A plant is moved from a cool room at 15°C to a warmer room at 28°C. The light intensity, humidity and air movement are kept the same in both rooms. Which statement correctly describes and explains the effect of this change on the plant's rate of transpiration?

IGCSE Biology 0610: Effect of Temperature on the Rate of Transpiration

Step 1: Recall what drives transpiration

Transpiration is the loss of water vapour from a leaf, which begins with water evaporating from the surfaces of mesophyll cells into the leaf’s air spaces, followed by diffusion of this water vapour out through the stomata.

Step 2: Apply the effect of a temperature rise

Raising the temperature gives water molecules more kinetic energy. This means they evaporate from the mesophyll cell surfaces more readily, and the resulting water vapour molecules also diffuse faster. Since light intensity, humidity and air movement are unchanged, temperature is the only factor responsible for the change, so the rate of transpiration increases.

Step 3: Why the other options are wrong

B wrongly assumes heat closes the stomata; nothing in the scenario suggests this, and evaporation and diffusion actually speed up with heat.
C wrongly claims temperature has no effect on water loss, ignoring its direct effect on evaporation and diffusion rates.
D confuses the air’s capacity to hold water vapour with the rate at which water evaporates from the leaf itself; the leaf’s own evaporation rate is what increases here.

Final answer

A rise in temperature increases the rate of transpiration, because water molecules evaporate and diffuse out of the leaf faster, option A.