Measuring the rate of transpiration

Question 1

What assumptions are made when measuring the rate of transpiration?

Answer 1

• Rate of water uptake = rate of water loss (transpiration)
• Photosynthesis/respiration involve relatively negligible quantities of water

Question 2

What are the means by which rate of transpiration can be measured?

Answer 2

• Indirectly by a potometer => measure rate of uptake of water
• Directly (a) => measure mass of water loss from the plant
• Directly (b) => measure volume of water loss from the plant

Question 3

What precautions need to be taken with a potometer?

Answer 3

• Leaves need to be dried after cutting under water
• Make an oblique cute in the stem to increase surface area for uptake of water

Question 4

What precautions need to be taken when setting up a potometer to ensure no air is present?

Answer 4

Precautions:
- Making an oblique cut in the stem of the plant underwater
- Have apparatus underwater
- Insert stem underwater
- Joints must be sealed tight

Explanation:
- Prevents air from entering stem/shoot/xylem/apparatus
- Air/bubble could block xylem
- Obtain a continuous column of water

Question 5

What precautions need to be taken when using a potometer to ensure no air is present?

Answer 5

Precautions:
- Do not allow the bubble to move too far
- Use syringe to move bubble
Explanations:
- So air bubble does not eneter xylem/stem
- So same air bubble can be re-used

Precautions:
- Place open end in water
Explanations:
- So no air/(new) bubble, is introduced

Precautions:
- Keep shoot still/supported
Explanations:
- To avoid breaking seal/water column

Overall Explanations:
- To meausre transpiration accurately
- Ensure validity

Question 6

Describe how to conduct an experiment using a potometer to investigate the effect of
one, named, environmental factor (or experimental manipulation) on the rate of
transpiration

Answer 6

• Light => lamp (with screen to prevent heat), dark
• Humidity => put plant in bag
• Temperature => heater
• Wind => fan

Question 7

Describe how a potometer can be used to measure the rate of transpiration?

Answer 7

• Potometer apparatus must be airtight and watertight
• Leaves must be dry
• Cut shoot under water with an oblique cut
• Measure distance air bubble travels per (named) time interval OR measure time for air bubble to travel known distance
• Calculate the volume of water uptake
• Maintain constant conditions