Transcript Water Potential Slide Show (needs edits)

Water Potential
in Plants
Y W = ΨP + Ψ S
Joyce Payne & Tracy Sterling
© 2004 New Mexico State University
Department of Entomology,
Plant Pathology, and Weed Science
The slide set is divided into two parts.
Part one investigates the significance
of water potential in plants and explains
the equation for estimating water potential in plants.
Part I
What is Water Potential?
 Importance
 The Water Potential Equation
Part two reviews the effect solutes and pressure
have on water potential and presents some of the
methods and instruments plant scientists use to
estimate the water potential of plant tissues.
Part II
Measuring Water Potential
 Solutes and Pressure
 Methods and Instruments
Part I
What is Water
Potential (ΨW)?
It is a quantitative description of the free energy states
of water.
The concepts of free energy and water potential are
derived from the second law of thermodynamics.
In thermodynamics, free energy is defined as the potential
for performing work.
A water fall is a good example. The water at the top of the fall has
a higher potential for performing work than the water at the base of
the fall. The water is moving from an area of higher free energy to
an area of lower free energy. The free energy from water is the
power source for waterwheels and hydroelectric facilities.
Water potential is a useful measurement to determine water-deficit stress in
plants. Scientists use water potential measurements to determine drought
tolerance in plants, the irrigation needs of different crops and how the
water status of a plant affects the quality and yield of plants.
Water potential affects plants in many ways. Atmospheric water
potential is one of the factors that influences the rate of
transpiration or water loss in plants. Soil water potential
influences the water available for uptake by plant roots.
Atmospheric
Water
Potential
Water available for
uptake by plant roots
Water Potential
is based on the ability of water to do work.
Let’s step back a bit and look at the potential for
a chemical to do work. Thermodynamics define
several forces act on any molecule which reduce
its ability or potential to do work. These forces
are pressure, concentration, electrical and
gravity; added together they make up the
Chemical Potential. So essentially,
Chemical Potential = Pressure + Concentration + Electrical + Gravity
Water Potential (ΨW)
The greek symbol for Water Potential, ΨW, is the
letter ‘psi’ (pronounced ‘sigh’).
Several forces act on water to alter its ability or
potential to do work. Again, these forces are
pressure, concentration, electrical and gravity;
added together they make up the Water
Potential. So essentially,
Water Potential = Pressure + Concentration + Electrical + Gravity
but only the first two of these are important…
Current Convention Defines Ψw as:
Y W = ΨP + ΨS + ΨE + ΨG
Where,
ΨP = pressure potential
ΨS = osmotic or solute potential
ΨE = electrical potential
- ignore because water is uncharged
ΨG = gravitational potential
- ignore because gravity is not a
large force for small trees
Simplified Definition of Ψw:
Y w = ΨP + ΨS
Where,
ΨP = pressure potential
- represents the pressure in addition to
atmospheric pressure
ΨS = osmotic or solute potential
- represents the effect of dissolved solutes on
water potential; addition of solutes will always
lower the water potential
SUMMARY:
Water Potential of Plant Tissue
has two components
and is always negative
 Pressure Potential
 Positive
 Negative
Turgor (in cells with
membranes)
Tension (in xylem)
 Osmotic or Solute Potential
- Negative
Some Principles Described in this Slide Show
• Water moves spontaneously only from places of higher water potential to
places of lower water potential
• Between points of equal water potential, there is no net water movement
• The zero point of the water potential scale is defined as the state of Pure
Water (no solutes) at normal pressure and elevation where, Ψw = 0
• Water potential values are always negative
– for example, all plant cells contain solutes which will always lower the water potential
• Ψw is increased by an increase in pressure potential (ΨP)
• Ψw is decreased by addition of solutes which lowers the solute potential (ΨS )
Plant scientists measure the water potential of plant tissue using a
variety of tools. Before we look at some of the methods and
instruments, we will review the effect of pressure and solutes on the
water potential of a solution.
