Measuring Transpiration

Name Date

Plants transport water in the form of a continuous column of liquid inward from the root surface to

the xylem and upward in the stem from root to shoot and out into the leaves. In the leaves liquid

water turns to vapour and diffuses out into the atmosphere by the process of transpiration. A

continuous supply of water is needed to replace water lost from aerial parts of the plant by

transpiration. This replacement water moves upward in plant tissue, xylem that is adapted for rapid,

long distance transport. The rate of transpiration, and hence of upward movement of water in the

plant, depends on several factors, temperature, humidity and wind.

The rate of water movement up a cut stem of a shoot (the stem is part of a shoot bearing leaves)

caused by transpiration can be quantitatively estimated by a potometer. The potometer is a device

designed for measuring rates of water uptake by a stem. The cut end of a stem with leaves is inserted

into a chamber containing water. The chamber is a sealed, flexible, uncalibrated tube connected to a

pressure sensor. Uptake of water by the plant from the chamber causes a decrease in pressure in the

water column (a negative pressure) and this decrease is detected and quantified by the pressure

sensor.

It is important to realize that the cut shoot takes up water faster than the intact plant under the same

conditions. This is because a major resistance to water uptake located at membranes of root cells has

been removed. Thus a potometer connected to a cut shoot can estimate the influence of

environmental and internal factors on uptake but not the true value of uptake for an intact plant.

Light causes potassium ions to be pumped into the guard cells which lowers their water potential and

so water diffuses into the guard cells causing them to go turgid and so open. At night potassium

moves out of the guard cells into the surrounding cells so the water diffuses out of the guard cells

causing them to close.

In hot climates the water loss by transpiration can exceed the water uptake from the roots which

causes the plants to suffer from water stress. To combat this the plant closes the guard cells and so

reduces the water loss by transpiration.

A photometer, sometimes known as a transpirometer is a device used for measuring the rate of

water uptake of a leafy shoot. The causes of water uptake are photosynthesis and transpiration.

Materials

- Plant shoot - Distilled water - Large basin

- Potometer - Plasticine

Procedures

1. Fill a basin with water.

2. Remove the plunger from the mounted syringe on the potometer. See Figure

3. Immerse the apparatus in the basin of water.

4. Re-insert the plunger and move the plunger in and out to remove any air bubbles trapped in the

tube

5. Carefully trim a plant shoot by cutting off the bottom under water with a sharp razor blade at

an angular cut. Keep the leaves out of the water.

6. Carefully insert the newly cut stem into the soft rubber tube and seal off by moulding plasticine

around the stem. It must be water tight so that no

leakage of water occurs.

7. Re-insert the plunger into the syringe up to the 2ml mark.

8. Remove the apparatus from the basin. Be careful not to dislodge the specimen from the rubber tube, if necessary support the specimen with your hand.

9. Let the potometer equilibrate for 5 minutes before recording the time zero reading. If the photometer does not equilibrate, slowly pull out the plunger until the water line reads 0.

10. Expose the plant in the tubing to one of the following treatments (you will be assigned a treatment by your teacher). After each reading, push the plunger into the syringe body in

order to return the water in the capillary tube to zero.

a. Heated

b. Floodlight

c. Fan (place at least 1 meter from the plant, on low speed, creating a gentle breeze).

11. Read the level of water in the pipette at the beginning of your experiment (time zero) and record your finding in the Table

Potometer Readings

Time (min) Beginning

(0) v3ss fff6ff 9 12 15 18 21 24 27 30

Light

4nnnnnnn 4nnnnnn nnnn4

Wind

Humidity

Temperature

Conclusions

1. Which changes of conditions resulted in (a) an increase and (b) a decrease in the rate of water uptake?

2. Suggest reasons for the changes of rate in each case.

3. Of the conditions which caused an increased rate, which one seemed to have the greatest effect?

4. (a) How might an increase in light intensity affect the rate of photosynthesis in the shoot?

(b) What influence might this change in photosynthetic rate have on the rate of water uptake?

(c) On what grounds could you justify the assumption that a change in uptake resulting from

an increase in light intensity is due largely to a change in the rate of transpiration rather than

in the rate of photosynthesis?

5. It is usually emphasized that the potometer does not measure the rate of transpiration but only

the rate of uptake.

(a) Why do you think this distinction is made?

(b) Why is it still reasonable to use the potometer to compare rates of transpiration of the

same shoot in different conditions?