59 - _____ = _____ /59 = ______.____
Name ______________________________
Transpiration
In the previous exercise you studied the process of osmosis and learned about water potential and its role in
determining the movement of water. Transpiration, the movement of water from root through stem and leaf to
atmosphere, can be studied in a similar way.
In this exercise you will study the movement of water through a plant by two different approaches
(transpiration and conduction). This exercise requires that you work well as a team and be well organized. Start
promptly, assemble the apparatus quickly and carefully, and do not let huge lapses of time go by between projects!
You will have two systems (A and B) going simultaneously; you cannot sit and watch one system at work.
Project Logic: The major motive force for lifting water through the shoot in this potometer
is root pressure capillarity evaporation . That occurs primarily through the stomata, which are surrounded
by two guard cells in the lower epidermis. The guard cells are different from other epidermal cells in
having chloroplasts mitochondria nuclei which, in light, produce sugars from water and carbon dioxide to
decrease the pressure solute water potential of the guard cells. Water therefore enters leaves the guard
cells by active transport osmosis bulk flow . Because the guard cells were already full, this water movement
increases the pressure solute water potential of the guard cells. Because the two guard cells are
connected end-to-end side-to-side they open close the stoma. Therefore, light should cause transpiration
to increase decrease . We can increase the rate of evaporation by blowing off the boundary layer of air with
some wind, particularly if it blows strongly across the upper lower epidermis.
A. Measuring Transpiration
How can we measure the rate of transpiration (ml min-1) through a plant? Your team will
construct a potometer. This is a shoot of a plant attached by soft tubing to a pipette. The
potometer has to be assembled with the tubing completely filled with water, the shoot severed
from its root system under water. We cannot have ANY air embolisms in the xylem.
1. Record the height of your geranium plant from soil to highest leaf tip:
_______cm
2. Fill a dishpan with room-temperature water. Attach the 2 mL (in 1/100 mL) pipette to the
medium-length piece of Tygon R-1000 (3/8”OD ¼”ID) tubing. Lower this slowly and
gradually into the dishpan so that it fills completely with water and is completely under
the water.
3. Lay one of your shorter potted geranium (Pelargonium graveolens) plants sideways (pot
and all) into the dishpan. Keeping the cut area under water, use a razor blade to sever the
shoot from the root just above a node along the stem, leaving an internode extension at
the bottom of the shoot. The cut should be slightly diagonal so that the internode
extension will be easy to insert into the water-filled tubing.
4. Slip a Snap Grip nylon hose clamp around the free end of the soft tubing. Keeping it
underwater, insert the cut end of the shoot into the water-filled soft tubing and work the
tubing up the internode extension until it gets very snug as it approaches the nodal region.
Secure the connection with the hose clamp.
5. Observe that there are NO air bubbles in the potometer. If any are found, lift the pipette
end of the potometer out of the water, straightening out the tubing so that the bubble can
rise toward the open end of the pipette (pointing to the ceiling). When the bubble comes
close to the open end, slip the short piece of tubing over the open end of the pipette and
fill it with water from a wash bottle. Finally, squeeze the tubing of the potometer to push
the bubble into the water-filled short tubing; as you release your hand, the water in the
short tubing will be drawn into the pipette to replace the expelled bubble. This method is
how you will refill your pipette as needed.
This lab exercise 1994 Ross E. Koning. Permission granted for not-for-profit instructional use.
Available at: plantphys.info/plant_physiology/labdoc/transpiration.doc
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6. Tape your potometer (pipette tip up, shoot tip up) onto the reagent shelf. Assure that the
meniscus is at the same height as about half-way along the length of the shoot. Position a
lamp close to the shoot. Position a fan to strongly blow across the lower epidermis of the
leaves to reduce the boundary layer.
Record the mL of water transpired through the shoot for 15 minutes: _________ mL. 
Divide by 15 to find the transpiration rate: _________ mL min-1.
Using the Li-Cor Quantum Radiometer,
record the output of your lamp at the level of the leaf: __________ µmol photons m-2 s-1. 
Using the Anemometer,
record the output of your fan at the level of the leaf: ___________ m s-1. 
Record the length of your shoot segment from cut end (inside tubing) to leaf tip: ______cm
7. Turn off the fan, but leave the light on.
Record the mL of water transpired through the shoot for 15 minutes: _________ mL. 
Divide by 15 to find the transpiration rate: _________ mL min-1.
8. Turn off the light, but turn the fan back on.
Record the mL of water transpired through the shoot for 15 minutes: _________ mL. 
