Explain the effect of environmental factors on the rate of water uptake by…

The idea

Water uptake by roots is driven mainly by transpiration pull, so any factor that speeds transpiration tends to raise the rate of uptake. Increasing light intensity opens stomata and warms the leaf, increasing evaporation from mesophyll surfaces and so increasing the transpiration stream and uptake. Greater air movement removes humid air from around the leaf, steepening the water vapour concentration gradient and increasing the rate. Higher temperature increases the kinetic energy of water molecules, so they evaporate faster, again raising uptake. Each factor works through the same mechanism: a steeper gradient or faster evaporation increases transpiration, which pulls more water up the xylem. Uptake is measured, for example, with a potometer, and results should be compared at controlled values of the other variables.

What earns the marks

• Higher light intensity opens stomata and warms the leaf, increasing evaporation and therefore the transpiration stream and water uptake.
• Increased air movement removes humid air near the leaf surface, maintaining a steep water vapour gradient and raising the rate of uptake.
• Higher temperature increases the kinetic energy of water molecules so they evaporate faster, increasing transpiration and uptake.
• All three factors act by increasing the rate of transpiration, which pulls water up the xylem from the roots.
• Fair comparisons require other variables, such as humidity and wind speed, to be controlled when one factor is changed.

Common mistakes and how to put them right

• Saying light directly pulls water up the stem.

  Put it right: Light acts by opening stomata and warming the leaf, which increases transpiration and hence uptake.

• Claiming still air increases transpiration.

  Put it right: Still, humid air reduces the gradient, so moving air increases the rate.

• Treating temperature and light as the same variable.

  Put it right: They act through different mechanisms, kinetic energy and stomatal opening respectively.

• Forgetting that uptake is a consequence of transpiration.

  Put it right: The driving force is water loss from leaves creating tension in the xylem.

Exam tips

• State the mechanism for each factor, not just the direction of change; link to the water vapour gradient or kinetic energy.
• Use comparative language such as 'increases' or 'decreases' and identify the factor being changed.
• Mention that other environmental variables must be kept constant to make a valid comparison.