5.3 Transpiration

This interactive website explains transpiration pull.

Investigation: To determine the effect of environmental conditions on transpiration rate (using a simple photometer.

The four groups should have leafy twigs of the same type of plant and about the same size, so that results in different environmental conditions can be reliably compared at the end. They should choose twigs with stems likely to fit tightly into a drinking straw and the stems should be strong enough not to be crushed when forced into the straws.

ALTERNATIVE METHOD:

Alternatively, if available, learners can use narrow clear plastic tubing filled with water that can be cut open / slit at one end to insert the twig. Plastic tubing is easier to seal with tape than drinking straws and it can be taped in place horizontally along a desk or table once an air bubble has been introduced at the open end. One learner will have to hold the plant off the desk. The tube is then placed into water and a ruler is taped in place next to it to track the movement of the bubble over time. They won’t be able to reset it, but that’s fine – take 3 or 4 measurements of how far the bubble moves in 2 minutes and get an average for each environmental condition. This is faster than doing it for an hour in each condition.

Learners can create the various environmental conditions in the laboratory.

• The conditions in the laboratory can serve as the "control" environment.
• Learners can use a fan to mimic 'windy' conditions.
• Learners can use an electric heater to mimic 'hot' conditions.
• Learners can place the apparatus under a bell jar or under a plastic bag to mimic 'humid' conditions.

Precautionary Measures:

• The stem must be cut under water as this will prevent air obstructions in the xylem tissue
• The system must be airtight (use Vaseline to ensure this)
• Use the same stem during the whole experiment to get the same readings
• If using a potometer, and the bubble gets too close to the plant, one can open the stopcock (tap), to allow extra water into the tube and push the bubble back to the start. This resets the potometer for further readings. Be careful not to push the bubble too far, otherwise it may escape completely and one has to wait with the tube out of the beaker, until a new air bubble has been sucked into the tube.
• It is important that learners WAIT TEN MINUTES after setting it up for the plant’s transpiration rate to stabilise before taking measurements.

Observations and Results:

Teachers should be able to guide the class in drawing up tables to compare the transpiration rate in the four conditions, but the following table is suggested as a guideline:

Learners should find the following transpiration rates:

• Fastest transpiration rate in the sun with wind.
• Second fastest in the sun, but no wind.
• Third fastest in the shade with no wind.
• Slowest transpiration rate in the plant that had the bag around it.

Bar graphs should have headings similar to the table heading. Environmental conditions will go onto the horizontal axis and rate of transpiration on the vertical axis. Since measurements were taken every 10 minutes for an hour, the total cumulative transpiration at the end of the 60 minute period will be the transpiration rate per hour.

Conclusions:

• Different environmental conditions have an effect on transpiration rate in plants.
• Warm conditions, wind and bright sunlight speed up transpiration rate.
• Cool conditions, no wind and humidity slow down transpiration rate.

Improving the accuracy of findings: Accept any 2 of the following:

• Do not take measurements directly after changing the conditions.
• Use the same plant and take it to different places to keep the leaf surface area constant.
• If using different plants, they must be of the same species and the same size.
• Do not get water on the leaves, as this will block stomata and reduce transpiration rate.
• Use a woody stem, to stop the xylem being crushed when it is inserted into the tube / straw.
• Cut the stem under water to stop air bubbles entering the xylem.
• Take several readings in each condition and average the findings.

Questions:

1. Why is it important to cut the stem at an angle under the water?
2. Which part of the stem does the straw represent?
3. Which three factors are you investigating?
4. Under which condition is the highest rate of transpiration?
5. Name one possible error that could have occurred in your investigation.
6. What are the potential limitations of this investigation?

Answers:

1. To prevent air from getting into the xylem. Cut at an angle so there is a large surface area through which the water can enter the plant. It is important to remember that cutting straight across the stem may crush and block the xylem.

2. The xylem of the stem.

3. Humidity, wind, light and moisture.

4. Learners should find the sunny area with wind to have the highest rate of transpiration. However, learners are to assess their own experiments and base answer on results.

5. Accept any error:
   • We didn’t measure for exactly the same amount of time every time
   • The plants were not exactly the same size
   • The tube was too wide and the bubble hardly moved
   • The tube wasn’t sealed, so the bubble didn’t move at all and the water ran out
   • Wind speed varied in our area
   • We had wind on our plant and couldn’t stop it – supposed to be no wind there
   • ANY other error that may have occurred during this investigation.
6. The following are some examples of answers that learners may provide:
   • We can’t be sure that we were accurate, because we couldn’t keep the wind speed the same.
   • We had wind when we didn’t want any.
   • We were comparing the transpiration rates of completely different plants so different numbers of leaves on the twigs could have caused differences in their transpiration rates
   • Since we were using different plants it was not only the environmental conditions that differed, but the plants themselves.