Part II
Measuring Water Potential
 Solutes and Pressure
 Methods and Instruments
Review
 YW = 0 MPa
Definition of Pure H2O,
under no pressure
 Y W = Ψ P + ΨS
- Pressure potential increases
water potential
- Solute Potential decreases
(gets more negative) with
increasing solute concentration,
thus, lowering water potential
To illustrate the effect that solutes have on water potential, let’s
calculate the water potential of a 0.10 molal (m) solution of sucrose.
The hydrostatic potential (ΨP) of this solution is equal to zero because
the beaker is open to atmospheric pressure and no excess pressure is
being applied. For a 0.10 m sucrose solution, the solute potential (ΨS)
of the solution, is -0.244 MegaPascals. This conversion is made using
the Van’t Hoff equation.
ΨP = 0 MPa
0.10 m
Sucrose
ΨS= - 0.244 MPa
When we plug these values into our equation and solve, we find that
the water potential of the 0.10 m solution of sucrose is - 0.244 MPa.
Y w = ΨP + ΨS
0.10 m
Sucrose
Ψw = 0 MPa + (-0.244 MPa)
Yw = - 0.244 MPa
So, by adding the solute sucrose to pure water we have lowered the
water potential of that pure water.
Yw
Yw
(Pure H20)
(0.10 m Sucrose)
= 0 MPa
= - 0.244 MPa
Methods and Instruments
YW = Ψ P - ΨS
• Constant Volume Method
• Pressure Chamber
• Cryoscopic Osmometer
• Psychrometer
Constant Volume Method
The constant volume method uses the known water
potentials of molar solutions to estimate the water
potential of plant tissue. This method assumes two
things. First, the hydrostatic pressure is zero since
the test tubes are open to the atmosphere and no
excess pressure is being applied and secondly, the
water potential of the plant tissue can be assumed
equal to the water potential of the solution when there
is no net water movement between the plant tissue
and the solution. Note that even when there is no net
water movement, water movement does not cease,
merely equal amounts of water are moving between
the plant tissue and the solution.
Constant Volume Method
• ΨP = 0 MPa
• The Yw of the plant tissue can
be assumed equal to the Ψw of the
solution when there is no net water
movement between the plant tissue
and the solution
0.15
0.15mm
0.15 m PEG
Yw = - 0.367 MPa
0.20
0.20mm
0.20 m PEG
Yw = - 0.489 MPa
0.25
0.25 m
m
0.25 m PEG
Yw = - 0.612 MPa
We begin by preparing solutions with increasing molality (or decreasing ΨW)
using polyethylene glycol (PEG), as our solute. PEG is a large hydrocarbon
that does not move across plant membranes.
Next, we prepare our plant tissue. Disks are cut from a leaf with a cork
borer and weighed. The weight for each disk is recorded on a chart.
One pre-weighed leaf disk is placed on each molar solution. The leaf disks
are kept in the solutions for approximately one hour to allow the water in
the leaf disks and the water in the molar solutions to come to equilibrium.
Equilibrium is defined as the point where net water movement is zero.
 Leaf disc in
each test tube
After one hour, the leaf disks are individually removed from the
solutions, blotted dry and reweighed.
Yw(soln)
-0.367 MPa
-0.489 MPa
-0.612 MPa
Initial
0.005 g
0.005 g
0.005 g
Final
0.006 g
0.005 g
0.004 g
+/- Wgt
+ 0.001 g
0g
- 0.001 g
The final weights are recorded on the same chart with the initial
weights and the difference between the initial and final weights is
calculated.
If the leaf disk gained weight, then the water moved from
the solution into the leaf disk. The water potential of the
solution was higher than the water potential of the leaf disk.
Yw(soln)
Initial
-0.367 MPa
0.005 g
Final
0.006 g
+/- Wgt
+ 0.001 g
Yw(leaf) < Yw(soln)
So, water moved down a water potential gradient.
