Students should be able to explain the effect of changing temperature,…

The idea

Transpiration is the loss of water vapour from leaves, mainly through stomata. The rate depends on water vapour diffusing out, driven by the concentration gradient between the leaf's interior and the surrounding air. Temperature raises the kinetic energy of water molecules, increasing evaporation and steepening the gradient. Humidity reduces the gradient because moist external air slows diffusion. Air movement removes humid air from around the leaf, maintaining a steep gradient and increasing the rate. Light intensity stimulates stomatal opening for photosynthesis, so more stomata are open and the rate increases. A useful method is to compare water uptake of a leafy shoot using a potometer, or mass loss using a balance.

What earns the marks

• Temperature: higher temperature increases the kinetic energy of water molecules, so evaporation from mesophyll cells and diffusion through stomata are faster, increasing the rate of transpiration.
• Humidity: higher humidity reduces the water vapour concentration gradient between the air spaces inside the leaf and the outside air, so diffusion of water vapour is slower and the rate decreases.
• Air movement: wind or a fan removes water vapour from the leaf surface, keeping the external concentration low and maintaining a steep diffusion gradient, so the rate increases.
• Light intensity: brighter light causes stomata to open for carbon dioxide uptake in photosynthesis, providing more open pores for water vapour to diffuse out, so the rate increases.
• A valid explanation links each factor to the steepness of the diffusion gradient or to stomatal aperture, rather than simply stating that the rate goes up or down.

Common mistakes and how to put them right

• Saying that high humidity increases transpiration because there is more water in the air.

  Put it right: A smaller concentration gradient slows diffusion, so the rate decreases.

• Claiming that light directly heats the leaf and that this alone explains the effect.

  Put it right: Light mainly affects stomatal opening, while temperature affects molecular kinetic energy and evaporation.

• Stating that air movement pushes water out of the leaf.

  Put it right: Moving air removes water vapour from near the leaf surface, helping to maintain a steep diffusion gradient.

Exam tips

• Name the factor, state whether the rate rises or falls, then explain the effect on the diffusion gradient or on stomatal opening.
• Use the phrase concentration gradient accurately: a steeper gradient means faster diffusion of water vapour.
• When describing an experiment, identify the independent variable, the control variables such as temperature and light, and how water uptake or mass loss is measured.