1. YW = ΨP + ΨS Water Potential in Plants
Joyce Payne & Tracy Sterling
© New Mexico State University
Department of Entomology,
Plant Pathology, and Weed Science
Slide 1
Water potential in plants was produced at New Mexico State University by the Department of Entomology, Plant Pathology and Weed Science. This slide set is divided into two parts.
The slide set is divided into two parts.

2. What is Water Potential?
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?
Slide 2
Part one investigates the significance of water potential in plants and the conversion of the equation for the chemical potential of a molecule to an equation for estimating water potential in plants.
Importance
The Water Potential Equation

3. Measuring Water Potential
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
Slide 3
Part two reviews the affect solutes have on water potential and presents some of the methods and instruments plant scientist use to estimate the water potential of plant tissues.
Solutes and Pressure
Methods and Instruments

4. 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.
Slide 4
What is water potential? The chemical potential of water or water potential 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.

5. 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.

6. 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.

7. Atmospheric Water Potential Water available for uptake by plant roots
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
Slide 6
Water potential affects plants in many ways. Atmospheric water potential is one of the factors that influences the rate of transpiration in plants. Soil water potential illustrates the water available for uptake by plant roots.
Water available for
uptake by plant roots

8. is based on the ability of water to do work.
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,
Slide 7
This presentation focuses on the water potential of plant tissues. Hydrostatic pressure and osmotic pressure are the two major factors that influence water potential in plant tissue.
Chemical Potential = Pressure + Concentration + Electrical + Gravity

9. Water Potential (ΨW) but only the first two of these are important…
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,
Slide 7
This presentation focuses on the water potential of plant tissues. Hydrostatic pressure and osmotic pressure are the two major factors that influence water potential in plant tissue.
Water Potential = Pressure + Concentration + Electrical + Gravity
but only the first two of these are important…

10. YW = ΨP + ΨS + ΨE + ΨG Current Convention Defines Ψw as: 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
Slide 1
Water potential in plants was produced at New Mexico State University by the Department of Entomology, Plant Pathology and Weed Science. This slide set is divided into two parts.

11. Yw = ΨP + ΨS Simplified Definition of Ψw: 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
Slide 1
Water potential in plants was produced at New Mexico State University by the Department of Entomology, Plant Pathology and Weed Science. This slide set is divided into two parts.

12. Water Potential of Plant Tissue
SUMMARY:
Water Potential of Plant Tissue
has two components
and is always negative
Pressure Potential
Positive Turgor (in cells with
membranes)
Negative Tension (in xylem)
Osmotic or Solute Potential
- Negative
Slide 7
This presentation focuses on the water potential of plant tissues. Hydrostatic pressure and osmotic pressure are the two major factors that influence water potential in plant tissue.

13. 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 )

14. Measuring Water Potential
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
Slide 20
Plant scientists measure the water potential of plant tissue using a variety of tools. Before we look at some of the methods and instruments, let’s review the basics of molar solutions and osmotic pressure.
Solutes and Pressure
Methods and Instruments

15. Review YW = 0 MPa Definition of Pure H2O,
under no pressure
YW = ΨP + ΨS - Pressure potential increases
water potential
- Solute Potential decreases (gets more negative) with
increasing solute concentration,
thus, lowering water potential
Slide 21
The water potential of pure water is defined as being equal to zero MegaPascals.
Osmotic pressure increases with increasing solute concentrations.
Remember, the addition of solutes to pure water always lower the water potential of that water.

16. 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 MegaPascals. This conversion is made using the Van’t Hoff equation.
ΨP = 0 MPa
ΨS= MPa
Slide 22
To illustrate the effect that solutes have on water potential, let’s calculate the water potential of a 0.10 molal solution of sucrose.
“P”, the hydrostatic pressure of this solution is equal to zero because the beaker is open to atmospheric pressure and no excess pressure is being applied.
“Pi”, the osmotic pressure of the solution is MegaPascals.
0.10 m
Sucrose

17. Yw = ΨP + ΨS Ψw = 0 MPa + (-0.244 MPa) Yw = - 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 MPa.
Yw = ΨP + ΨS
Ψw = 0 MPa + ( MPa)
Yw = MPa
0.10 m Sucrose
Slide 23
When we plug these values into our equation and solve, we find that the water potential of the 0.10 molal solution of sucrose is negative MegaPascals.

