Lecture #5 – Plant Transport
Image of waterfall
1
Key Concepts:
• The importance of water
• Water potential: Ψ = P - s
• How water moves – gradients,
mechanisms and pathways
• Transpiration – water movement from soil
to plant to atmosphere
• The pressure flow model of phloem
transport
2
WHY WATER???
• Required for metabolism
and cytoplasm
• Nutrients are taken up
and transported in
water-based solution
• Metabolic products are
transported in waterbased solution
• Water movement
through the plant affects
gas exchange and leaf T
Diagram – movement of
water through a tree
3
Water Potential (Ψ):
• Controls the movement of water
• A measure of potential energy
• Water always moves from an area of HIGH
water potential to an area of LOW water
potential
• Controlled by physical pressure, solute
concentration, adhesion of water to cell
structures and to soil particles, temperature,
and gravity
Ψ=P-s
4
Diagram – water moves from high
water potential to low water
potential, sometimes toward a
negative value; same next 3 slides
5
6
minus 4 is MORE NEGATIVE than minus 1
7
High
Low
8
Diagram – water potential is universal, including with waterfalls
9
Water Potential (Ψ):
• Controls the movement of water
• A measure of potential energy
• Water always moves from an area of HIGH
water potential to an area of LOW water
potential
• Controlled by physical pressure, solute
concentration, adhesion of water to cell
structures and to soil particles, temperature,
and gravity
Ψ=P-s
10
P – Pressure Potential
• By convention, set to zero in an open
container of water (atmospheric pressure
only)
• In the plant cell, P can be positive, negative
or zero
A cell with positive pressure is turgid
A cell with negative pressure is plasmolyzed
A cell with zero pressure is flaccid
11
Turgid
P>0
Plasmolyzed
P<0
Flaccid
P=0
12
What are the little green things???
Micrograph – photosynthetic cells: turgid on left,
plasmolyzed on right; same on next 3 slides
13
Turgid
Plasmolyzed
14
Critical Thinking
• How can you tell this
tissue was artificially
plasmolyzed?
15
Critical Thinking
• How can you tell this
tissue was artificially
plasmolyzed?
• Observe the cell on
the far right – it is still
turgid 
16
Crispy means plasmolyzed
beyond the permanent
wilting point 
Image – turgid plant on left, plasmolyzed on right
17
s – Solute Potential
• s = zero for pure water
Pure H2O = nothing else, not a solution
• Adding solutes ALWAYS decreases the
potential energy of water
Some water molecules now carry a load – there
is less free water
s
s
s
18
Remember,
Ψ=P–s
Diagram – effect on water potential of adding salts to
solutions separated by semi-permeable membrane
19
Ψ=P–s
Pressure can be +, -, or 0
Solutes always have a negative effect
Simplest way to calculate Ψ is by this
equation
20
Flaccid cell in pure water – what happens???
…..what do you know???
….what do you need to know???
21
Flaccid cell in pure water – what happens???
Ψ=?
22
Flaccid cell in pure water – what happens???
Ψ=?
P = ?.......s = ?
23
Flaccid cell in pure water – what happens???
Ψ=?
P = 0.......s = about 0.7 MPa
24
Flaccid cell in pure water – what happens???
Ψ = -0.7 MPa
P = 0.......s = about 0.7 MPa
25
Flaccid cell in pure water – what happens???
…..what do you know???
Ψ=?
….what do you need to know???
26
Flaccid cell in pure water – what happens???
Ψ=?
P = ?.......s = ?
27
Flaccid cell in pure water – what happens???
Ψ=?
P = 0.......s = 0
28
Flaccid cell in pure water – what happens???
Ψ = 0 MPa
P = 0.......s = 0
29
Flaccid cell in pure water – what happens???
Ψ = -0.7 MPa
Ψ = 0 MPa
Will water move into the cell or out of the cell???
30
Flaccid cell in pure water – what happens???
Ψ = -0.7 MPa
Ψ = 0 MPa
Water moves from high Ψ to low Ψ
31
Then what happens???
Ψ = -0.7 MPa
Ψ = 0 MPa
32
Then what happens???
Ψ = -0.7 MPa
Ψ = 0 MPa
P in cell goes up…..
33
Then what happens???
Ψ = 0 MPa
Ψ = 0 MPa
Dynamic equilibrium!
34
Hands On
• Prepare a section of plump celery and stain
with T-blue
• Examine and describe
• Introduce a drop of salt water
• Any change???
• Examine the stalk of celery that was in salt
water vs. one that was in fresh water
• Explain your observations in your lab notes.