Divide by 15 to find the transpiration rate: _________ mL min-1.
9. Turn off the light, and turn off the fan.
Record the mL of water transpired through the shoot for 15 minutes: _________ mL. 
Divide by 15 to find the transpiration rate: _________ mL min-1.
Using the Li-Cor Quantum Radiometer, record the
“light-off” ambient light at the level of the leaf: __________ µmol photons m-2 s-1. 
Using the Anemometer, record the
“fan-off” ambient wind speed at the level of the leaf: ___________ m s-1. 
10. Use a razor blade to cut off all of the leaf blades.
Record the mL of water transpired through the shoot for 15 minutes: _________ mL. 
Divide by 15 to find the transpiration rate: _________ mL min-1. B. Measuring Water Conduction (Bulk Flow)
How can we measure conduction, and how is it different from transpiration? You have learned
about Poiseuille’s equation and the factors affecting bulk flow through a pipe. The flow of water
through a stem is basically the same concept. So to study transpiration another way, you will set
up a second apparatus to provide a pressure differential along a segment of a stem. This is similar
to a potometer, except that the shoot is replaced by just a stem segment cut at both ends.
1. Using the 1m-long piece of Tygon tubing, another 2 mL pipette, and the taller geranium,
assemble another potometer under water in your dishpan. Make sure the joints are
watertight and the tubing is bubble-free.
2. Remove the potometer from the water. Use a metric ruler to cut the apical end of the shoot off
so that the potometer only has a 10cm segment of stem attached. This step opens the
xylem vessel and tracheid columns at the top; it converts a more-or-less closed shoot
vascular system, depending on transpiration from leaf surface area and out through
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stomata for water movement, into the equivalent of a pipe conducting water out the open
end. We can now apply a known pressure to the attached end and observe flow out the
freshly cut end.
3. Tape the pipette to the upper shelf or to the bench edge and lower the stem segment to the
benchtop or the floor. Use a meter stick to assure that the meniscus is 1 meter above the
cut end of the 10 cm stem segment.
Record the mL of water conducted through the 10 cm segment in 15 minutes: _________ mL. 
Divide by 15 min to find the segment conduction rate: _________ mL min-1.
Divide by 10 cm to find the specific conduction rate: _________ mL min-1 cm-1.
Multiply meniscus height: 1 m by 0.0098 MPa m-1 to find the imposed p: _________ MPa.
Divide the specific conduction rate by the imposed p: _________ mL min-1 cm-1 MPa-1.
This value is called the specific conductivity of the stem.
4. Position so that the stem segment is resting just 0.5 meters below the meniscus.
Record the mL of water conducted through the 10 cm segment in 15 minutes: _________ mL. 
Divide by 15 min to find the segment conduction rate: _________ mL min-1.
Divide by 10 cm to find the specific conduction rate: _________ mL min-1 cm-1.
Multiply meniscus height: 0.5 m by 0.0098 MPa m-1 to find the imposed p: _________ MPa.
Divide the specific conduction rate by the imposed p: _________ mL min-1 cm-1 MPa-1.
This value is a second estimate of the specific conductivity of the stem.
5. Cut the stem segment to just 5 cm long, but leave the segment 0.5 m below the meniscus.
Record the mL of water conducted through the 5 cm segment in 15 minutes: _________ mL. 
Divide by 15 min to find the segment conduction rate: _________ mL min-1.
Divide by 5 cm to find the specific conduction rate: _________ mL min-1 cm-1.
Multiply meniscus height: 0.5 m by 0.0098 MPa m-1 to find the imposed p: _________ MPa.
Divide the specific conduction rate by the imposed p: _________ mL min-1 cm-1 MPa-1.
This value is a third estimate of the specific conductivity of the stem.
6. Reposition the 5 cm stem segment so it is again 1 meter below the meniscus.
Record the mL of water conducted through the 5 cm segment in 15 minutes: _________ mL. 
Divide by 15 min to find the segment conduction rate: _________ mL min-1.
Divide by 5 cm to find the specific conduction rate: _________ mL min-1 cm-1.
Multiply meniscus height: 1 m by 0.0098 MPa m-1 to find the imposed p: _________ MPa.
Divide the specific conduction rate by the imposed p: _________ mL min-1 cm-1 MPa-1.
This value is a fourth estimate of the specific conductivity of the stem.
7. Calculate the average specific conductivity of 4 estimates:
Data Analysis—Putting Our Calculations Together
_________ mL min-1 cm-1 MPa-1. -
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You have in-hand the experimental data and direct transpiration and conduction calculations
from the work done in the laboratory. At home you can now analyze the data that you have.
