Water Potentialin Plants

ΨW= ΨP + ΨS

Joyce Payne & Tracy Sterling© 2004 New Mexico State

UniversityDepartment of Entomology,

Plant Pathology, and Weed Science

The slide set is divided into two parts.

Part I

What is Water Potential?

Importance The Water Potential Equation

Part one investigates the significance of water potential in plants and explains

the equation for estimating water potential in plants.

Part II

Measuring Water Potential

Solutes and Pressure Methods and Instruments

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 I

What is WaterPotential (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 available foruptake by plant roots

Atmospheric

Water Potential

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.

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…

ΨW= ΨP + ΨS + ΨE + ΨG

Where, ΨP = pressure potentialΨS = osmotic or solute potentialΨE = electrical potential

- ignore because water is uncharged

ΨG = gravitational potential- ignore because gravity is not a large force for small trees

Current Convention Defines Ψw as:

Ψw= 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

Simplified Definition of Ψw:

Pressure Potential Positive Turgor (in cells with

membranes) Negative Tension (in xylem)

Osmotic or Solute Potential- Negative

SUMMARY: Water Potential of Plant Tissue

has two components and is always 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 )

Part II

Measuring Water Potential

Solutes and Pressure Methods and Instruments

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.

Review

ΨW = 0 MPa Definition of Pure H2O,

under no pressure

ΨW = ΨP + ΨS - Pressure potential

increaseswater potential- Solute Potential decreases

(gets more negative) with

increasing solute concentration,

thus, lowering water potential

0.10 m Sucrose

ΨP = 0 MPa

ΨS= - 0.244 MPa

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.

Ψw = ΨP + ΨS

Ψw = 0 MPa + (-0.244 MPa)

Ψw = - 0.244 MPa

0.10 m Sucros

e

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.

Ψw (Pure H20) = 0 MPa

Ψw (0.10 m Sucrose) = - 0.244 MPa

So, by adding the solute sucrose to pure water we have lowered the water potential of that pure water.

Methods and Instruments

• Constant Volume Method• Pressure Chamber• Cryoscopic Osmometer• Psychrometer

ΨW = ΨP - ΨS

Constant Volume MethodThe 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 sincethe 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 thereis 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 Ψwof the plant tissue can

be assumed equal to the Ψw of thesolution when there is no net watermovement between the plant tissueand the solution

0.15 m PEG 0.20 m PEG 0.25 m PEG

Ψw = - 0.367 MPa Ψw = - 0.489 MPa Ψw = - 0.612 MPa

0.15 m 0.20 m 0.25 m 0.15 m 0.20 m 0.25 m

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.

Ψw(soln) Initial Final +/- Wgt

-0.367 MPa 0.005 g 0.006 g + 0.001 g-0.489 MPa 0.005 g 0.005 g 0 g-0.612 MPa 0.005 g 0.004 g - 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.

Ψw(leaf) <Ψw(soln)

Ψw(soln) Initial Final +/- Wgt

-0.367 MPa 0.005 g 0.006 g + 0.001 g

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.

So, water moved down a water potential gradient.

-0.367 MPa 0 MPa

Ψw(soln) Pure Water

(+)(-)

H2O movement down a Ψw gradient

Ψw(leaf) <Ψw(soln)

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.

Ψw(soln) Initial Final +/- Wgt

-0.612 MPa 0.005 g 0.004 g - 0.001 g

Ψw(leaf) >Ψw(soln)

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.

Ψw(soln) Pure H2O

-0.367 MPa 0 MPa

Ψw(leaf) >Ψw(soln) Ψw(leaf)

<Ψw(soln)

Ψw(soln)

-0.612 MPa

(+)(-)

Ψw(leaf)

We now know that the water potential of our leaf disk is somewhere between - 0.367 and - 0.612 MPa.

Ψw(soln) Initial Final +/- Wgt

-0.489 MPa 0.005 g 0.005 g 0 g

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.

Ψw(leaf) Ψw(soln)

Water is in equilibrium

Ψw(soln) Initial Final +/- Wgt

-0.124 MPa 0.017 g 0.018 g +0.001 g-0.247 MPa 0.016 g 0.015 g - 0.001 g-0.370 MPa 0.017 g 0.015 g - 0.002 g-0.494 MPa 0.017 g 0.014 g - 0.003 g-0.618 MPa 0.017 g 0.013 g - 0.004 g-0.741 MPa 0.018 g 0.013 g - 0.005 g

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.

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

When using this method we make two assumptions:

• The solute potential is assumed to be zero, since few dissolved solutes are in the xylem sap.

• 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.

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.

Pressure Chamber

• ΨS is assumed to be zero, since fewdissolved solutes are in the xylem sap

• The positive pressure required

toreverse the xylem sap flow estimatesthe water potential of the plant because the xylem is in intimate contact with most of the plant’s cells

TranspirationPull

Xylem Sap

Negative HydrostaticPressure or

Tension

(-ΨP)

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.

PP

Positive Pressure Applied, Xylem sap exudes from cut

surface

Excised Leaf

H2O column in xylem recedes (red)

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).

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.

ΨP(air) + ΨP(xylem) = 0

Positive Pressure Needed toReverse 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 surfaceoccurs, the value on the pressure gauge is read.

Cut end of tissue with sap exuding (oversized)

10 Bars = 1 MPa

45 Bars = 4.5 MPa ΨP(air) + ΨP(xylem) = 0

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.

4.5 MPa + Ψxylem = 0

4.5 MPa – 4.5 MPa + Ψxylem = 0 – 4.5 MPa

ΨW(xylem) = - 4.5 MPa

The estimated water potential is - 5 MPa because:

Ψair + Ψxylem = 0

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 anappropriate 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 areapplied in the chamber

Pressure Chamber - Summary

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

Temperature MonitoringDevice

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.

Oil Cell Sap

Thermal Stage

Temperature MonitoringDevice

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.

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 ΨS

Psychrometer

• Estimates ΨW by measuring the changein temperature due to:

- evaporation (cooling)- condensation (warming)

“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.

Thermocouple

Temperature Gaugeand Controls

PlantTissue

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 should be noted that we have greatly enlarged the chamber size for demonstration purposes.

Thermocouple with drop of solution

Temperature Gaugeand Controls

PlantTissue

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.

Thermocouple with drop of solution

Temperature Gaugeand Controls

PlantTissue

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.

Thermocouple with drop of solution

Temperature Gaugeand Controls

PlantTissue

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.

(+)

0

(-)

T

Ψsoln on thermocouple (MPa)

-1 -2 -3 -4

Ψsol < Ψtissue (Temp )

Ψsoln > Ψtissue (Temp )

ΨtissueΨsoln

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.

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 ΨW of excised and intact planttissue and solutions

• Equipment is sensitive to temperaturefluctuations

• Controlled laboratory conditions

Water Potential - Summary

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 livingthings, 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.

ΨW = ΨP + ΨS

ReferencesNobel, 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 BowersTracy M. Sterling

Department of Entomology, Plant Pathology, and Weed Science

© 2004 New Mexico State University