8: Investigate factors affecting water uptake by plant shoots using a potometer.

Question 1

How is transpiration rate determined indirectly in this practical?

Answer 1

it is assumed that the plant shoot takes up an equal volume of water to that lost by transpiration.

Question 2

How is water taken up by plants?

Answer 2

Plants draw up water through the xylem, due to the transpiration stream and cohesion forces between water molecules.

Water is a polar molecule, due to the uneven distribution of charge in the molecule (hydrogen=slightly positive, oxygen=slightly negative).
This causes hydrogen bonds to form between the hydrogen atom of one water molecule and the oxygen atom of another. This causes COHESION of water.

Water evaporates from the surface of the mesophyll cells out of the stomata on the leaves of the plants. Because of cohesion, a column of water molecules are drawn through the xylem.

Question 3

What is the independent variable in this experiment?

Answer 3

Can be any of the following:

-light intensity
(use a lamp at different distances and a light meter to record intensity)

-air movement
(use a fan at different distances or speed settings)
-temperature
(use an incubator set to different temperatures)

-temperature
(use an incubator set to different temperatures)

-humidity
(increase humidity by covering shoot with a clear plastic bag)

Question 4

What is the dependent variable?

Answer 4

The rate of transpiration

Question 5

What are the control variables?

Answer 5

same shoot for each condition; ensures same leaf area, stomatal density.

(light intensity, air movement, temperature, humidity)

Question 6

What is the effect on respiration if the light intensity is increased?

Answer 6

Increased light intensity leads to an increase in the rate of photosynthesis. This leads to an increase in the stomatal opening to allow gas exchange required for photosynthesis to occur. Leads to a greater rate of transpiration.

Question 7

What is the effect on transpiration if air movement is increased?

Answer 7

Increased air movement leads to the humid air around the stomatal opening being removed and being replaced by drier air, leading to a greater diffusion gradient and therefore increase in the rate of transpiration.

Question 8

What is the effect of temperature on transpiration?

Answer 8

Increased temperature increases kinetic energy of water molecules, increasing the rate of evaporation of water.

Question 9

What is the effect of increased humidity on transpiration?

Answer 9

Increased humidity decreases the concentration gradient between the exterior air and the air inside the leaf, lowering the transpiration rate.

Question 10

Method:

Answer 10

1. Lay capillary tube and rubber connector under water and fill both parts.
2. Select leafy shoot with diameter as close to rubber connector as possible. Keep shoot end under water at all times to prevent bubbles entering the xylem and disrupting the transpiration stream, and trim if necessary.
3. Under water, insert shoot into rubber connector of potometer.
4. Clamp capillary tube to the stand with the shoot facing upwards. Place the bottom end of the capillary tube into the beaker of water.
5. Smear petroleum jelly around the join between the shoot and the rubber connector to ensure an airtight seal. Dry leaves by blotting gently with paper towels.
6. Leave apparatus for 5 mins-allows water to be drawn up into the end of the capillary tube. A small air bubble should be present.
7. With a stopwatch, time the movement of the air bubble along the tube for a set distance-eg 2cm.
   Or… measure the distance moved by the bubble in a set time (e.g. 5 mins). Record results in a table.
8. Repeat readings using different environmental conditions. Reset air bubble to the start each time if necessary. Allow shoot to settle in new conditions 2-3 minutes before testing.
9. Work out leaf area of shoot by drawing around each leaf on graph paper and counting up the squares.

Question 11

Analysis of results:

Answer 11

Convert results into volume of water taken up:

• Measure radius of the hole in the capillary tubing.
• Use formular for the VOLUME OF A CYLINDER: v = πr2h to calculate the volume of the water. (h= distance moved by the bubble).

Calculate the RATE of water uptake as volume taken up per unit of time. Formula: unit in mm3/min.

Question 12

Calculation for rate of water uptake:

Answer 12

Rate of water uptake=
Volume of water taken up/
Time taken for the bubble to move.

Question 13

How can different shoots be compared?

Answer 13

take into account LEAF AREA of each shoot: convert results into volume of water per unit of leaf area per unit time (mm3/m2/hour). Divide water uptake rate results by leaf area for measurements.

Question 14

Why is an airtight seal needed?

Answer 14

Without airtight seal, water will not be drawn up the capillary tube by cohesion and no air bubble movement would be possible. Air bubbles prevent cohesive forces and stop the upward tension being transmitted through the water column.

Question 15

Limitations to the experiment.

Answer 15

Factors that influence transpiration are difficult to control:
eg. size of shoot, number of leaves, total surface area of leaves, leaves that have not fully dried.

Factors that reduce the accuracy of the equipment:
eg. difficulty in making a seal between the shoot and the apparatus.

Question 16

Effects of the limitations:

Answer 16

-transpiration rate will increase if a larger leaf SA is used or if there are more leaves present. Wet leaves reduce the diffusion gradient. Lack of airtight seal will slow air bubble movement and may stop transpiration. Therefore lower value for water uptake.

Question 17

To reduce limitations:

Answer 17

Shoots should be the same size, with the same leaf number. Leaf area should be measured so that water loss per unit area can be calculated.

seal between the shoot and apparatus must be airtight. Add sealant.

Repeat experiment a large number of times to give a measure of reliability and calculate mean.