This document discusses plant water relations and transport. It covers several topics:
1. It describes how turgor pressure in plant cells is sensitive to small changes in cell volume due to the rigid cell wall. As cell volume decreases, turgor pressure and water potential decrease sharply at first and then more gradually.
2. It explains how water and minerals are transported through the root system, entering through root hairs, moving through the apoplast and symplast of root cells, and being restricted by the casparian strip in endodermal cells before entering the xylem vessels for transport upward.
3. It discusses the mechanisms of cohesion and adhesion that allow for the ascent of water through
10. BISC 367
Yp is sensitive to small changes in
cell volume
• Relates to rigid cell wall, illustrated by
Hofler diagram
– Plot of Yw & its components
against relative cell vol.
• Initial drop in cell vol (5%) is
accompanied by a sharp drop in Yp
and Yw
• As cell vol falls <90%, decreased Yw is
accounted for by a lowered Ys as
[solute] increases
11. BISC 367
Yp is sensitive to small changes in
cell volume
• Slope of Yp curve yields the
volumetric elastic modulus (e)
– e is a function of the rigidity of
the cell wall
– High value indicates a rigid wall
for which a small vol. change
translates into a large drop in Yp
– e decreases as Yp falls b/c walls
are rigid only when Yp is high
12.
13.
14.
15. • Lateral transport of minerals and water in roots
Figure 36.9
1
2
3
Uptake of soil solution by the
hydrophilic walls of root hairs
provides access to the apoplast.
Water and minerals can then
soak into the cortex along
this matrix of walls.
Minerals and water that cross
the plasma membranes of root
hairs enter the symplast.
As soil solution moves along
the apoplast, some water and
minerals are transported into
the protoplasts of cells of the
epidermis and cortex and then
move inward via the symplast.
Within the transverse and radial walls of each endodermal cell is the
Casparian strip, a belt of waxy material (purple band) that blocks the
passage of water and dissolved minerals. Only minerals already in
the symplast or entering that pathway by crossing the plasma
membrane of an endodermal cell can detour around the Casparian
strip and pass into the vascular cylinder.
Endodermal cells and also parenchyma cells within the
vascular cylinder discharge water and minerals into their
walls (apoplast). The xylem vessels transport the water
and minerals upward into the shoot system.
Casparian strip
Pathway along
apoplast
Pathway
through
symplast
Plasma
membrane
Apoplastic
route
Symplastic
route
Root
hair
Epidermis Cortex Endodermis Vascular cylinder
Vessels
(xylem)
Casparian strip
Endodermal cell
4 5
2
1
27. Fig. 36-15
Outside air ψ
= −100.0 Mpa
Leaf ψ (air spaces)
= −7.0 Mpa
Leaf ψ (cell walls)
= −1.0 Mpa
Trunk xylem ψ
= −0.8 Mpa
Trunk xylem ψ
= −0.6 Mpa
Soil ψ
= −0.3 Mpa
Xylem
sap
Mesophyll
cells
StomaStoma
Water
molecule
Transpiration
Atmosphere
Adhesion
by hydrogen
bonding Cell
wall
Xylem
cells
Cohesion and
adhesion in
the xylem
Cohesion
by hydrogen
bonding
Water
molecule
Root
hair
Soil
particle
Water
Water uptake
from soil
Waterpotentialgradient
28. • Ascent of xylem sap
Xylem
sapOutside air Y
= –100.0 MPa
Leaf Y (air spaces)
= –7.0 MPa
Leaf Y (cell walls)
= –1.0 MPa
Trunk xylem Y
= – 0.8 MPa
Waterpotentialgradient
Root xylem Y
= – 0.6 MPa
Soil Y
= – 0.3 MPa
Mesophyll
cells
Stoma
Water
molecule
Atmosphere
Transpiration
Xylem
cells Adhesion Cell
wall
Cohesion,
by
hydrogen
bonding
Water
molecule
Root
hair
Soil
particle
Water
Cohesion
and adhesion
in the xylem
Water uptake
from soilFigure 36.13
29. BISC 367
Measuring Yw 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
Scholander’s pressure bomb
From Plant Physiology on-line (http://4e.plantphys.net/)
30.
31.
32.
33.
34.
35. Fig. 10.10
Phloem solution moves along a gradient of pressure
generated by a solute concentration difference
between source and sink ends of the pathway
42. BISC 367
Movement of water into a plant cell
occurs by osmosis
• 2 mechanisms:
– Diffusion across
the membrane
– Bulk flow across
aquaporins (water
filled pores)