H2O movement down a Yw gradient
The number line helps when you are working with water
potential gradients. Water moves from areas with high water
potentials, less negative numbers, to areas with lower water
potentials, more negative numbers. At this point, we know that
the water potential of the leaf disk is a more negative number
than - 0.367 MPa because water moved from the solution to the
leaf disk down a water potential gradient.
-0.367 MPa 0 MPa
(-)
Yw(leaf) < Yw(soln)
(+)
Yw(soln) Pure Water
If the leaf disk lost weight, then water moved from the leaf disk down
a water potential gradient into the solution. So, the water potential of
the leaf disk is a less negative number than - 0.612 MPa.
Yw(soln)
-0.612 MPa
Yw(leaf) > Yw(soln)
Initial
0.005 g
Final
0.004 g
+/- Wgt
- 0.001 g
We now know that the water potential of our leaf disk is
somewhere between - 0.367 and - 0.612 MPa.
Yw(leaf)
(-)
Yw(leaf) > Yw(soln)
Yw(soln)
-0.612 MPa
Yw(leaf) < Yw(soln)
Yw(soln)
-0.367 MPa
(+)
Pure H2O
0 MPa
In theory, it is the leaf with no net weight gain that gives
us our estimate of the water potential of the plant tissue.
As much water is moving into the disc, as is moving out.
In reality, when this method is used you very rarely see
leaf disks with no net weight gained or lost.
Yw(soln)
-0.489 MPa
Initial
0.005 g
Water is in equilibrium
Yw(leaf)  Yw(soln)
Final
0.005 g
+/- Wgt
0g
What normally occurs is that there is a point in the data where the difference
between initial and final weights changes from positive to negative. The
water potential of the plant tissue is between these points where the leaf disk
quit gaining weight and started losing weight. An estimate of the water
potential of the plant tissue can be made by averaging the two water
potentials of the solutions or with linear interpolations.
Yw(soln)
-0.124 MPa
-0.247 MPa
-0.370 MPa
-0.494 MPa
-0.618 MPa
-0.741 MPa
Initial
0.017 g
0.016 g
0.017 g
0.017 g
0.017 g
0.018 g
Final
0.018 g
0.015 g
0.015 g
0.014 g
0.013 g
0.013 g
+/- Wgt
+0.001 g
- 0.001 g
- 0.002 g
- 0.003 g
- 0.004 g
- 0.005 g
Constant Volume Method - Summary
The constant volume method is a simple and
straight forward method for estimating the water
potential of plant tissue that requires minimal
equipment and expense. However, this method
does have low resolution results.
Constant Volume Method - Summary
• Simple method
• Requires minimal equipment
• Low resolution results
Pressure Chamber
The pressure chamber, or pressure ‘bomb’ as it is commonly called, is an
instrument for estimating the water potential of a plant by reversing the
negative hydrostatic potential (-ΨP), or tension, in a plant’s xylem sap.
When using this method we make two assumptions:
1. The solute potential is assumed to be zero, since
few dissolved solutes are in the xylem sap.
2. The xylem is in intimate contact with the majority
of cells in the entire plant because only two to three
cells separate vascular bundles. Therefore,
measuring the positive potential required to reverse
the xylem sap flow will give us a good estimate of
the water potential of the plant.
Pressure Chamber
• ΨS is assumed to be zero, since few
dissolved solutes are in the xylem sap
• The positive pressure required to
reverse the xylem sap flow estimates
the water potential of the plant
because the xylem is in intimate
contact with most of the plant’s cells
The water column in the xylem is under tension or negative hydrostatic
pressure because transpiration is drawing water through the plant from
the soil to the atmosphere.
Xylem Sap
Negative Hydrostatic
Pressure or Tension
(-ΨP)
Transpiration
Pull
When a stem is cut, the water
column recedes away (red)
from the cut surface.
The pressure chamber applies
positive pressure to bring the
xylem sap back to the cut surface
(blue).
Excised Leaf
H2O column in
xylem recedes (red)
P
P
Positive Pressure Applied,
Xylem sap exudes from cut surface
When the air pressure of the chamber causes the exudation of xylem
sap at the cut end, the resulting pressure of the sap is zero.
