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. 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.

3. 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.

4. 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.

5. 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

6. 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.
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

7. YW = ΨP + ΨS Current Convention Defines Ψw as:
ΨP = pressure potential – where water wants to go based on the physical pressure being applied
ΨS = osmotic or solute potential – where water wants to go based on concentration gradients of solutes
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.

8. Yw = ΨP + ΨS Simplified Definition of Ψw:
The rule is water will move from High overall water potential to LOW overall water potential.
So… figure out where water would go based on pressure alone, figure out where water would go based on solute concentration alone, add those two together and you have your answer!
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.

9. Yw = ΨP + ΨS Simplified Definition of Ψw:
Do not get hung up on numbers or signs. The pos/neg signs are more accurately “higher” “lower” water potential indicators.
The numbers will usually be provided.
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.

10. 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.

11. 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 )

12. 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

13. 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.

14. 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.

15. 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

16. 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).

17. 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.

18. 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.

19. 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.