O-Level Nutrition and Transport in Flowering Plants: worked solution

Question

An ordinary garden plant, plant R, is placed inside a newly sealed terrarium and set under a strong lamp. Draw a sketch graph showing how the rate of transpiration in plant R changes with time during the first hour after it is put into the terrarium.

Axes: transpiration rate (y) against time (x) over the first hour. Under the strong lamp the plant initially transpires rapidly (stomata open, warmth from lamp). Because the terrarium is sealed, the water vapour cannot escape and the humidity of the enclosed air rises quickly. As the air approaches saturation the water potential gradient between leaf and air falls, so the transpiration rate decreases. The curve therefore starts high and falls, levelling off to a low value (approaching zero) as the air becomes saturated. Sketch: a curve beginning at a high point on the y-axis, declining, then flattening into a low plateau.