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. Several hours prior
to class your plant will be placed in the dark to reduce the rate of transpiration. This exercise
requires that you work in teams and be well organized. Work quickly and carefully.
1. How can we measure the rate of transpiration (ml min-1) through a plant? You will be given a
potted geranium plant with a leafy shoot. Figure out a way to measure water uptake by a
shoot. If you remember that project with Eosin Y and think about volume instead of
color, you are probably on the right track. Design your set-up and clear your protocol with
Dr. Koning before proceeding.
2. Assemble your apparatus being careful to avoid the critical problems. What are those?
Determine the “control” rate of transpiration in your shoot.
3. Now using your shoot set-up, apply some environmental variables. What variables might
increase or decrease the rate of transpiration?
 ∆C
Think about Fick’s law: -Ds ∆x  .


In the case of evaporation from leaves, what factor alters Ds (the diffusion coefficient)?
What factor alters ∆C (or is it ∆)? What factor alters ∆x? Dr. Koning will try to supply
what you suggest might be needed or added to your shoot to alter the rate of transpiration.
Think about the order of how you will apply the variables...it could be critical. One
treatment might alter or prevent a response to the next.
4. Since each iteration of the project may take ±20 minutes, while you are waiting, you can set up
another apparatus. Think about the variables influencing flow through a stem. Perhaps the
equation: Jv = Lp • ∆ [rate=hydraulic conductivity•water potential difference] or the
 πr4 • ∆P 
Poiseuille equation 
 [r=radius, =viscosity] would help? The viscosity of
8 • length
water is 0.009325 poise. A poise is g cm-1 sec-1. A Pa is 10 g sec-2 cm-1. How might you
determine resistance to water flow in the xylem? How could you impose a known
pressure on water going through a piece of stem? How could you then alter the rate of
that flow? Should you call this transpiration or conduction?
5. From this work you should be able to give a value for specific conductivity for the stem (ml
cm-1 min-1 MPa-1). From that you should be able to give a conductivity value for the
whole shoot (ml min-1 MPa-1). Then you should be able to combine the shoot
conductivity value and the highest transpiration rate you found in parts 1-3 to determine
the potential drop needed to lift the water at that rate through the stem!
6. Can water flow in the xylem when there is no evaporation from leaves? This is something you
should be able to observe...if not within the time-frame of class, then surely overnight.
Think of a way to determine that.
7. If possible, measure the osmotic potential of xylem sap. Try to reduce other variables in
obtaining a sample of xylem sap. Clear your procedure with Dr. Koning before
proceeding. Assuming that all of this dissolved material is sucrose, can you determine the
potential viscosity of the xylem sap. Is this viscosity higher or lower than pure water?
This lab exercise 1994 Ross E. Koning. Permission granted for not-for-profit instructional use.
Available at: plantphys.info/plant_physiology/labdoc/transpiration.doc
wrong 0
right 39
score 100
1
38
97.4
2
37
94.9
3
36
92.3
4
35
89.7
5
34
87.2
6
33
84.6
7
32
82.1
8
31
79.5
9
30
76.9
10
29
74.4
11
28
71.8
12
27
69.2
13
26
66.7
14
25
64.1
15
24
61.5
16
17 18 19 20 21 22 23 24
23
22 21 20 19 18 17 16 15
59.0 56.4 53.8 51.3 48.7 46.2 43.6 41.0 38.5
Effect of light, wind, and pressure on water
movement in Pelargonium graveolens shoot—rubric for: ______________________________
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 (9)









No outside frame
No title above plot frame
Transpiration Rate (mL/min) axis title
Treatment Axis Title (Control, Wind, Light, Both)
Axis fonts large and bold
Plot area boxed boldly
No background shading or shadows
No background gridlines
Use of bar shading intuitive
Results Figure One Legend (4)




Legend present below Figure
Figure 1.
Title: “Effect of Light and Wind…”
Clarification: PFD, wind speed values
Results Figure Two (12)












No outside frame
No title above plot frame
Water Movment (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 large and bold
Plot area boxed boldly
No background shading or shadows
No background gridlines
Use of bar or symbol shading intuitive
Results Figure Two Legend (4)




Legend present below Figure
Figure 2.
Title: “Effect of Pressure and Segment Length…”
Clarification: water pressure applied