Factors Affecting the Rate of Transpiration

The rate at which transpiration occurs is affected by several environmental factors

Environmental factors & transpiration table

Environmental factor	Effect on transpiration rate	Explanation
Light intensity	As light intensity increases, the transpiration rate increases	At high light intensities the rate of photosynthesis increases, so stomata open to allow gas exchange; this allows more water to diffuse out of the stomata
Temperature	As temperature increases, the transpiration rate increases	At high temperatures water vapour molecules have more kinetic energy, increasing the number of water molecules that diffuse out of the stomata per unit time
Wind speed/air movement	As wind speed increases, the transpiration rate increases	Wind moves water vapour away from the leaf surface after it diffuses out of the stomata; this increases the water vapour concentration gradient between the inside and the outside of the leaf
Humidity	As humidity increases, the transpiration rate decreases	Humid air contains a high concentration of water vapour, decreasing the water vapour concentration gradient between the inside and the outside of the leaf

Specified Practical: Investigating Factors Affecting Transpiration

It is possible to investigate the effect of environmental conditions on the rate of transpiration using a piece of apparatus called a potometer
A bubble potometer measures the uptake of water by a plant shoot; this can be used as a measure of the volume of water lost in transpiration
- This experiment assumes that water uptake and water loss by transpiration are equal
- In reality, there will be a very small difference between the volume of water taken up and the volume of water lost in transpiration, as some water is used in photosynthesis; this means that potometers provide an indirect measure of transpiration rate

Potometer diagram

Bubble potometers can be used to measure the uptake of water by a plant shoot; this can be used to represent water loss in transpiration

Investigating the effect of environmental conditions on transpiration rate

Equipment

Potometer
Plant shoot
Basin or sink filled with water
Scalpel or sharp scissors
Vaseline
Paper towel
Beaker
Timer
Means of controlling an environmental factor

Method

Cut a shoot underwater using a diagonal cut
- A diagonal cut creates a larger surface area for the uptake of water
Assemble the potometer underwater
- Cutting and assembling underwater prevents air bubbles from entering the xylem where they could block the movement of water
Make sure that the apparatus is airtight, using Vaseline to seal any gaps
- This prevents the entry of air into the system while the experiment is running
Dry the leaves of the shoot using a paper towel
- Water on the leaves will block the stomata and affect transpiration
Allow a single air bubble to form inside the potometer and place the end of the tube into a beaker of water
- The air bubble allows water movement in the tube to be observed
- The beaker of water provides a supply of water to replace water that is lost during transpiration
Set up the environmental factor to be assessed (see below) and allow the plant shoot to adjust to the new environment for 5 minutes
Record the starting location of the air bubble
Leave the potometer for a set period of time, e.g. 15 minutes
Record the end location of the air bubble
Reset the bubble to its starting position by opening the tap below the reservoir
Repeat steps 7-10 twice more under the same environmental conditions before repeating the experiment under changed environmental conditions
Calculate the rate of transpiration:

rate-of-transpiration-calculation

Risk assessment

A hazard in an experiment is something that could potentially harm you
A risk is the harm that could occur as a result of the risk
A control measure is the action taken to reduce the chance of the hazard causing a problem

Risk assessment for transpiration practical table

Hazard	Risk	Control measure
Sharp scalpel or scissors	Cuts to hands when cutting the plant shoot	Cut away from fingers
Delicate glassware may break, resulting in sharp edges	Cuts to hands while assembling apparatus or clearing up breakages	Be gentle while assembling equipment

Variables

The dependent variable in this investigation is the distance moved by the air bubble in a set time period
- This can be used to calculate a rate of transpiration
There are a range of independent variables that could be tested for this investigation; some examples are given in the table below
The control variables depend on the independent variable being investigated:

Independent and control variables table

Possible environmental factor	Means of controlling environmental factor	Control variables
Light intensity	Lamp placed at varying distances from potometer	Temperature, wind speed, humidity, leaf surface area
Temperature	Heater with fine temperature control / an incubator	Light intensity, wind speed, humidity, leaf surface area
Wind speed	Fan with a range of speed options	Temperature, light intensity, humidity, leaf surface area
Humidity	Plastic bags placed over the shoot	Light intensity, temperature, wind speed, leaf surface area
Leaf surface area	Repeating the experiment with different numbers of leaves	Light intensity, temperature, wind speed, humidity

Results

We would expect to see the following results for the different independent variables:
- Light intensity
  - As light intensity increases so does the rate of transpiration
- Temperature
  - As temperature increases so does the rate of transpiration
- Wind speed
  - As wind speed increases so does the rate of transpiration
- Humidity
  - As humidity increases the rate of transpiration decreases
- Leaf surface area
  - As leaf surface area increases so does the rate of transpiration

Examiner Tip

You are expected to be able to evaluate practical methods such as this, so make sure that you are aware of the following:

How to keep any relevant control variables constant, e.g. maintaining a single light source while investigating the effects of temperature
How to ensure that your results are reliable, e.g. by carrying out repeats for each environmental condition
How to improve on the suggested method, e.g. always using fresh plant cuttings that are still transpiring
Hazards, risks, and corresponding control measures

Note that setting up potometers can be quite tricky, so it is possible that your teacher or lab technician may do it in advance; you still need to understand the purpose of the under-water set-up process even if you don't carry out this part of the practical yourself!

Factors Affecting the Rate of Transpiration (WJEC GCSE Biology)

Revision Note