Learning focus
Develop accurate biological explanations, interpret plant diagrams and data, and connect practical evidence with theory.
Temperature
Higher temperature increases kinetic energy and evaporation from mesophyll surfaces. Vapour diffuses faster, usually increasing transpiration.
Wind speed
Wind removes the humid boundary layer around the leaf and maintains a steep water-vapour concentration gradient. Strong wind therefore usually increases transpiration.
Humidity
High humidity reduces the difference in water-vapour concentration between leaf and air, lowering diffusion. Low humidity steepens the gradient.
Light intensity
Light commonly causes stomata to open for carbon dioxide uptake. This increases the route for water loss. In very hot or dry conditions, stomata may close despite bright light.
Interactions
Factors act together. A hot, dry, windy day strongly promotes water loss. A still, humid environment reduces it. Conclusions should be based on measured conditions rather than one factor in isolation.
Practical or data skill
Predict the direction of change, then explain using evaporation, stomatal aperture, boundary layer or concentration gradient.
Examination tip
For humidity, refer to the water-vapour gradient between the leaf and surrounding air.
Review questions and suggested answers
Question 1
How does wind increase transpiration?
Suggested answer
It removes humid air around the leaf and maintains a steep gradient.
Question 2
Why does high humidity reduce transpiration?
Suggested answer
It reduces the water-vapour concentration gradient.
Question 3
How can light increase transpiration?
Suggested answer
It causes stomata to open for carbon dioxide uptake.