35
Water Movement
• Osmosis – the diffusion of water one
molecule at a time across a semi-permeable
membrane
Controlled by both P and s
• Bulk Flow – the movement of water in bulk –
as a liquid
Controlled primarily by P
36
Osmosis
Diagram – osmosis across a semi-permeable membrane; next slide also
Critical Thinking: Where does water
move by osmosis in plants???
37
Osmosis
Critical Thinking: Where does water
move by osmosis in plants???
Cell membrane is semi-permeable
38
Water Movement
• Osmosis – the diffusion of water one
molecule at a time across a semi-permeable
membrane
Controlled by both P and s
• Bulk Flow – the movement of water in bulk –
as a liquid
Controlled primarily by P
39
Water Movement
• Osmosis – the diffusion of water one
molecule at a time across a semi-permeable
membrane
Controlled by both P and s
• Bulk Flow – the movement of water in bulk –
as a liquid
Controlled primarily by P – no membrane, no
solute gradient!
40
Critical Thinking
• Where does water
move by bulk flow
in plants???
41
Critical Thinking
• Where does water
move by bulk flow
in plants???
• Primarily in the
xylem, also in
phloem and in the
cell walls
42
Routes of water transport
soil  root  stem  leaf  atmosphere
Cell Wall
Cell Membrane
Diagram – apoplast, symplast and transmembrane
pathways; same on next slide
Cytoplasm
43
Routes of water transport
soil  root  stem  leaf  atmosphere
Cell Wall
Cell Membrane
Cytoplasm
44
Diagram – Casparian strip; same on next 2 slides
45
The Casparian Strip is a band of suberin in the
transverse and radial (but not the tangential) walls of
the endodermis cells
Water CANNOT PASS THROUGH the Casparian Strip
Water must GO AROUND the Casparian Strip –
through the tangential face of the endodermis
46
The Casparian Strip is a band of suberin in the
transverse and radial (but not the tangential) walls of
the endodermis cells
Water CANNOT PASS THROUGH the Casparian Strip
Water must GO AROUND the Casparian Strip –
through the tangential face of the endodermis
47
Critical Thinking
• Apoplast water is forced into the symplast
at the Casparian Strip
• What does this mean for the water???
• What is the function of the Casparian
Strip???
48
Critical Thinking
• Apoplast water is forced into the symplast
at the Casparian Strip
• What does this mean for the water???
• It has to cross a cell membrane (easy for
water!)
• What is the function of the Casparian
Strip???
49
Critical Thinking
• Apoplast water is forced into the symplast
at the Casparian Strip
• What does this mean for the water???
• It has to cross a cell membrane (easy for
water!)
• What is the function of the Casparian
Strip???
• Solute uptake is regulated at the
membrane!!!
50
Membrane Transport
(review in text if necessary)
Diagram – review of membrane transport proteins
51
Water is on the move
52
Transpiration
Diagram – transpiration
• Movement of water from
soil  plant  atmosphere
• Controlled by HUGE water
potential gradient
• Gradient controlled by P
Very little s contribution
Ψ=P-s
53
Stomates are the Valves:
as long as the stomata are open, water
will move through the plant
Micrograph – stomata
54
Transpiration
Diagram – transpiration
• Movement of water from
soil  plant  atmosphere
• Controlled by HUGE water
potential gradient
• Gradient controlled by P
Very little s contribution
Ψ=P-s
55
Solar Heating Drives
the Process
• Air is dry because of solar heating
The air molecules bounce around more which
causes air masses to expand
Warm air has tremendous capacity to hold
water vapor
• Warm, dry air dramatically reduces the Ψ
of the atmosphere
• Daytime gradient is commonly 30+ MPa
56
Critical Thinking
• Why do we have life on this planet and not
the others in our solar system???
57
Critical Thinking
• Why do we have life on this planet and not
the others in our solar system???
• Liquid water!
• Why do we have liquid water???
58
Critical Thinking
• Why do we have life on this planet and not
the others in our solar system???
• Liquid water!
• Why do we have liquid water???
• 3rd rock from the sun!
The Goldilocks Zone – not too hot, not too cold
Plus, we have enough gravity to hold our
atmosphere in place
It’s our atmosphere that holds the warmth
59
Life is Random
Model – our solar system
60
Solar Heating Drives
the Process
• Air is dry because of solar heating
The air molecules bounce around more which
causes air masses to expand
Warm air has tremendous capacity to hold
water vapor
• Warm, dry air dramatically reduces the Ψ
of the atmosphere
• Daytime gradient is commonly 30+ MPa
61
- 200
asymptotic
Atmospheric
water
potential
(MPa)
- 30
0
0
80
100
Relative Humidity (%)
62
Critical Thinking
• Under what conditions does atmospheric
water potential approach zero???
