Good for undergraduates

1. WATER TRANSPORT/TRANSLOCATION
• Here we shall Learn about:
• Water absorption from surrounding soils to the xylem vessels
• Water ascent up in plants
Dr. Omony JB

2. Water transport in plants:
• The same way we drink soda from a straw!
• Water’s great cohesive forces (molecules
sticking to each other)
and adhesive forces (attaching to walls of xylem
cells)

3. Transpiration-cohesion Theory
for water transport in the xylem
• Evaporation of water in the leaves (through
stomates) generates the ‘sucking force’ that pulls
adjacent water molecules up the leaf surface

4. Water transport (cont’d…)
• Like a long chain, water molecules pull each other up
the column.
• The column goes from roots to leaves.
• What’s amazing is that the water moves up by using the
sun’s evaporative energy…
• Plants control transpiration by opening/closing stomata.

5. Water Moves through the Soil by Bulk Flow
• Water moves through soils predominantly by bulk flow
driven by a pressure gradient, although diffusion also
accounts for some water movement.
• As a plant absorbs water from the soil, it depletes the
of water near the surface of the roots.
Water absorption by the root

6. Root tip—the water absorption zone

7. • Physical forces drive the transport of materials in plants over a range
of distances
• Transport in vascular plants occurs on three scales
– Transport of water and solutes by individual cells, such as root hairs
– Short-distance transport of substances from cell to cell at the levels of
tissues and organs
– Long-distance transport within xylem and phloem at the level of the
whole plant

8. The overall scheme of water movement through the plant
1- From soil to root epidermis
– Diffusion to the intercellular space
• Capillary movement of soil water to plant roots. Plant root
removes water. Tension in the soil right around the root increases
gradient flow of water from low tension to high. This keeps a
source of capillary water flowing to the plant root.
– Osmosis to the epidermis cells

9. 1 Apoplast pathway: water moves exclusively through the cell wall
without crossing any membranes. (The apoplast is the continuous
system of cell walls and intercellular air spaces in plant tissues.)
2 Symplast pathway: water moves through the symplast, traveling from
one cell to the next via the plasmodesmata (The symplast consists of
entire network of cell cytoplasm interconnected by plasmodesmata.)
3 Transmembrane pathway: water sequentially enters a cell on one side,
exits the cell on the other side. In this pathway, water crosses at least
membranes for each cell in its path.
Symplast pathway and transmembrane pathway are two
components of cellular pathway,
2- From epidermis to and through cortex

12. • Lateral transport of minerals and water in roots
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.
Pathway along
apoplast
Pathway
through
symplast
Plasma
membrane
Apoplastic
route
Symplastic
route
Root hair
Epidermis Cortex Endodermis 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.
Vessels
(xylem)
Casparian strip
Casparian strip
Endodermal cell
4
5
2
1

13. Transversing endodermis
• Casparian strip?
– Casparian strip is a band of cell wall material
deposited on the radial and transverse walls
of the endodermis, which is
chemically different from the rest of the cell
wall. It is used to block the passive flow of
materials, such as water and solutes into the
stele of a plant.
– T
o transverse casparian strip, apoplast
pathway does not work (blocked), only
cellular pathway works
Stele is the central part of the root or stem containing the tissues derived from the
procambium. These include vascular tissue, in some cases ground tissue (pith) and a
pericycle, which, if present, defines the outermost boundary of the stele. Outside the stele
lies the endodermis.

14. 3 From endodermis to root vessel
apoplast pathway and cellular pathway (diffusion or
osmosis).
4 From root vessel to stem vessel to leaf vessel apoplast
pathway (mass flow).
5 From leaf vessel → leaf mesophylls and
intercellular space →stomatal cavity → stomata →air
(diffusion or osmosis).

15. ASCENT OF WATER UP IN A TALL PLANT

16. H2O
Minerals
CO2
O2
CO2 O2
H2O Sugar
Light
The ascent of xylem sap: Rises to heights of more than 100m
in the tallest plants
• A variety of physical processes are involved in the different
types of transport
Sugars are produced by
photosynthesis in the leaves.
5
Sugars are transported as
phloem sap to roots and other
parts of the plant.
6
Through stomata, leaves
take in CO2 and expel O2.
The CO2 provides carbon for
photosynthesis. Some O2
produced by photosynthesis
is used in cellular respiration.
4
3 Transpiration, the loss of water
from leaves (mostly through
stomata), creates a force within
leaves that pulls xylem sap upward.
Water and minerals are
transported upward from
roots to shoots as xylem sap.
2
Roots absorb water
and dissolved minerals
from the soil.
1 Roots exchange gases with
the air spaces of soil, taking in
O2 and discharging CO2. In
cellular respiration, O2 supports
the breakdown of sugars.
7

17. Factors Affecting the Ascent of Xylem Sap
Xylem sap Rises to heights of more than 100 m in the tallest plants
1. Pushing Xylem Sap: Root Pressure
• At night, when transpiration is very low
– Root cells continue pumping mineral ions into the
xylem of the vascular cylinder, lowering the water
potential
• Water flows in from the root cortex
– Generating root pressure.

18. Root pressure sometimes results in guttation, the
exudation of water droplets on tips of grass blades
or the leaf margins of some small, herbaceous
eudicots

19. 2. Pulling Xylem Sap: The Transpiration-
Cohesion-Tension Mechanism
• Water is pulled upward by negative pressure in
the xylemcalled capillary force

20. 3. Transpirational Pull
• Water vapor in the airspaces of a leaf
– Diffuses down its water potential gradient and exits
the leaf via stomata.