18. Yw (Pure H20) = 0 MPa Yw (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.
Yw (Pure H20) = 0 MPa
Yw (0.10 m Sucrose) = MPa
Slide 24
So, by adding the solute sucrose to pure water we have lowered the water potential of that pure water.

19. Methods and Instruments
YW = ΨP - ΨS
Constant Volume Method
Pressure Chamber
Cryoscopic Osmometer
Psychrometer
Slide 25
Keep in mind what we have just reviewed as we look at some of the methods and instruments plant scientists use to estimate water potential.
We will examine the following methods and instruments:
The constant volume method,
the pressure chamber,
the Cryoscopic Osmometer,
and the Psychrometer.

20. 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.
Slide 26
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.

21. 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
Slide 26
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.

22. Yw = - 0.367 MPa Yw = - 0.489 MPa Yw = - 0.612 MPa
Slide 27
We begin by preparing solutions with increasing molarity. Five milliliters of each solution is placed in a labeled test tube. Here, we are using polyethylene glycol, abbreviated as PEG, as our solute. PEG is a large hydrocarbon that does not move across plant membranes.
0.15 m PEG m PEG m PEG
Yw = MPa Yw = MPa Yw = 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.

23. Next, we prepare our plant tissue
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.

24.  Leaf disc in each test tube
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

25. After one hour, the leaf disks are individually removed from the solutions, blotted dry and reweighed.

26. Yw(soln) Initial Final +/- Wgt
MPa g g g
MPa g g g
MPa g g g
Slide 31
The final weights are recorded on the same chart with the initial weights and the difference between the initial and final weighs is calculated.
The final weights are recorded on the same chart with the initial weights and the difference between the initial and final weights is calculated.

27. Yw(soln) Initial Final +/- Wgt
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 Final +/- Wgt
MPa g g g
Slide 32
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(leaf) < Yw(soln)
So, water moved down a water potential gradient.

28. 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 MPa because water moved from the solution to the leaf disk down a water potential gradient.
Slide 33
So, water moved down a water potential gradient. It is helpful to remember the number line when you are working with water potential gradients. Water moves from areas with high water potentials, less negative, to areas with lower water potentials, more negative.
At this point, we know that the water potential of the leaf disk is a more negative number than negative MegaPascals.
MPa 0 MPa
(-)
(+)
Yw(leaf) < Yw(soln)
Yw(soln) Pure Water

29. Yw(leaf) > Yw(soln) Yw(soln) Initial Final +/- Wgt
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 MPa.
Yw(leaf) > Yw(soln)
Yw(soln) Initial Final +/- Wgt
MPa g g g
Slide 34
If the leaf disk lost weight, then water moved from the leaf disk down a water potential gradient into the solution.

30. We now know that the water potential of our leaf disk is somewhere between - 0.367 and - 0.612 MPa.
Yw(soln) Pure H2O
MPa MPa
Yw(leaf) > Yw(soln) Yw(leaf) < Yw(soln)
Yw(soln)
MPa
(+)
(-)
Yw(leaf)
Slide 35
We now know that the water potential of our leaf disk is somewhere between negative and negative MegaPascals.

31. Yw(soln) Initial Final +/- Wgt
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(leaf)
Yw(soln)
Yw(soln) Initial Final +/- Wgt
MPa g g g
Slide 36
In theory, it is the leaf with no net weight gain that gives us our estimate of the water potential of the plant tissue.
In reality, when this method is used you very rarely see leaf disks with no net weight gained.
Water is in equilibrium

32. Yw(soln) Initial Final +/- Wgt
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) Initial Final +/- Wgt
MPa g g g
MPa g g g
MPa g g g
MPa g g g
MPa g g g
MPa g g g
Slide 37
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 the points where the leaf disk quit gaining weight and started loosing weight. An estimate of the water potential of the plant tissue can be made by averaging the two water potentials of the solutions or linearly interpreted.

33. 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.
Slide 38
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.

34. Constant Volume Method - Summary
Simple method
Requires minimal equipment
Low resolution results
Slide 38
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.

