Investigating transpiration - Part three

Insufficient water affects the yields of crops. Different factors affect transpiration and therefore water uptake. They can be investigated using a potometer.

Laboratory experiments are designed to simulate conditions a plant may encounter:

ConditionWhen encountered by plantHow the condition is produced in the laboratory
Air movementWindy conditionsFan
Coatings on leaves that block stomataAirborne pollution; horticultural use on cuttings and Christmas trees Smear leaf surface(s) with petroleum jelly
High light intensitySunny climates; artificial lighting in greenhouses Artificial lighting giving a high light intensity
High temperatureWarm and hot climatesHeater or heated greenhouse

When investigating the effect of light intensity on water uptake, what are the control variables?

Temperature, no air movement or draughts. It is also important to use the same species of plant, and shoots with a similar leaf surface if different plant shoots are used.

Example results

Here is a graph with some data collected from four laboratory experiments on factors affecting water uptake by a plant.

Graph showing factors affecting water uptake by a type of plant

Under which conditions is the rate of transpiration greatest?

At high temperatures. The graph indicates the greatest rate of transpiration has the steepest gradient.


What is the rate of movement of the bubble in the potometer when the air is moved by the fan?

6.8 mm/min.

This could be calculated from 205 ÷ 30, or use any other two points.

Calculate the gradient:

Calculating the gradient from a line graph

Value of x = 0 minutes

Value of y = 0 millimetres

Value of x = 30 minutes

Value of y = 205 millimetres

\textup{Gradient~of~line} = \frac{\textup{increase~in}~y}{\textup{increase~in}~x} = \frac{(205-0)~\textup{mm}}{(30-0)~\textup{min}}\: = 6.8~\textup{mm/minute}