through roots.

CoreRequired practicalCalculation

What you need to know

  • Describe evaporation from mesophyll cells and diffusion through stomata.
  • Predict effects of light, temperature, humidity and air movement.
  • Explain how stomatal closure affects water loss and photosynthesis.
  • Use a potometer and calculate water-uptake rate.
  • Relate xerophyte adaptations to reduced transpiration.

The transpiration pathway

Xylem deliveryWater reaches leaf veins through xylem.
Mesophyll surfacesWater moves onto moist cell walls.
EvaporationLiquid water becomes water vapour.
Air spacesVapour enters internal leaf spaces.
DiffusionWater vapour moves through stomata to drier air.

Transpiration is an unavoidable consequence of opening stomata for carbon-dioxide uptake. The rate depends on the water-vapour concentration gradient and stomatal opening.

Environmental factors

Factor increasesTypical effectExplanation
Light intensityRate usually increases, then levels off.Stomata often open for photosynthesis; once fully open another factor limits.
TemperatureRate increases.Evaporation and particle movement are faster; warm air can hold more water vapour.
Air movementRate increases.Moving air removes the humid boundary layer around the leaf.
HumidityRate decreases.The water-vapour concentration gradient between leaf and air becomes smaller.

These predictions assume other variables stay constant. Very hot or dry conditions may cause stomata to close, reversing a simple trend.

Stomatal control and water balance

Guard cells regulate stomatal apertures. When water is plentiful and conditions favour photosynthesis, open stomata allow carbon dioxide to enter. During water shortage, closing stomata conserves water but restricts carbon-dioxide uptake.

If water loss exceeds root uptake, leaf cells lose turgor and the plant wilts. Continued water deficit can stop growth, reduce photosynthesis and damage tissues.

Misconception AlertTranspiration is the movement of water up the xylem.Select to reveal the correctionSelect to hide the correction
Correct understanding

Transpiration is the loss of water vapour from a plant's aerial parts. Water movement through the xylem replaces the water that is lost.

Required practical: a bubble potometer

Bubble potometer with an upright leafy shoot sealed into a vertical water-filled tube, a reset reservoir with tap, a ruler beside a capillary containing one air bubble, and the capillary end submerged in water.
A bubble potometer estimates water uptake by tracking movement of an air bubble along a water-filled capillary tube.
  1. Cut a leafy shoot under water to reduce air entering xylem.
  2. Assemble the water-filled apparatus under water and seal every joint.
  3. Introduce one air bubble into the capillary.
  4. Record the distance moved in a measured time.
  5. Use the reservoir to return the bubble to its starting point.
  6. Change one environmental variable, allow acclimatisation, repeat and calculate means.
water-uptake rate = distance moved × capillary cross-sectional area ÷ timeIf only distance is measured, report distance per unit time as a relative rate.

A potometer measures water uptake, not transpiration directly. Some absorbed water is used in photosynthesis, growth or maintaining turgor, although most is often lost by transpiration under the test conditions.

Control and reliability

Keep leaf area, shoot species, temperature and exposure time constant unless one is the independent variable. Check for leaks, repeat readings and calculate a mean.

Adaptations to dry environments

Xerophytes are plants adapted to conditions where water is scarce. Possible adaptations include:

  • a thick waxy cuticle that reduces evaporation;
  • fewer stomata or stomata sunk in pits that trap humid air;
  • leaf hairs that reduce air movement near stomata;
  • rolled leaves that enclose a humid space;
  • small leaves or spines that reduce surface area;
  • succulent tissues that store water;
  • extensive shallow or deep root systems that improve water capture.
Exam tip

Name the feature, then state how it changes the diffusion gradient, exposed area or air movement.

Quick retrieval check

1. Where does water evaporate inside a leaf?
From moist mesophyll cell walls into the internal air spaces.
2. Why does moving air usually increase transpiration?
It removes humid air around the leaf and maintains a steep water-vapour concentration gradient.
3. Why is a potometer an indirect measurement?
It measures water uptake; not all absorbed water is immediately lost by transpiration.
4. A bubble moves 36 mm in 12 minutes. Calculate the relative rate.
36 ÷ 12 = 3 mm min−1.

Exam connection

Question

Explain why increasing temperature from 20°C to 30°C may increase potometer bubble speed, but increasing it further to 45°C may not.

Show the mark points
  • Higher temperature initially increases evaporation and diffusion.
  • This increases water loss and therefore water uptake.
  • At very high temperature, water loss can exceed supply.
  • Guard cells lose turgor or stomata close.
  • The smaller stomatal opening reduces further water loss.