Measuring water uptake - potometers

Investigating transpiration

The uptake of water can be measured using a potometer. Under normal circumstances, the rate of water uptake gives a measure of the rate of transpiration.

A simple potometer is a piece of capillary tubing to which a plant has been connected. The water uptake is measured by recording the time taken for a bubble in the tube to move a set distance.

Aim of the experiment

To find the rate of water uptake of a plant.

Method

Diagram of how to conduct an experiment of how to find the rate of water uptake in a plant

The potometer is filled with water.

Variables

  • Independent variable - time
  • Dependent variable - the distance moved by the bubble along the capillary tube
  • Control variables - temperature, air flow or draughts, adequate supply of water

Risks

Care must be taken when inserting the plant shoot into the rubber tubing attached to the capillary tube and with the scissors used to cut the plant shoot.

Example results

Time in minutesDistance moved by bubble in mm
00
516
1032
1547
2064
2580
3095

Analysis of results

Plot a line graph of the data.

Graph showing the rate of water uptake

The rate of water uptake is best found using the gradient of the graph.

You need to calculate the gradient of the graph - and not take values from the table of results.

Choose any two points on straight line:

  • choose as wide an interval along the x-axis as is possible
  • try to make sure that these correspond with values of y that you can read accurately

The most accurate results will be obtained if these points are as far apart as possible. Here, the two points on the line, labelled P and Q, are at either end of the line.

Calculate the gradient:

P Value of x = 0 minutes

P Value of y = 0 millimetres

Q Value of x = 30 minutes

Q Value of y = 96 millimetres

Gradient\; of\; line\: =\: \frac{increase\: in\: y}{increase\: in\: x }\: =\: \frac{(96-0)\: mm)}{(30-0)\: min)}\: =\: 3.2\: mm/minute

The distance moved by the bubble is 3.2 mm/minute.

Note that rate is always a compound measurement. It is made up of two - or more - measurements. In this case, millimetres per minute.

curriculum-key-fact
Note that the gradient/rate is calculated from the line of best fit and not from the table of data.
Question

When will the water uptake of a plant not equal the rate of transpiration?

When it is short of water, eg it has wilted.