The rate of transpiration through the shoot can be thought of at some level by Fick’s law:
Jv = -Ds • ∆C • x-1. But, of course, this law is used to understand water flowing through
membranes having a diffusion coefficient (Ds) and fixed thickness (x) driven by a concentration
difference (∆C) across the membrane. Of course our situation is water flowing through a shoot
having a specific conductivity (Lp) a fixed length (l) and a water potential difference (∆)
driving the flow. So for our geranium plant our revised rate formula is: Jv = -Lp • ∆ • l-1.
1. The fastest transpiration rate (Page 2 probably 2nd value of step A6): _________ mL min-1.
2. The average specific conductivity on page 3 (step B7):
_________ mL min-1 cm-1 MPa-1.
3. The length of the shoot segment on page 2 (end of step A6):
4. Plug these three values into our formula and then solve for ∆:
________ cm.
_________ MPa.
This is the water potential difference required to push the water through this stem at the
maximum transpiration rate observed in the shoot that was attached to our potometer.
Repeat step 4 above using the height of the plant above the soil (page 1) as the value for step
3 above. The water potential difference required:
to push the water through the whole plant _________ MPa.
Our project with the stem segment (10 and 5 cm) can also help us understand a whole stem in
another way. We can also apply the Poiseuille equation: Jv = r4 • p • ( • -1 to the one
situation (step 3) on page 3 to solve for the combined pipe radius (r) representing all of the
tracheids and vessels of the vascular bundles in the stem.
Jv is the page 3 step 3 segment conduction rate: _________ mL min-1.
The weight of 1 ml (= 1 cm3) of water is 1 g.
Obviously 60 sec can substitute for one minute.
Use 3.14159 for 
For p, use the imposed p value from page 3 step 3: _________ MPa = ________ g sec-2 cm-1.
FYI: 1 Pa is also known as 10 g sec-2 cm-1.
For  the viscosity of water, use 0.00899 poise (A poise is also known as: 1 g cm-1 sec-1).
For x, use the length of the stem segment from page 3 step 3: _________ cm
Hopefully, if you have handled all the conversion factors correctly, your calculated radius will
have units of cm, and the coefficient will make some sense given the diameter of the segment
you inserted into the tubing:
The effective combined radius of all vascular pathways through the segment: ________ cm
Now convert this to proper scientific notation in meters: _______  10____ m
For this exercise, we are handing in JUST this worksheet!
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score 100 97.7 95.3 93.0 90.7 88.4 86.0 83.7 81.4 79.1 76.7 74.4 72.1 69.8 67.4 65.1 62.8 60.5 58.1 55.8 53.5 51.2 48.8 46.5 44.2 41.9
Effect of light, wind, and pressure on water
movement in Pelargonium graveolens stems—rubric for: _____________________________
Title (3)
 The effect of light, wind, and pressure
 on water movement through
 Pelargonium graveolens stems.
Author (3)
 primary author
 lab partners, alphabetical by last name
 contact information
Abstract (10)
 introduction sentence
 methods in 1-2 sentences only
 scented geranium shoot
 water-filled tubing-with 2 mL pipette
 leafless stem segment
 results without xs detail or data
 results: control vs treatment with shoot
 results: segment length vs pressure with stem
 discussion: additive/synergistic effect
 discussion: Poiseuille factors
Results Figure One (8)
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No outside frame or title above plot frame
Transpiration Rate (mL/min) axis title
Treatment Axis Title (Control, Wind, Light, Both)
Axis fonts sans serif, large, bold
Plot area boxed boldly in black
No plot area shading, shadows, gridlines
Axis lines and tick-marks match plot area box
Use of bar fills intuitive
Results Figure One Legend (4)
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Legend present below Figure
Figure 1.
Title: “Effect of Light and Wind…”
Clarification: PFD, wind speed values
Results Figure Two (11)
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No outside frame or title above plot frame
Water Movement (mL/min) axis title
Segment Length (cm) axis title
Key: Applied Pressure Bar Groups (m or MPa)
Key: located inside plot frame, w/o own frame
Overlapped bars or grouping of bars
Axis fonts sans serif, large, bold
Plot area boxed boldly in black
No background shading, shadows, or gridlines
Axis lines and tick-marks match plot area box
Use of bar or symbol fills intuitive
Results Figure Two Legend (4)
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Legend present below Figure
Figure 2.
Title: “Effect of Pressure and Segment Length…”
Clarification: water pressure applied