- 200
asymptotic
Atmospheric
water
potential
(MPa)
- 30
0
0
80
100
63
Relative Humidity (%)
Critical Thinking
• Under what conditions does atmospheric
water potential approach zero???
• Only in the pouring rain
asymptotic
- 200
Atmospheric
water
potential
(MPa)
- 30
0
0
80
100
64
Relative Humidity (%)
Gradient is HUGE
• Pressure plumbing ~ 0.25 MPa
• Fully inflated car tire ~ 0.2 MPa
• Only in the pouring rain does atmospheric
Ψ approach zero
• Soil Ψ is ~ zero under most conditions
• Remember – gradient is NEGATIVE
• Water is pulled into plant under TENSION
65
Gradient is HUGE
• Pressure plumbing ~ 0.25 MPa
• Fully inflated car tire ~ 0.2 MPa
• Only in the pouring rain does atmospheric
Ψ approach zero
• Soil Ψ is ~ zero under most conditions
• Remember – gradient is NEGATIVE
• Water is pulled into plant under TENSION
66
- 200
asymptotic
Atmospheric
water
potential
(MPa)
- 30
0
0
80
100
Relative Humidity (%)
67
Gradient is HUGE
• Pressure plumbing ~ 0.25 MPa
• Fully inflated car tire ~ 0.2 MPa
• Only in the pouring rain does atmospheric
Ψ approach zero
• Soil Ψ is ~ zero under most conditions
• Remember – gradient is NEGATIVE
• Water is pulled into plant under TENSION
68
The tension gradient is
extreme, especially
during the day
Diagram – transpiration
gradient from soil to
atmosphere
Sunday, 1 October 2006
8 am – RH = 86%
Noon – RH = 53%
4 pm – RH = 36%
8 pm – RH = 62%
5am, 23 September – 94% in light rain
69
- 200
asymptotic
Atmospheric
water
potential
(MPa)
- 30
0
0
80
100
Relative Humidity (%)
70
Critical Thinking
• Tension is a strong force!
• Why doesn’t the water stream break???
• Adhesion and cohesion
• Why doesn’t the xylem collapse???
• Lignin!
71
Critical Thinking
• Tension is a strong force!
• Why doesn’t the water stream break???
• Adhesion and cohesion
• Why doesn’t the xylem collapse???
72
Critical Thinking
• Tension is a strong force!
• Why doesn’t the water stream break???
• Adhesion and cohesion
• Why doesn’t the xylem collapse???
• Lignin!!!
73
Diagram – transpiration gradient plus pathways
74
Table – water use by various crops
One hectare (2 football fields) of corn transpires about 6 million liters of water
per growing season – the equivalent of 2’ of water over the entire hectare…
75
Transpiration is a powerful force!
• A single broadleaf tree can move 4000 liters
of water per day!!! (about 1000 gallons)
• If humans had to drink that much water we
would drink about 10 gallons per day!
• Transpiration accounts for 90% of
evapotranspiration over most terrestrial
surfaces
• Plants are the most important component of
the hydrological cycle over land!!!
76
Tropical deforestation is leading to
ecological and social disaster
• Poverty, famine and forced migration
• 250 million victims of ecological destruction –
that’s about how many people live in the US!
….and just a tiny fraction of the world’s
impoverished people
You can help
change this!!!
Image – deforestation snaps
water cycle and also results in
erosion
77
Panama
Guatemala
Tropical deforestation is leading to
ecological and social disaster
• Poverty, famine and forced migration
• 250 million victims of ecological destruction –
that’s about how many people live in the US!
….and just a tiny fraction of the world’s
impoverished people
You MUST help
change this!!!
78
Panama
Guatemala
Social Justice
I’m not
angry
with
you
……
79
Social Justice
But I do
expect
you to DO
something
!!!
80
Hands On
• Examine variegated plant
Water with dye solution
What do you expect???
• Set up experiments with white carnations
Vary conditions of light, temperature and air
flow
Re-cut stems and place in dye solution – why?
• Be sure to develop hypotheses
• Discuss findings with team and be prepared
to share conclusions with the class
81
Hands On
• Work with team to develop hypotheses
about how different species might vary in
water transport – rely on locally available
plant species, and vary species only (not
environmental conditions)
• As a class, develop several hypotheses
• Collect plant samples
• Set up potometers, record data
• Summarize results and discussions in lab
82
notes
Transpiration is a Natural Process
• It is a physical process that occurs as long
as the gradient exists and the pathway is
open
• Under adequate soil moisture conditions
the enormous water loss is not a problem
for the plant
83
Critical Thinking
• What happens when soil moisture
becomes limited???