35. 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:
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.
Slide 39
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 pressure, or tension, in a plants xylem sap. When using this method we make two assumptions.
First, since there are few dissolved solutes in the xylem sap, the osmotic pressure is assumed to be zero. Secondly, since the xylem is in intimate contact with the majority of cells in the entire plant, only two to three cells separate vascular bundles, measuring the positive pressure required to reverse the flow of xylem sap will give us a good estimate of the water potential of the plant.

36. 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
Slide 39
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 pressure, or tension, in a plants xylem sap. When using this method we make two assumptions.
First, since there are few dissolved solutes in the xylem sap, the osmotic pressure is assumed to be zero. Secondly, since the xylem is in intimate contact with the majority of cells in the entire plant, only two to three cells separate vascular bundles, measuring the positive pressure required to reverse the flow of xylem sap will give us a good estimate of the water potential of the plant.

37. 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.
Transpiration
Pull
Xylem Sap
Negative Hydrostatic
Pressure or Tension
(-ΨP)
Slide 40
The water column in the xylem is under tension or negative hydrostatic pressure because transpiration pull is drawing water through the plant from the soil to the atmosphere.

38. Positive Pressure Applied, Xylem sap exudes from cut surface
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). P
Excised Leaf
H2O column in xylem recedes (red)
Slide 41
When a stem is cut, the water column recedes away from the cut surface. The pressure chamber applies positive pressure to bring the xylem sap back to the cut surface.
Positive Pressure Applied,
Xylem sap exudes from cut surface

39. Positive Pressure Needed to Reverse the Xylem Sap Flow
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).

40. 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.

41. 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.

42. occurs, the value on the pressure gauge is read.
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)

43. YP(air) + YP(xylem) = 0 10 Bars = 1 MPa 45 Bars = 4.5 MPa
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
Slide 46
In order to estimate the water potential, we must first convert the positive pressure from bars into MegaPascals. Ten bars is equal to one MegaPascal, so 50 bars equals 5 Mega Pascals. We now plug our hydrostatic pressure value into the equation and solve. The estimated water potential is negative five MegaPascals. Note that a negative sign is placed in the final answer. This is because the xylem sap is under negative hydrostatic pressure.

44. Yair + Yxylem = 0 4.5 MPa + Yxylem = 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) = MPa
Slide 46
In order to estimate the water potential, we must first convert the positive pressure from bars into MegaPascals. Ten bars is equal to one MegaPascal, so 50 bars equals 5 Mega Pascals. We now plug our hydrostatic pressure value into the equation and solve. The estimated water potential is negative five MegaPascals. Note that a negative sign is placed in the final answer. This is because the xylem sap is under negative hydrostatic pressure.

45. 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.
Slide 47
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 the equipment is heavy and bulky. 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.

46. 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
Slide 47
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 the equipment is heavy and bulky. 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.

47. 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.
Slide 48
The Cryoscopic Osmometer estimates the water potential of plant tissue by estimating the osmotic pressure in a plants cell 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 pressure is zero after the cell membranes are damaged from freezing.

48. 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)
Slide 48
The Cryoscopic Osmometer estimates the water potential of plant tissue by estimating the osmotic pressure in a plants cell 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 pressure is zero after the cell membranes are damaged from freezing.

49. Temperature Monitoring
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
Slide 49
The Cryoscopic Osmometer apparatus consists of a thermal-controlled stage with a temperature monitoring device attached to a microscope. The thermal stage contains depressions. A drop of plant cell sap is suspended in oil in a depression to prevent evaporation. The temperature of the thermal stage is rapidly lowered. By looking through the microscope it can be determined when the cell sap freezes. The temperature of the stage 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 osmotic pressure of the cell sap is then calculated using the freezing point depression.
Temperature Monitoring
Device

50. Temperature Monitoring
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.
Oil
Cell Sap
Thermal Stage
Slide 49
The Cryoscopic Osmometer apparatus consists of a thermal-controlled stage with a temperature monitoring device attached to a microscope. The thermal stage contains depressions. A drop of plant cell sap is suspended in oil in a depression to prevent evaporation. The temperature of the thermal stage is rapidly lowered. By looking through the microscope it can be determined when the cell sap freezes. The temperature of the stage 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 osmotic pressure of the cell sap is then calculated using the freezing point depression.
Temperature Monitoring
Device

51. 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.
Slide 50
The Cryoscopic Osmometer is an expensive piece of equipment that can only be operated by trained technicians under laboratory conditions. Only rough estimates of water potential can be obtained with this method because the presence of antifreeze compounds in plant cells can affect the freezing point depression estimates.

