Comparison of transpiration rates

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[Genetics and Adaptation (Plant adaptations for maintaining a water balance)]

Comparison of transpiration rates

STUDENT ACTIVITY GUIDE

Introduction

A potometer is a piece of apparatus used to measure the rate of water loss from a plant (transpiration). The rate of transpiration can be estimated in two ways:

1) Indirectly - by measuring the distance the water level drops in the graduated tube over a measured length of time. It is assumed that this is due to the cutting taking in water which in turn is necessary to replace an equal volume of water lost by transpiration.
2) Directly - by measuring the reduction in mass of the potometer over a period of time. Here it is assumed that any loss in mass is due to transpiration.

Although many metabolic processes (e.g. respiration and photosynthesis) also change water content and mass of the plant their effects are usually ignored when conducting potometer experiments. This is because the volume of water involved in metabolic processes is insignificant compared to the large volume constantly flowing through the plant in the transpiration stream.

	When setting up a potometer, several precautions must be followed: e.g. 1. the plant stem must be cut under water to prevent airlocks forming in the xylem. 2. all fittings must be tight so that the only way the apparatus can lose water is by the plant transpiring.

	When the plant is replaced by a porous pot, the apparatus is called an ATMOMETER. Atmometers are set up alongside potometers. They provide a non-living surface to compare with the potometer. When potometer and atmometer results are different this indicates that stomata in the leaves are reacting to their surroundings and controlling water loss while the non-living surface of the atmometer simply reflects the evaporating power of the atmosphere. The size of the atmometer is important. Why?

Experiment

Below is a list of situations where you can put your potometer / atmometer.

NB 1) Remember to change just one environmental factor at a time.
2) Allow the plant time to get used to its new environment (at least 2-3 minutes) before starting to take measurements.

Situations for potometer / atmometer - take measurements over a 5 minute period each time.

	1. In the open lab.
	2. Under a strong light source.
	3. In a dark cupboard.
	4. In a warm incubator.
	5. Windy conditions on upper surfaces of leaves.
	6. Windy conditions on lower surfaces of leaves.
	7. Enclosed in a polythene bag.

Results

These can be recorded in a table as below:

Alternatively, results can be obtained by measuring loss of mass at each situation.

Conclusions

By comparing potometer results, write down your conclusions. Use the atmometer results to see when stomata seem to be controlling water loss in the potometer.

Student Notes

1. What is a potometer?

2. Give two ways that results can be obtained from a potometer?

3. (i) What biological process produces water and also decreases mass of a plant?

(ii) What biological process uses water as a raw material and also increases the mass of a plant?

4. Why are the effects of these processes usually ignored when taking potometer measurements?

5. What is an atmometer?

6. What is an atmometer measuring?

7. What factors affect transpiration rate?

8. In what situation(s) do potometer and atmometers respond differently? Explain the difference.

9. What precautions must be followed when setting up a potometer?

10. If you were comparing transpiration rates of several leaves, what leaf feature should be measured to obtain a fair comparison?

11. What steps were taken while carrying out the experiment to ensure that results were both meaningful and reliable?

TECHNICAL GUIDE

Setting up a Simple Potometer

Apparatus required

Small glass jar
Rubber bung (to fit jar) with two 4 mm holes and one 2mm hole
2 cm³ graduated pipette or 30cm length of capillary tubing
10 cm³ or 20 cm³ 'Gillette' plastic syringe

Method

1. Collect a 2 cm³ graduated glass syringe, or piece of capillary tubing, and carefully push it into the larger holes in the bung. (If the hole needs to be enlarged, temporarily insert a no. 3 cork borer into the hole until the capillary tubing is fitted.)

2. Fill a sink with water.

3.Take a plant cutting at least 15-20 cm long with a stem diameter of about 5 mm. Immediately plunge the cut end of the stem in the sink of water and remove the bottom 2-3 cm of stem with sharp scissors. Leave the cutting in the sink.

4. To temporarily open the bung hole further, put the no.3 cork borer through one of the larger holes starting from the narrow end of the bung. Immerse it in the water.

5. Keeping the bung immersed, gently insert the cutting into the cork borer far enough so that the cutting goes all the way through the bung. Carefully remove the cork borer. The stem of the cutting should now fit tightly in the hole.

6. Immerse the syringe and small jar in the water, making sure both are full of water.

7. Place the bung in the neck of the small jar. Insert the syringe nozzle into the small hole of the bung.

8. Now remove the entire apparatus from the sink and remove surface moisture with a paper towel.

9. Push syringe plunger down until the capillary tubing is full of water or the water level is at zero in the graduated pipette. The potometer is now ready for use.

Equipment used by SAPS:

Vessels for plant tissue culture, 100 cm³ capacity, 66mm height
Cat. no. V8630 Sigma Biosciences (Tel. 0800 373731)

Two-hole rubber stoppers 41x47x42 mm long (make a third hole using a no.1 cork borer)
Cat. no. Y59032/3 Philip Harris

Reference

Burgess J.A. (1983) An improved potometer, School Science Review , 64 (229) 699-701.

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