21. • Transpiration produces negative pressure (tension) in the leaf
– Which exerts a pulling force on water in the xylem, pulling
water into the leaf Evaporation causes the air-water interface to retreat farther into the
cell wall and become more curved as the rate of transpiration
increases. As the interface becomes more curved, the water film’s
pressure becomes more negative. This negative pressure, or
tension, pulls water from the xylem, where the pressure is greater.
Cuticle
Upper
epidermis
Mesophyll
Lower
epidermis
Cuticle
Water vapor
CO2 O2 Xylem CO2 O2
Water vapor
Stoma
Evaporation
At first, the water vapor lost by
transpiration is replaced by evaporation
from the water film that coats mesophyll
cells.
In transpiration, water vapor (shown as blue
dots) diffuses from the moist air spaces of the
leaf to the drier air outside via stomata.
Airspace
Cytoplasm
Cell wall
Evaporation
Vacuole
Water film
Low rate of
transpiration
Cell wall
Airspace
 = –0.15 MPa  = –10.00 MPa
Air-water
interface
High rate of
transpiration
3
1 2
Air-
space

22. Cohesion and Adhesion in the Ascent of Xylem
Sap
• The transpirational pull on xylem sap
– Is transmitted all the way from the leaves to the
root tips and even into the soil solution
– Is facilitated by cohesion and adhesion

23. • Ascent of xylem sap
Outside air  = –100.0
MPa
Leaf  (air spaces)
= –7.0 MPa
Leaf  (cell walls) = –
1.0 MPa
Trunk xylem  = – 0.8
MPa
Water
potential
gradient
Root xylem = – 0.6
MPa
Soil  = – 0.3
MPa
Xylem
sap
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 soil

24. Summery of the Driving Forces of Water
absorption and movement in the xylem sap
1 Root Pressure
2 Transpiration pull

25. 1- Root Pressure
• Solute Accumulation in the Xylem Generates “Root Pressure”
• The root absorbs ions from the dilute soil solution and
transports them into the xylem. The buildup of solutes in the
xylem sap leads to a decrease in the xylem osmotic potential
(Ψs) and thus a decrease in the xylem water potential (Ψw).
lowering of the xylem Ψw provides a driving force for water
absorption.

26. Guttation
Appearance of xylem sap drops on
the tips or edges of leaves e.g.
grasses
•Sugars, mineral nutrients and
potassium
Dew?
•Transpiration stops at night time due to stomata closing
• High soil moisture level
• Lower root water potential
•Accumulation of water in plants
•Plants will start bleeding through leaf tips and edges

27. 2-Transpiration Pull
Transpiration-
cohesion theory
Transpiration is the loss of water through the
stomata in leaves. This loss of water causes an
area of low pressure within the plant and
water moves from where it is at high pressure
to low pressure. The cohesion part is what
allows water to do this against gravity.

28. • Stomata help regulate the rate of transpiration
• Leaves generally have:
– Broad surface areas
– High surface-to-volume ratios
Both of these characteristics
Increase photosynthesis
Increase water loss through stomata
20 µm

29. Effects of Transpiration on Wilting and Leaf Temperature
• Plants lose a large amount of water by
transpiration
• If the lost water is not replaced by absorption
through the roots
– The plant will lose water and wilt
Stomata: Major Pathways for Water Loss

30. • Transpiration also results in evaporative cooling
– Which can lower the temperature of a leaf and prevent the
denaturation of various enzymes involved in photosynthesis and
other metabolic processes
• About 90% of the water a plant loses
– Escapes through stomata

31. • Each stoma is flanked by guard cells
– Which control the diameter of the stoma by changing
shape
Cells flaccid/Stoma closed
Cells turgid/Stoma open
Radially oriented
cellulose microfibrils
Cell
wall
Vacuole Guard cell
(a) Changes in guard cell shape and stomatal opening
and closing (surface view). Guard cells of a typical angiosperm
are illustrated in their turgid (stoma open) and flaccid (stoma
closed) states. The pair of guard cells buckle outward when
turgid. Cellulose microfibrils in the walls resist stretching and
compression in the direction parallel to the microfibrils. Thus, the
radial orientation of the microfibrils causes the cells to increase
in length more than width when turgor increases. The two guard
cells are attached at their tips, so the increase in length causes
buckling.

32. • Changes in turgor pressure that open and close
stomata
– Result primarily from the reversible uptake and loss of
potassium ions by the guard cells
H2O
H2O
H2O
H2O
2
H O
K+
Role of potassium in stomatal opening
and closing.
The transport of K+ (potassium ions,
symbolized here as red dots) across the
plasma membrane and vacuolar membrane
causes the turgor changes of guard cells.
(b)
H2O H2O
H2O
H2O
H2O

33. Xerophyte Adaptations That Reduce Transpiration
• Xerophytes
– Are plants adapted to arid climates
– Have various leaf modifications that reduce the rate of
transpiration

34. • The stomata of xerophytes
– Are concentrated on the lower leaf surface
– Are often located in depressions that shelter the pores
from the dry wind.
Lower epidermal
tissue
Cuticle Upper epidermal tissue
Trichomes Stomata
(“hairs”)
100 m

35. Summary of the plant water relationships
Dissolving sucrose in the water to a concentration of 0.1 M:
Lowers the osmotic potential to –0.244 Mpa (fig. B)
Decreases water potential to –0.244 Mpa
If Cell is flaccid, the internal pressure is the same as ambient pressure, so the
hydrostatic pressure is 0 MPa
If this cell is placed in the beaker containing 0.1 M sucros
the water potential of the sucrose solution is greater than the water potential of
the cell water will move from the sucrose solution to the cell (from
high to low water potential).
A slight increase in cell volume causes a large increase in the hydrostatic pressure within the
cell.

36. • Turgor loss in plants causes wilting
– Which can be reversed when the plant is watered