Download Bio426Lecture6Feb1

How do changes in the components of Yw affect
each other and the total value of Yw?
Turgor of living cells changes depending on solute
(.1M, or -0.244
MPa)
concentration and total water
potential.
cell
cell
solution
Reducing cell volume concentrates solutes
and reduces YS.
Water potential gradient tells us which direction
water will move, but how do we understand how rapidly
water moves?
Several concepts for understanding speed of water flow
1. Diffusion rates
2. Bulk flow
3. Ideal flow through tubes: Hagen-Poiseuille equation
4. Hydraulic conductivity
1. Back to diffusion - net movement of molecules from
regions of higher concentration to lower concentration.
Diffusive flux = diffusion coefficient x concentration
gradient
J = -Ds DCs/Dx
Fick’s first law
J is flux rate, moles per m2
DCs/Dx is the concentration gradient, moles m-3/m
Ds is the diffusion coefficient, m2 s-1
Values of D depend on the type of molecule
and the medium
Larger, heavier molecules have lower D.
D values are higher in air than water
DCO2 in air = 1.51 x 10-5 m2 s-1
DO2 in air = 1.95 x 10-5 m2 s-1
DH2O in air = 2.42 x 10-5 m2 s-1
How effective is diffusion for transport
• across membranes?
•from roots to leaves?
Diffusion time = L2/Ds
Double the distance means 4X the time.
Compare 50µm membrane to 1 m long corn leaf
Dglu in water is 10-9 m2 s-1
(50 x 10-6m)2/10-9 m2 s-1 = 2.5 seconds!
(1 m)2/10-9 m2 s-1 = 109 seconds
About 32 years!
So how does water move long distances through plants?
Bulk flow
Bulk flow
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Let’s build an equation that
describes the various influences on
the rate of liquid moving through a
straw.
“Volume flow rate”
m3 s-1 =
Hagen - Poiseuille Equation
m3 s-1 = pr4 DYP
8h Dx
Water flow in xylem “pipes”
Pressure gradients
Diameter of tracheids or vessel elements
The viscosity of xylem fluid - does it vary?
Conductive Vessel Element in Mountain Mahogany
Wood (SEM x750). This image is copyright Dennis Kunkel at
www.DennisKunkel.com
Water movement in the soil-plant-atmosphere
continuum. Chapter 4
Water moves from higher to
lower water potential, so
Yatmos < Yleaf < Ystem < Yroot < Ysoil
Fig. 4.1
Soil water potential
Soil water adheres to soil particles of different sizes and kinds.
This adhesion represents a “tension”, or YP < 0.
In most soil solutions, solutes are dilute so YS ≈ 0.
Exceptions: saline soils, salt marshes.
YW = YS + YP + Yg
YS ≈ 0
YP < 0
Y g≈ 0
So, for most soils
YW = YP
Fig. 4.2
What determines the value of Yw (YP) of soils?
What dries out faster, a bucket of sand
or a bucket of clay?
Why?
Soils differ in characteristic particle size.
How might particle size affect soil water potential?
The more contact a volume of water has with the
soil surface, the greater the tension with which it
is held.
Water is held more tightly in small crevices.
YP = -2T/r
Where r = radius (m) of curvature of meniscus,
and
T = the surface tension of water,7.28 x 10-8 MPa
m
r1
r2
YP = -2T/r
1. As soils dry, water is held in small pore
spaces (r decreases) so soil water
potential decreases
2. Soils with smaller characteristic particle
size (e.g. clay vs. sand) tend to have lower
water potential.
3. More difficult for plants to extract
water from clay than sand
YP = -2T/r
.
YP = -2T/r
Example: calculate YP for r = 1 x 10-6 m and 1 x 10-7 m.
About -0.15MPa for 1µm, and -1.5 MPa for 0.1 µm.
Getting water from the soil into the plant.
Yroot < Ysoil
What is the pathway for
water movement into the
xylem of the roots?
Water can travel from the soil to the root xylem by two
distinct pathways - the symplastic and apoplastic pathways.
Fig. 4.3
The less-suberized growing tips of roots have higher
water uptake rates than older portions of the root.
Fig. 4.4
What is the pathway for water movement from roots
to leaves?
Water flows from roots to leaves via the xylem, a
network of specialized cells called tracheary elements.
Gymnosperms have tracheids.
Angiosperms have vessel elements & sometimes
tracheids.
Note special anatomical features.
Fig. 4.6
Xylem cavitation
Embolisms that stop water transport can form in tracheary elements
when xylem pressure is sufficiently negative to pull in air through a pit.
Fig. 4.7
May 17, 2003 North of San Francisco Peaks
September 20, 2003 North of San Francisco Peaks
PJ Woodland
Juniper Woodland
The xylem network is extremely intricate in leaves.
Fig. 4.8
OK, we’ve got water from the soil, into the roots,
and up to to the leaves.
Where does water evaporate inside leaves?
How does water at sites of evaporation have a
lower water potential than xylem “upstream”?
The wet walls of leaf cells are the sites of
evaporation.
Fig. 4.9
As for soils, a more negative YP develops as leaf cell walls
dehydrate and water is held in smaller pore spaces.
YP = -2T/r
Fig. 4.9
Putting it all together:
A model for water movement through the plant.
The Cohesion-Tension Model
The most widely accepted model of water transport
through the xylem is the “cohesion-tension model”.
(note web essay)
1. A negative pressure or tension is generated in
leaf cell walls by evaporation (transpiration).
2. The cohesive property of water means this tension
is transmitted to water in adjacent xylem and throughout
the plant to the roots and soil.