52. Cryoscopic Osmometer - Summary
Expensive instrument
Trained technicians operate under laboratory conditions
Anti-freeze compounds in plant cells may affect the estimate of YS
Slide 50
The Cryoscopic Osmometer is an expensive piece of equipment that can only be operated by trained technicians under laboratory conditions. Only rough estimates of water potential can be obtained with this method because the presence of antifreeze compounds in plant cells can affect the freezing point depression estimates.

53. Psychrometer Estimates YW by measuring the change
“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)
Slide 51
“Psychro” is from the Greek word for “to cool”. The Psychrometer estimates water potential by measuring the change of temperature due to evaporation.

54. Temperature Gauge and Controls
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
Slide 52
The Psychrometer apparatus 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 placed int he bottom of the chamber. The chamber is sealed and 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.
Temperature Gauge
and Controls
Plant
Tissue

55. Temperature Gauge and Controls
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.
Thermocouple with drop of solution
Temperature Gauge
and Controls
Plant
Tissue
Here, the water is moving from the drop to the tissue, down water potential gradient. This evaporation cools the thermocouple.
Slide 52
The Psychrometer apparatus 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 placed int he bottom of the chamber. The chamber is sealed and 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.

56. 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.
Thermocouple with drop of solution
Temperature Gauge
and Controls
Plant
Tissue
Here, the water is moving from the tissue to the drop, down water potential gradient. This condensation warms the thermocouple.
Slide 52
The Psychrometer apparatus 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 placed int he bottom of the chamber. The chamber is sealed and 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.

57. 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.
Thermocouple with drop of solution
Temperature Gauge
and Controls
Plant
Tissue
Here, the water is in equilibrium between the tissue to the drop. Thus, there is no change in temperature.
Slide 52
The Psychrometer apparatus 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 placed int he bottom of the chamber. The chamber is sealed and 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.

58. Ytissue Ysoln Ysol < Ytissue (Temp ) T Ysoln > Ytissue (Temp )
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
Ysoln on thermocouple (MPa)
Ysol < Ytissue (Temp )
Ysoln > Ytissue (Temp )
Ytissue
Ysoln
Slide 53
If the plant tissue has a higher water potential than the drop of solution, the thermocouple will register a rise in temperature. If the drop of solution has a higher water potential than the plant tissue, the thermocouple will register a drop in temperature. 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.

59. 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.
Slide 54
The Psychrometer can be used to estimate the water potentials of excised and intact plant tissue and solutions. The equipment is very sensitive to temperature fluctuations and must be operated under controlled constant conditions in the laboratory.

60. Psychrometer - Summary
Estimates YW of excised and intact plant
tissue and solutions
Equipment is sensitive to temperature
fluctuations
Controlled laboratory conditions
Slide 54
The Psychrometer can be used to estimate the water potentials of excised and intact plant tissue and solutions. The equipment is very sensitive to temperature fluctuations and must be operated under controlled constant conditions in the laboratory.

61. 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.
Slide 54
The Psychrometer can be used to estimate the water potentials of excised and intact plant tissue and solutions. The equipment is very sensitive to temperature fluctuations and must be operated under controlled constant conditions in the laboratory.

62. References
Nobel, P. S Physicochemical and Environmental Plant Physiology. Academic Press, Inc., San Diego, CA pp.
Salisbury, F. B. and C. W. Ross Plant Physiology. 4th Edition. Wadsworth Publishing Co., Belmont, CA pp.
Taiz, L. and E. Zeiger Plant Physiology. 3rd Edition. Sinauer Associates, Inc., Sunderland, MA pp.
Slide 1
Water potential in plants was produced at New Mexico State University by the Department of Entomology, Plant Pathology and Weed Science. This slide set is divided into two parts.

63. Water Potential in Plants © 2004 New Mexico State University
Joyce Payne Bowers
Tracy M. Sterling
Department of Entomology, Plant Pathology,
and Weed Science
© 2004 New Mexico State University
Slide 1
Water potential in plants was produced at New Mexico State University by the Department of Entomology, Plant Pathology and Weed Science. This slide set is divided into two parts.