84
Critical Thinking
• What happens when soil moisture
becomes limited???
• Water stress causes stomata to close
• What then???
85
Critical Thinking
• What happens when soil moisture
becomes limited???
• Water stress causes stomata to close
• What then???
• Gas exchange ceases – no CO2 = no
photosynthesis
86
What happens when soil moisture
becomes limited???
• Water stress causes stomata to close
• Closed stomata halt gas exchange
 P/T conflict  P/T compromise
• Stomata are generally open during the day,
closed at night
 Abscissic acid promotes stomata closure daily, and
under water stress conditions
 Other structural adaptations limit water loss when
stomata are open
 Other metabolic pathways (C4, CAM) limit water loss 87
Normally, stomata open during the day and
close at night in response to changes in K+
concentration in stomata guard cells
• K+ accumulation is triggered by increased light,
low carbon dioxide, circadian rhythms
• High [K+] does
what to Ψ???
Micrograph – turgid guard
cells; same next 4 slides
88
Normally, stomata open during the day and
close at night in response to changes in K+
concentration in stomata guard cells
• K+ accumulation is triggered by increased light,
low carbon dioxide, circadian rhythms
• High [K+] lowers
water potential in
guard cells
• What does water
do???
89
Normally, stomata open during the day and
close at night in response to changes in K+
concentration in stomata guard cells
• K+ accumulation is triggered by increased light,
low carbon dioxide, circadian rhythms
• High [K+] lowers
water potential in
guard cells
• Water enters, cells
swell and buckle
90
Normally, stomata open during the day and
close at night in response to changes in K+
concentration in stomata guard cells
• K+ accumulation is triggered by increased light,
low carbon dioxide, circadian rhythms
• High [K+] lowers
water potential in
guard cells
• Water enters, cells
swell and buckle
• Pore opens
91
Normally, stomata open during the day and
close at night in response to changes in K+
concentration in stomata guard cells
• K+ accumulation is triggered by increased light,
low carbon dioxide, circadian rhythms
• High [K+] lowers
water potential in
guard cells
• Water enters, cells
swell and buckle
• Pore opens
• Reverse at night
closes the pores
92
Diagram – open and closed stomata
93
Abscissic acid is the hormone that
mediates this response
Diagram – hormone mediated stomatal opening and closing
94
Cellulose orientation determines
shape of turgid cells
Diagram – spoke-like orientation of cellulose microfibrils
95
What happens when soil moisture
becomes limited???
• Water stress causes stomata to close
• Closed stomata halt gas exchange
 P/T conflict  P/T compromise
• Stomata are generally open during the day,
closed at night
 Abscissic acid promotes stomata closure daily, and
under water stress conditions
 Other structural adaptations limit water loss when
stomata are open
 Other metabolic pathways (C4, CAM) limit water loss 96
Micrograph – location of stomatal gradient
This is the
gradient that
counts
97
Images – structural adaptations to dry environments
98
Images and diagrams – metabolic
adaptations to dry environments
Spatial separation
helps C4 plants be
more efficient in hot
climates
Temporal separation
does the same for
CAM plants
Both use an enzyme
that can’t fix O2 to
first capture CO2
Both adaptations
allow photosynthesis
to proceed with
stomata largely
closed during the
99
day
Hands On
• Work with your team to make hypotheses
about stomata number and placement on
various types of leaves
• Use nail polish to make impressions of
stomata
Put a tab of paper under the polish
Make a dry mount of the impression
• Count stomata in the field of view and
estimate the number of stomata per mm2
• Be prepared to discuss your findings
100
Phloem Transport
• Most of phloem sap is water (70% +)
• Solutes in phloem sap are mostly carbohydrates,
mostly sucrose for most plant species
• Other solutes (ATP, mineral nutrients, amino
acids, hormones, secondary metabolites, etc)
can also be translocated in the phloem
• Phloem transport driven by water potential
gradients, but the gradients develop due to
active transport – both P and s are important
101
The Pressure
Flow Model For
Phloem Transport
• Xylem transport
is uni-directional,
driven by solar
heating
Diagram – pressure flow model
of phloem flow; this diagram is
repeated throughout this section
• Phloem flow is
multi-directional,
driven by active
transport –
source to sink
102
The Pressure
Flow Model For
Phloem Transport
• Sources can be
leaves, stems or
roots
• Sinks can be
leaves, stems,
roots or
reproductive
parts (especially
seeds and fruits)
103
The Pressure
Flow Model For
Phloem Transport
• Sources and
sinks vary
depending on
metabolic activity,
which varies daily
and seasonally
• Most sources
supply the
nearest sinks, but
some take priority
104
Active transport (uses ATP) builds
high sugar concentration in sieve
cells adjacent to source
Diagram – the transport proteins that actively transport
sucrose into the phloem cells from the leaf cells
105
The Pressure
Flow Model For
Phloem Transport
• High [solute] at
source end does
what to Ψ???