YP(air) + YP(xylem) = 0
Positive Pressure Needed to
Reverse the Xylem Sap Flow
At that point, ΨP(air)
= - ΨP(xylem).
Because there are few solutes
in the xylem, ΨS(xylem) is zero.
Thus,
ΨP(xylem) = ΨW(xylem).
The pressure chamber
apparatus consists of a
pressure gauge (mid left), a
pressure chamber (mid right),
a rubber gasket for holding
plant material and creating a
pressure seal (lower right),
and lid that holds the rubber
gasket and seals the pressure
chamber lower right. A hose
connection (upper left)
attaches the pressure chamber
to a compressed gas source.
An excised leaf is inserted through a
slit in the rubber gasket and placed
into the pressure chamber lid. The
lid is then placed on the pressure
chamber and sealed.
Once the pressure chamber has been sealed, compressed gas is slowly
released into the chamber thus increasing the hydrostatic pressure. The cut
end of the stem is closely watched. When the cut end is wet, the xylem sap
has been pushed back to the surface of the cut. When wetting of the surface
occurs, the value on the pressure gauge is read.
Cut end of tissue

with sap exuding (oversized)
The positive pressure reading from the plant tissue tested in the previous
slide was 45 bars, a very stressed plant. To estimate the water potential,
we must first convert the positive pressure from bars into MegaPascals
(MPa). Ten bars is equal to one MegaPascal, so 45 bars equals 4.5
MegaPascals. We now plug our hydrostatic potential value into the
equation and solve on the next slide.
10 Bars = 1 MPa
45 Bars = 4.5 MPa
YP(air) + YP(xylem) = 0
The estimated water potential is - 5 MPa because:
Yair + Yxylem = 0
4.5 MPa + Yxylem = 0
4.5 MPa – 4.5 MPa + Yxylem = 0 – 4.5 MPa
YW(xylem) = - 4.5 MPa
Pressure Chamber - Summary
The pressure chamber is a quick method for
estimating the water potential of plants and is
commonly used by plant scientists. It can be
transported to the field but some models are
heavy and bulky. The pressure ‘bomb’ is an
appropriate nickname for this piece of
equipment because of the dangerous pressure
levels in the chamber. Great care should
always be used when operating a pressure
bomb.
Pressure Chamber - Summary
• Quick method, commonly used
• Equipment can be used in the
field; can be heavy and bulky
• Dangerous pressure levels are
applied in the chamber
Cryoscopic Osmometer
The Cyroscopic Osmometer estimates the water potential
of plant tissue by estimating the solute potential in a plant
cell’s sap. This method is based on the Colligative
Property of Solutions which states that as the solute
concentration of a solution increases, the freezing point
decreases. When using this method we assume that the
hydrostatic potential in the cell is zero because the cell
membranes are damaged from freezing.
Cryoscopic Osmometer
• Colligative Property of Solutions As the solute concentration of a solution
increases, the freezing point decreases
• Assume ΨP = 0
(when membranes are damaged from freezing)
Oil
Cell Sap
Thermal Stage
The Cryoscopic
Osmometer consists of a
temperature-controlled
thermal stage attached to a
microscope. A drop of
plant cell sap is suspended
in a depression on the
stage. Oil is included to
prevent evaporation.
Temperature Monitoring
Device
Oil
Cell Sap
Thermal Stage
The temperature is rapidly lowered
to freeze the cell sap. The
temperature is then slowly raised and
the melting process is observed
through the microscope until the last
ice crystal in the plant cell sap melts.
The temperature is then noted and
recorded. Remember that melting
and freezing points are the same.
The solute potential of the cell sap is
then calculated using the freezing
point depression.
Temperature Monitoring
Device
Cryoscopic Osmometer - Summary
The Cryoscopic Osmometer is an expensive piece of
equipment that can only be operated by trained
technicians under laboratory conditions.
