Investigating transpiration

cannot be measured directly so instead it is measured indirectly, either by measuring the decrease in of a plant due to water loss, or by measuring the volume of water absorbed.

Decrease in mass

The decrease in mass of a plant due to transpiration can be investigated using the following method:

Select four leaves of similar size.
Cover the surfaces of the leaves with petroleum jelly as shown in the diagram.
Record the initial mass of the leaves.
After 24 hours record the mass of the leaves and calculate the percentage decrease in mass.

4 leaves tied to a wire. 1 has not been covered with petroleum jelly. 2 has the upper surface only covered. 3 has the lower surface only covered. 4 has both surfaces covered.

The results are shown here.

	Leaf 1	Leaf 2	Leaf 3	Leaf 4
Surface coated with petroleum jelly	None	Upper only	Lower only	Both
% decrease in mass	40	36	4	2

Surface coated with petroleum jelly
Leaf 1	None
Leaf 2	Upper only
Leaf 3	Lower only
Leaf 4	Both

% decrease in mass
Leaf 1	40
Leaf 2	36
Leaf 3	4
Leaf 4	2

Question

What conclusions can you make from the results above? Give two pieces of evidence to support your conclusion.

Potometer

A can be used to measure the rate of transpiration that’s proportional to water uptake. Transpiration cannot be measured directly as some of the water will be used in photosynthesis. The rate of transpiration can be calculated by measuring the distance travelled by an air bubble in a capillary tube over a given time. The faster the bubble moves, the greater the rate of water uptake – and so the greater the assumed rate of transpiration.

The potometer is set up underwater to avoid unwanted air bubbles in the xylem of the plant which may disrupt the transpiration stream. All joints are sealed with petroleum jelly to make it as airtight as possible.

Method

A single air bubble is introduced into the capillary tubing.
The tap on the reservoir is opened to add water to push the air bubble back to zero on the scale.
A timer is started and a set time measured.
The distance the air bubble has travelled along the scale is recorded.
The experiment can be repeated with different environmental conditions.

The faster the bubble moves, the greater the rate of water uptake – and so the greater the rate of transpiration.

Factors affecting rate

The rate of transpiration is affected by several factors, including:

temperature
humidity
wind speed
light intensity

This table shows the effects of different factors on the rate of transportation.

Factor	Effect	Explanation
Temperature	Increased	Evaporation and diffusion are faster at higher temperatures
Humidity	Decreased	Humidity decreases the concentration gradient between the inside and outside of the leaf – this reduces transpiration
Wind speed	Increased	Moving air removes water vapour, increasing the rate of diffusion of water vapour from the leaf
Light intensity	Increased	The stomata open wider to allow more carbon dioxide into the leaf for photosynthesis

Factor	Temperature
Effect	Increased
Explanation	Evaporation and diffusion are faster at higher temperatures

Factor	Humidity
Effect	Decreased
Explanation	Humidity decreases the concentration gradient between the inside and outside of the leaf – this reduces transpiration

Factor	Wind speed
Effect	Increased
Explanation	Moving air removes water vapour, increasing the rate of diffusion of water vapour from the leaf

Factor	Light intensity
Effect	Increased
Explanation	The stomata open wider to allow more carbon dioxide into the leaf for photosynthesis

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Structure of plants – WJECInvestigating transpiration