106
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What happens to Ψ as s increases???
107
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What happens to Ψ as s increases???
• Water potential is reduced
• This is what happens at the source end of
the phloem
108
The Pressure
Flow Model For
Phloem Transport
• High [solute] at
source end
decreases Ψ
• What does water
do???
109
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What does water do when Ψ decreases???
110
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What does water do when Ψ decreases???
• Water moves toward the area of lower water
potential
• This is what happens at the source end of
the phloem
• Where does the water come from???
111
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What does water do when Ψ decreases???
• Water moves toward the area of lower water
potential
• This is what happens at the source end of
the phloem
• Where does the water come from???
• The adjacent xylem – remember structure
112
and function are related!
The Pressure
Flow Model For
Phloem Transport
• High [solute] at
source end
decreases Ψ
• Water moves into
the source end of
the phloem
• What does this
do to P at the
source end?
113
Critical Thinking
• What will happen to water pressure in any
plant cell as water moves in???
114
Critical Thinking
• What will happen to water pressure in any
plant cell as water moves in???
• It increases
• Why???
115
Critical Thinking
• What will happen to water pressure in any
plant cell as water moves in???
• It increases
• Why???
• The cell wall limits expansion – it “pushes
back”
116
The Pressure
Flow Model For
Phloem Transport
• High [solute] at
source end
decreases Ψ
• Water moves into
the source end of
the phloem
 This increases
the pressure
117
The Pressure
Flow Model For
Phloem Transport
• Increased
pressure at
source end
causes phloem
sap to move to
any area of lower
Ψ = sinks
118
The Pressure
Flow Model For
Phloem Transport
• At sink end, the
sugars are
removed by
metabolism, by
conversion to
starch, or by
active transport
119
The Pressure
Flow Model For
Phloem Transport
• What then
happens to the Ψ
at the sink end of
the phloem???
120
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What happens to Ψ as s decreases???
121
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What happens to Ψ as s decreases???
• Water potential is increased
• This is what happens at the sink end of the
phloem
122
The Pressure
Flow Model For
Phloem Transport
• Ψ goes up at the
sink end of the
phloem
• What does water
do???
123
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What does water do when Ψ increases???
124
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What does water do when Ψ increases???
• Water moves away from the area of higher
water potential
• This is what happens at the sink end of the
phloem
• Where does the water go???
125
Critical Thinking
• Remember the water potential equation
Ψ=P-s
• What does water do when Ψ increases???
• Water moves away from the area of higher
water potential
• This is what happens at the sink end of the
phloem
• Where does the water go???
• The adjacent xylem – remember structure
126
and function are related!
The Pressure
Flow Model For
Phloem Transport
• Ψ goes up at the
sink end of the
phloem
• Water leaves the
phloem at the
sink end, thus
reducing Ψ
• Adjacent xylem
provides and
accepts the water
127
The Pressure
Flow Model For
Phloem Transport
• Thus the
phloem sap
moves – from
source to sink
Some xylem
water is cycled
into and out of
the phloem in
the process
128
The Pressure
Flow Model For
Phloem Transport
• Active transport
is always
involved at the
source end, but
only sometimes
at the sink end
129
Critical Thinking
• What about the structure of
the sieve cells facilitates
the movement of phloem
Micrograph – sieve cells;
sap???
same next slide
130
Critical Thinking
• What about the structure of
the sieve cells facilitates
the movement of phloem
sap???
• The open sieve plate
• The lack of major
organelles
131
The Pressure
Flow Model For
Phloem Transport
Questions???
132
Key Concepts: Questions???
• The importance of water
• Water potential: Ψ = P - s
• How water moves – gradients,
mechanisms and pathways
• Transpiration – water movement from soil
to plant to atmosphere
• The pressure flow model of phloem
transport
133
Hands On
• For tomorrow – bring some soil from your
yard and/or garden
• Put it in a clear, water-tight container
(glass jar is easiest)
134