The presence of anti-freeze compounds in plant cells
may affect the freezing point depression estimates of
solute potential.
Cryoscopic Osmometer - Summary
• Expensive instrument
• Trained technicians operate under
laboratory conditions
• Anti-freeze compounds in plant
cells may affect the estimate of YS
Psychrometer
“Psychro” is from the Greek word for “to cool.” The
Psychrometer estimates water potential by measuring the
change in temperature due to evaporation or condensation.
• Estimates YW by measuring the change
in temperature due to:
- evaporation (cooling)
- condensation (warming)
The Psychrometer consists of a sealed chamber with a thermocouple attached
to a temperature gauge. A drop of a standard solution with known water
potential is placed on the thermocouple and a piece of plant tissue is place in
the bottom of the chamber.
The chamber is sealed.
The solution drop and the
plant tissue are allowed to
come to equilibrium. It
Thermocouple
should be noted that we
Temperature Gauge have greatly enlarged the
chamber size for
and Controls
demonstration purposes.
Plant
Tissue
If the drop of solution has a higher water potential than the plant tissue, water
will move from the drop of solution toward the leaf tissue causing the
temperature to drop because of evaporative cooling.
Here, the water is moving
from the drop to the tissue,
down water potential
gradient. This evaporation
cools the thermocouple.
Plant
Tissue
Thermocouple with drop of solution
Temperature Gauge
and Controls
If the plant tissue has a higher water potential than the drop of solution, water
will move from the leaf tissue and condense onto the drop of solution causing
a rise in temperature.
Here, the water is moving
from the tissue to the drop,
down water potential
gradient. This condensation
warms the thermocouple.
Plant
Tissue
Thermocouple with drop of solution
Temperature Gauge
and Controls
It is at the point where there is no net change in temperature that the water
potential of the drop of solution and the plant tissue are assumed to be equal.
Here, the water is in
equilibrium between the
tissue to the drop. Thus,
there is no change in
temperature.
Plant
Tissue
Thermocouple with drop of solution
Temperature Gauge
and Controls
Results can be graphed to determine the water potential of the tissue (Y axis)
from the change in temperature (X axis). Note the green line pointing from
zero temperature change, down to the water potential of the solution. This
value is the water potential of the tissue.
(+)
T
Ysol < Ytissue (Temp )
Ysoln Ytissue
0
Ysoln > Ytissue (Temp )
(-)
-1
-2
-3
-4
Ysoln on thermocouple (MPa)
Psychrometer - Summary
The Psychrometer can be used to estimate the water potentials
of excised an intact plant tissue and solutions. The equipment
is very sensitive to temperature fluctuations and must be
operated under controlled constant conditions in the
laboratory.
Psychrometer - Summary
• Estimates YW of excised and intact plant
tissue and solutions
• Equipment is sensitive to temperature
fluctuations
• Controlled laboratory conditions
Water Potential - Summary
ΨW = ΨP + ΨS
Water potential dictates the water status of
the plant. Water potential gradients drive
water movement in plants from the cellular
to the whole plant level. Long distance
transport of sucrose is another example of
processes driven by water potential
gradients in plants. All living
things, including humans, require input of
free energy to grow, reproduce and
maintain their structures. As autotrophs,
plants are able to convert light energy from
the sun into usable energy themselves.
References
Nobel, P. S. 1991. Physicochemical and Environmental Plant
Physiology. Academic Press, Inc., San Diego, CA. 635 pp.
Salisbury, F. B. and C. W. Ross. 1992. Plant Physiology. 4th
Edition. Wadsworth Publishing Co., Belmont, CA. 682 pp.
Taiz, L. and E. Zeiger. 2002. Plant Physiology. 3rd Edition.
Sinauer Associates, Inc., Sunderland, MA. 690 pp.
Water Potential in Plants
Joyce Payne Bowers
Tracy M. Sterling
Department of Entomology, Plant Pathology,
and Weed Science
© 2004 New Mexico State University