(6) An experiment to investigate the rate of transpiration - using a potometer

A much more sophisticated experiment to look at the rate of transpiration.

The potometer - a way of measuring the uptake of water by a plant - it measures the rate of transpiration

You are measuring the rate of uptake of water by the plant, which is an estimate of the rate of water loss by transpiration.

It is NOT completely accurate because some of the water is used up in photosynthesis in making glucose.

You can lose water from leaks, which is why the potometer system should be carefully sealed water tight.

 

A potometer consists of a vertical tube with a plant shoot sealed in it plus a system to feed in water and some means of measuring the water uptake of the plant.

The plant shoot should be cut under water to prevent air entering the xylem and at an angle to increase the surface area for water absorption.

Ideally, the whole apparatus is assembled under water.

The tube is connected to a reservoir of water, controlled by a tap, used to replace water lost by transpiration.

The horizontal capillary tubing, connected to the plant tube and reservoir, has a scale set up beside it.

Inside the capillary tube, adjacent to the scale is an air bubble which will move along to the left as water evaporates from the leaves.

The whole apparatus must be airtight and watertight or the readings will be inaccurate and initially have the tap open.

You need to let the plant acclimatise ('settle down') to the laboratory conditions before starting the experiment and then shut the tap.

Then remove the end of the capillary tube from the beaker of water and let one air bubble into the capillary tube which is then put back under the water in the beaker.

When the bubble reaches the scale, record the initial reading and start the stop-watch..

When the bubble has moved a significant distance along the scale, record the distance and the time.

relative rate of transpiration = distance bubble moved / time taken

 

At the start and after a measurement of transpiration has been made, the reservoir tap is opened to allow water to flow in and move the air bubble to the right near the start of the scale.

Excess water runs out into the beaker which also acts as a reservoir of water itself during an experimental run, rather than sucking in air.

 

Making a measurement

You note the starting position of the air bubble on the right.

You then measure the time it takes for the air bubble to move from right to left and note the total distance moved.

To repeat the experiment you let water in from the reservoir to bring the bubble back to near the start of the scale.

Estimated transpiration rate = distance air bubble travels / time taken

So your rate of transpiration units might be mm or cm/min (an arbitrary scale based on the experimental setup).

BUT, note that the experiment assumes the water uptake by the plant through its roots is directly related to the water loss by evaporation from the leaves.

 

The sorts of investigations you can do with a potometer

Remember

Wherever possible keep everything constant except the one factor you are investigating e.g. constant temperature, constant air humidity in the laboratory.

You can now investigate various environmental conditions by comparing the relative uptake of water.

1. Varying light intensity:

You can use a reasonably powerful light to increase light intensity, placing it quite close to the plant, with the bulb at the same height as the centre of the plant shoot.

You measure the distance of the centre of the lamp bulb to the centre of the plant shoot (d).

Run the experiment and measure the rate of transpiration.

You can then move the same lamp back, measuring the new longer distance and re-measure the transpiration rate. You should repeat all measurements for varying lamp distances.

The intensity of light on the plant is proportional to1/d² or you can measure the intensity with a light meter.

You can then plot a graph of transpiration rate against light intensity.

You must make sure the temperature and humidity are constant and there is no air flow over the plant (still air).

It would be easy to do comparative experiments in a brightly lit room and a darkened room.

 

2. Varying air flow:

You could use a hair dryer to blow cold air (room temperature) at different speeds over the plant to see if increased air flow increases transpiration rate. Difficult to get data to produce a meaningful graph, but no air blowing, low blowing setting and a high blowing setting should give you the trend.

You must make sure the temperature, light intensity and humidity are constant and the light intensity stays the same.

 

3. Varying temperature:

This is also tricky to get good quantitative data.

You can easily compare blowing cold air and warm air over the plant, and that should give a difference in transpiration rate.

BUT, not that easy to keep conditions constant.

You can measure the temperature of the air near the plant during the experiment.

It might be convenient to do the experiment in a cold room and then in a warm room.

You must make sure the air flow is constant (completely still air is the best condition), and the humidity and light intensity stay the same.

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Key points on an experiment to investigate the rate of transpiration with a potometer

Source of information is based on textbooks and syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10)

Investigating the Rate of Transpiration with a Potometer

A potometer is a device used to measure the rate at which a plant loses water through transpiration.

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Apparatus

• Potometer (e.g. bubble potometer or mass potometer)

• Healthy leafy shoot

• Beaker of water

• Capillary tubing with a scale

• Rubber bung or sealant

• Stopwatch

• Ruler

• Vaseline (optional – to seal joints)

• Environmental controls (lamp, fan, etc.)

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Method (Bubble Potometer Setup)

1. Set up the apparatus: Fill the potometer completely with water to remove air bubbles.

2. Cut a leafy shoot underwater at a slant to prevent air entering the xylem and increase surface area.

3. Insert the shoot firmly into the bung under water.

4. Ensure all joints are watertight (use Vaseline if needed).

5. Introduce a small air bubble into the capillary tube by lifting the tube briefly out of the water reservoir and back in.

6. Start a stopwatch and measure how far the air bubble moves in a set time (e.g. every 2 minutes).

7. Repeat readings and record results in a table.

8. Change environmental conditions (e.g. increase light or wind) to observe their effect on transpiration rate.

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Variables

• Independent variable: Environmental conditions (e.g. light intensity, temperature, wind)

• Dependent variable: Distance air bubble moves (volume of water taken up)

• Control variables: Same plant species, same leaf size/number, constant temperature/humidity

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Recording Results

e.g. create a results table like this:

(Note: mm³ is used if the capillary has a known cross-sectional area)

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Calculations

Rate of transpiration can be calculated using the formula:

Rate of transpiration in mm/min = Distance moved by air bubble (mm) / Time (minutes)

Or, if cross-sectional area of the capillary is known:

Volume = π r² h ] where:

• r = radius of capillary tube

• h = distance bubble moved

Then:

Rate of transpiration (mm³/min) = Volume of water taken up / Time

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Limitations and Considerations

• Potometer measures water uptake, which is an estimate of water loss (some water is used in photosynthesis or stored).

• Ensure no leaks in the system.

• Avoid air bubbles in the xylem.

• Keep environmental conditions constant unless deliberately changing them.

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Keywords, phrases and learning objectives for this part on transport systems in plants

Be able to describe, with the aid of a diagram, an experiment to investigate rate of transpiration using a potometer apparatus.

Know how to set up a potometer and the procedure for making measurements to measure the rate of transpiration in a plant.

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