Practical 10.X
Investigation of
the difference in
transpiration rate
between the two leaf
surfaces
Objective: To measure the
transpiration rate of a leafy shoot by using bubble photometer (1), and
(2 marks) to deduct the
difference in stomatal density between the upper and the lower leaf surface by
comparing the transpiration rate between the two surfaces of the leaf (1)
Biological
principle and experiment design: (8
marks)
Most of the water absorbed by a plant is eventually
lost in the form of water vapour (1)
from
the leaf surface. This process is called transpiration (1),
which creates a pulling force
to transport water and minerals (1) in the xylem vessels
upwards to the leaves. This is the main transport mechanism in plants.
To measure the rate of transpiration, a (bubble) potometer
(1) can be used. Since water is continuously absorbed and lost through
transpiration, the air bubble inside the connected capillary tube moves towards
the direction of the leafy shoot. Transpiration rate can be calculated as shown
below:
Transpiration rate = Volume of
absorbed water (1)
Time of measurement
(1)
The abundance of the stomata (1) on both surfaces
of a leaf is different. The difference can be deduced indirectly by comparing
the transpiration rates between both surfaces. To measure the transpiration
rate of one surface of a leaf, surface of the other side should be smeared with
Vaseline that is impermeable to water and air.
To ensure a valid experimental result, we have to
make an assumption that all the water uptake by the
leafy
shoot is lost through transpiration (2)
Procedures: (6
marks)
1.
Smear the
upper surface of a leaf from a plant with Vaseline.
2.
Cut the leafy
shoot from the plant and fit it tightly into the potometer tightly. This should
be done under
so as to prevent the air bubble from entering the xylem
vessles (1) that may cause blockage of water
transport and absorption (1).
3.
Set up the
apparatus as shown below. Seal off all connections with vaseline (1).
1.
Introduce an air bubble (1)
into the capillary tube so that the change in water absorption can be observed.
2.
Allow the setup to equilibrate for 10 minutes, so that the transpiration
rate would be stabilized.
3. Record the distance (1) travelled by the bubble in (actual time) (1)
minutes.
4.
Open the tap
of the reservoir to re-adjustment the position of the air bubble. Repeat steps
4 to 5 to take two more readings (if time allowed).
5.
Repeat steps 1
to 7, but use another leaf with similar size and smear the lower surface with
Vaseline.
Results: (7
marks)
|
Leafy shoots
|
Upper surface smeared
|
Lower surface smeared
|
|
Initial position of the air bubble (cm)
|
1.3 ml
|
4.3 ml (1)
|
|
Final position of the air bubble
(cm)
|
7.87 ml
|
2.7 ml (1)
|
|
Distance of the bubble movement
(cm)
|
6.57 ml
|
1.6 ml (2)
|
|
Time of the bubble movement (min)
|
54
|
48 (1)
|
|
Rate of the bubble movement
(cm / min)
|
0.085 ml min-1
|
0.033 ml min-1 (2)
|
Result
Interpretation: (7 marks)
1
According to
the experimental result, deduce the difference in the abundance of stomata on
both surfaces of a leaf. (3
marks)
The transpiration rate of the leaf with
lower surface smeared is much lower than that with upper surface smeared (1).
This shows that the transpiration rate is much more reduced by smearing the
lower leaf surface than smearing the upper surface (1).
It can be deduced that the lower leaf
surface may have higher stomatal density than the upper surface (1).
2
Describe and
explain the movement of the air bubble inside the capillary tube. (2 marks)
The air bubble moves towards the leafy
shoot along the capillary tube (1).
During the process of transpiration, water
is absorbed into the leafy shoot by transpiration pull, leading the air bubble
to moving towards the leafy shoot (1)
3
Explain why
the rate of the movement of the air bubble cannot directly show the rate of
transpiration?
(2 marks)
The rate of the movement of the air
bubble can only show the rate of water absorption directly (1). Actually, not
all the water absorbed is lost through transpiration (1)
Discussion:
(2 marks)
4 State
the limitations of this experimental setup or the source of errors of this
experiment. (1 mark)
The friction between the air bubble and the
capillary wall may hinder the movement of the air bubble (OR any other
reasonable answers) (1)
5 Suggest improvements for the limitations or the
errors as stated in question (4).
(1 mark)
To reduce the volume of the air bubble introduced
so as to reduce the friction during movement (1)
Conclusion: (2 marks)
The
transpiration rates of the upper leaf surface and the lower leaf surface are 0.033 ml min-1 and 0.085 ml min-1 respectively (1).
By comparing the transpiration rates
between the two leaf surfaces, it can be deduced that the lower leaf surface
may have a higher stomatal density than the upper leaf surface (1).
