1. Plant Water Relations Driving Force Process Osmosis, etc. DiffusionDY
Diffusion
Dcwv
Bulk Flow DP

2. Water Dipole, Hydrogen bonding, Adhesion, Cohesion,
high Specific Heat,
high Latent Heat of Vaporization,
etc.

3. Movement of water in plants
Bulk flow
Movement of water in response to a pressure gradient
Analogous to water flowing in a pipe
Affected by:
Radius of pipe (r)
Viscosity of liquid (h)
Pressure gradient

4. Mechanisms of Transport in organisms
Diffusion, by molecular motion,
good only at short distances,
Osmosis (water across membranes),
Bulk Flow,
efficient large-scale, mass movement.

5. Osmosis and Water Potential (Y) Free Energy Status of Water in Plants
The water potential of a sample is the sum of three major component potentials: gravitational, osmotic, and pressure.
Gravitational potential (YG) depends on the position of the water in a gravitational field,
negligible at the level of the cell. Is significant in taller plants and trees.
Osmotic potential (YS) depends on the concentration of dissolved substance in the water.
Pressure potential (YP) depends on the hydrostatic pressure on the water.
+ YP
Yg +
Y = YS
+ YP
Y = YS

6. YP = pressure potential Yg = gravitational potential
The concept of water potential, Y, brings together the influences of gravity, pressure, and concentration (solutes) in describing the energy state of water and the direction of water movement.
The water potential equation:
YW = YS + YP + Yg
YW = total water potential
YS = solute potential
YP = pressure potential
Yg = gravitational potential
All units will be pressure, pascals, Pa.
MPa is megapascal, 106 Pa

7. Aquaporins
…integral membrane proteins (IPP) that form a water pore across the membrane.
Aquaporins assemble as homo- or hetero-tetramers,each monomer acting as an independent water channel. The aquaporin monomer consists of six membrane-spanning α-helices connected by five loops, with both N and C termini facing the cytosol. Two loops B and D create a pore with high specificity to water (Murata et al. 2000).
Co-ordinated control of aquaporins regulate plant cell permeability to water.

9. PP03120.jpg

10. Y = YS + Yp Y (Units)
Y is the free energy of a water sample per unit mass,
J m-3,
…expressed as units of pressure,
1 megapascal (MPa) = 10 bars, ~ 10 atmospheres, 7500 mmHg.
Standard (Y0) = pure water at ambient pressure = 0 MPa.

11. Movement of water into a plant cell occurs by osmosis
2 mechanisms:
Diffusion across the membrane
Bulk flow across aquaporins (water filled pores)

12. Movement of water into a plant cell occurs by osmosis
Water uptake is driven by a free energy gradient composed of:
Concentration gradient
Pressure gradient
Free energy gradient for water movement is referred to as a Water Potential Gradient

13. Water Potential
Water potential (Yw) is equivalent to the free energy of water & influenced by:
Concentration (or activity)
Pressure
Gravity

14. Water Potential
Chemical potential of water (in pressure units) compared to the chemical potential of pure water (at atm. pressure and temp.) which is set to zero

15. Water Potential Yw = Ys + Yp + Yg
Ys = Solute component or osmotic potential
Result of dissolved solutes that dilute water (entropy effect)
Estimated using van’t Hoff’s eqtn (see p.44)
Yp = Pressure component or pressure potential
Yp inside a cell is positive = turgor pressure
Yp in the apoplast is negative
Note: Yp of pure water is zero, therefore not a measure of absolute pressure

16. Water Potential Yg = Gravity component
Ignored unless considering vertical water movement
> 5 m
Dependent on:
Height of water above ref. state (h)
density (rw)
acceleration due to gravity (g)
g = 0.01MPa m-1

17. Typical values for Yw Yw = -0.2 to -0.6 MPa Ys = -0.5 to -1.5 MPa
Yp = 0.1 to 1.0 MPa
Positive values needed to drive growth and provide mechanical rigidity

18. Water equilibration method

19. Measuring Yw Scholander’s pressure equilibration method
A leaf or shoot is excised and placed in the chamber
Cutting the leaf breaks the tension in the xylem causing water to retreat into the surrounding cells
Pressurizing the leaf chamber returns water to the cut surface of the petiole
The amount of pressure to return water to the cut surface equals the tension (Yp) present in the xylem (but is opposite in sign) before excision
Values obtained approximate the tension in the xylem and are used as a measure of Yw
Strictly speaking to know the actual Yw some xylem sap should be collected to measure Ys

20. Change in water status causes physiological changes
PP03120.jpg
Measure of the rate of passage of CO2 entering or water vapors exiting through stomata