BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.3. Transpiration and Water Movement Through Leaves

13.3.5. Measuring the Rate of Transpiration

Transpiration can be easily demonstrated by covering a potted plant with a bell jar while sealing the pot tightly in a plastic bag to prevent Water loss from the soil. As transpiration proceeds, liquid droplets collect on the inside of the jar. This liquid can be proven to be water using cobalt(II) chloride indicator paper (which changes from blue to pink) or anhydrous copper(II) sulfate crystals (which turn from white to blue).

Measuring The rate of transpiration can be challenging, but satisfactory results—at least for comparison purposes—can be obtained using two simple Methods described below.

Practical 13.4. Investigating and measuring The Effect of various factors on the rate of transpiration using a potometer

A potometer is a device designed to measure the rate of water uptake by leafy shoots or seedlings. While it cannot measure transpiration directly, the two processes are closely linked because almost all absorbed water is lost through transpiration. Although potometers can be purchased, a simple version is quite easy to assemble yourself, as shown in Fig. 13.10.

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Fig. 13.10. A simple potometer.

Materials and Equipment

Potometer (Fig. 13.10; Buchner/filter flask, short rubber connector, one-holed rubber bung, syringe with hypodermic needle, graduated capillary tube)

Large black polyethylene bag

Large transparent polyethylene bag

Small electric fan

Retort stand with clamps

Stopwatch

Thermometer

Vaseline

Leafy SHOOT, e.g., a lilac twig

Bucket of water

Method

1. Select a suitable leafy plant, cut a shoot, and immediately submerge the cut end in water to minimize air entry into the xylem. Quickly recut the stem underwater with a slanting cut a few centimeters above the first one. The stem must be thick enough to fit tightly into the potometer bung.

2. Submerge the conical flask in water so that it fills completely. Still underwater, insert the stem into the bung and seal the flask securely.

3. Submerge the capillary tube in water until it is also filled. Connect it to the side-arm of the flask using the rubber tubing adapter.

4. Remove the assembled device from the bucket, insert the syringe needle filled with water into the rubber adapter, and secure it in a vertical position on a stand, as shown in Fig. 13.10. Seal the joint where the stem enters the stopper with petroleum jelly to ensure the system is airtight.

5. As the shoot absorbs water, the fluid meniscus in the capillary tube will shift. It can be returned to its initial position by adding a small amount of water from the syringe. Allow the shoot to stand for 5 min, regularly pumping water into the system during this time.

6. Record the time it takes for the water Column in the capillary to move a specific distance, and calculate the rate of water uptake in suitable units, such as cm/min. Make several measurements to ensure that the rate remains relatively constant, and then calculate the average value. Note the air Temperature around the plant.

7. Each time the air bubble reaches the end of the graduated section of the capillary, return it to its starting position by injecting water with the syringe.

8. This technique can be used to investigate how the rate of water uptake by a shoot is affected by the following factors;

a) wind (use a small electric fan for this; the airflow should not be too strong, otherwise the Stomata will close);

b) humidity (enclose the shoot in a transparent plastic bag);

c) light intensity (enclose the shoot in a black plastic bag);

d) removal of half the leaves (does this reduce the rate of transpiration by half?)

e) coating the upper and/or lower leaf epidermis with petroleum jelly to prevent water loss.

In each case, allow sufficient time for the rate of transpiration to stabilize. It is not always possible to alter only a single factor; for instance, covering the shoot even with a transparent bag will also slightly reduce light intensity.

Absolute rate of water uptake

The obtained results can be expressed as the actual volume of water absorbed per unit of time, such as ml/h; this requires knowing the volume of water corresponding to a single division on the capillary scale.

The bulk of the absorbed water is lost through the leaves. The rate of water loss per unit of leaf surface area can be determined by first measuring water loss as described above, and then removing all the leaves and determining their total surface area. This area can be measured by placing the leaves on graph paper, tracing their outlines, and counting the number of squares covered by the leaves. The final result can be expressed in ml/h per 1 m² of leaf surface area.

Results

The effects of temperature, humidity, wind, and light intensity on the rate of transpiration are discussed in Section 13.3.6.