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Chapters 34 & 35
Lecture 9
Transport in Plants: Xylem
Dr. Alan McElligott
XYLEM
Aims:
• To introduce the structure of the xylem
• To study the transport of water and
minerals in the xylem
• To introduce transpiration and stomata
XYLEM
Aims:
• To introduce the structure of the xylem
• To study the transport of water and minerals in the xylem
• To introduce transpiration and stomata
These lecture aims form part of the knowledge
required for learning outcome 3:
Describe mechanisms for the life processes
(LOC3)
XYLEM
• 34.2 How Are Plant Cells Unique?
(pages 750-752)
Essential reading
• pages 750-752
• pages 769-774
Figure 34.6 Three Tissue Systems Extend Throughout the Plant Body
34.2 How Are Plant Cells Unique?
Xylem contains cells called tracheary elements—
die before assuming their function.
Gymnosperms have tracheids with pits in the
secondary walls that allow materials to move freely.
34.2 How Are Plant Cells Unique?
Angiosperms have vessel elements with pits. Larger
diameter than tracheids; lignin in secondary cell
walls; end walls break down after death, forming
hollow tubes.
Xylem of many angiosperms also contains tracheids.
Figure 34.9 Plant Cell Types
35 Transport in Plants
• 35.2 How Are Water and Minerals
Transported in the Xylem?
• 35.3 How Do Stomata Control the Loss of
Water and the Uptake of CO2?
Figure 35.1 The Pathways of Water and Solutes in the Plant
35.2 How Are Water and Minerals Transported in the Xylem?
Xylem transport: a maple tree in summer loses 220
liters of water per hour. Xylem must transport 220
litres per hour to prevent wilting.
The tallest trees exceed 110 meters. Xylem must
transport water to great heights.
Several models for xylem transport have been
proposed.
35.2 How Are Water and Minerals Transported in the Xylem?
First proposal was pumping action by
living cells.
Ruled out in 1893 by an experiment in
which cut trees were placed in a poison
solution. Solution rose through trunk to
leaves (which died), then stopped rising.
35.2 How Are Water and Minerals Transported in the Xylem?
Experiment established three points:
• Living, pumping cells were not involved.
• Leaves were crucial: solution continued
to rise until leaves were dead.
• Movement was not caused by the roots.
35.2 How Are Water and Minerals Transported in the Xylem?
Some hypothesized that xylem transport
is based on root pressure.
Higher solute concentration and more
negative water potential in roots than in
soil solution; water enters stele and
from there has no where to go but up.
35.2 How Are Water and Minerals Transported in the Xylem?
Guttation is evidence of root pressure; water is
forced out through openings in leaves.
Root pressure also causes sap to ooze from cut
stumps. But it cannot account for ascent of sap in
trees.
35.2 How Are Water and Minerals Transported in the Xylem?
If root pressure was pushing sap up the
xylem, there would be a positive
pressure potential in the xylem at all
times.
But xylem sap in most trees is under
tension; negative pressure potential.
35.2 How Are Water and Minerals Transported in the Xylem?
An alternative to pushing is pulling.
Leaves pull the xylem sap upwards.
Evaporative water loss from the leaves
creates a pulling force or tension on the
water in the apoplast of leaves.
Hydrogen bonding between water
molecules makes sap cohesive enough
to withstand the tension and rise by bulk
flow.
35.2 How Are Water and Minerals Transported in the Xylem?
Concentration of water vapour in the
atmosphere is lower than in the leaf.
Water vapour diffuses from leaf through
the stomata: transpiration.
Within the leaf, water evaporates from
walls of mesophyll cells, film of water on
cells shrinks, creating more surface
tension (negative pressure potential).
Draws more water into cell walls to
replace what was lost.
35.2 How Are Water and Minerals Transported in the Xylem?
Resulting tension in mesophyll cells
draws water from nearest vein.
Removal of water from veins results in
tension on the entire column of water in
the xylem, so that water is drawn up.
35.2 How Are Water and Minerals Transported in the Xylem?
Ability of water to be drawn up through
tiny tubes is due to cohesion: water
molecules stick together because of
hydrogen-bonding.
The narrower the tube, the greater the
tension the water column can withstand.
Water also adheres to the xylem walls.
35.2 How Are Water and Minerals Transported in the Xylem?
This transpiration–cohesion–tension
mechanism requires no energy from
the plant. Water moves passively
toward a region of more negative water
potential.
Dry air has the most negative water
potential, and soil solution has the least.
Mineral ions in xylem sap rise passively
with the water.
Figure 35.6 The Transpiration–Cohesion–Tension Mechanism
35.2 How Are Water and Minerals Transported in the Xylem?
Transpiration also helps cool plants. As
water evaporates from mesophyll cells,
heat is taken up from the cells, and leaf
temperature drops.
Important for plants in hot environments.
35.2 How Are Water and Minerals Transported in the Xylem?
A demonstration of the negative pressure
potential, or tension, in xylem sap was
done by measuring tension with a
pressure chamber.
Also determined that tension
disappeared at night in some plants,
when transpiration stopped.
Figure 35.7 A Pressure Chamber
35.2 How Are Water and Minerals Transported in the Xylem?
Xylem sap does not rise at night, when
there is no transpiration.
During the day, rate of ascension
depends on temperature, light intensity,
and wind velocity, which all affect
transpiration.
Rate of flow may also depend on
concentration of K+—seems to change
size of pits.
Figure 35.8 Potassium Ions Speed Transport in the Xylem (Part 1)
Figure 35.8 Potassium Ions Speed Transport in the Xylem (Part 2)
Figure 35.8 Potassium Ions Speed Transport in the Xylem
35.3 How Do Stomata Control the Loss of Water
and the Uptake of CO2?
Leaf and stem epidermis has a waxy cuticle to
minimize water loss, but it also prevents gas
exchange.
Stomata, or pores in the leaf epidermis, allow CO2 to
enter by diffusion. Guard cells control opening and
closing.
35.3 How Do Stomata Control the Loss of Water
and the Uptake of CO2?
Most plants open stomata when light
intensity is enough for moderate rate of
photosynthesis.
At night, stomata remained closed; CO2
not needed, and no water is lost.
During the day stomata close if water is
being lost too rapidly.
35.3 How Do Stomata Control the Loss of Water
and the Uptake of CO2?
Cues for stomatal opening include light,
and concentration of CO2 in intercellular
spaces in the leaf: low levels favor
opening of stomata.
If plant is under water stress or water
potential of mesophyll cells is too
negative, they release the hormone
abscisic acid—acts on guard cells and
causes them to close.
35.3 How Do Stomata Control the Loss of Water
and the Uptake of CO2?
Opening and closing of stomata is
controlled by K+ in the guard cells.
Blue light is absorbed by pigments in
guard cells and activates a proton
pump. Resulting gradient drives K+ into
guard cell, making its water potential
more negative. Water enters cell by
osmosis. Increased pressure potential
causes guard cells to change shape,
and a gap appears between them.
Figure 35.9 Stomata
35.3 How Do Stomata Control the Loss of Water
and the Uptake of CO2?
The process is reversed when active transport
of protons stops. K+ diffuse out of guard cells
passively, and water follows by osmosis.
Pressure potential decreases, and cells sag,
closing the gap between them.
Demonstration of how much K+ moves in and
out of guard cells was done by using an
electron probe microanalyzer.
Figure 35.10 Measuring Potassium Ion Concentration in Guard Cells (Part 1)
Figure 35.10 Measuring Potassium Ion Concentration in Guard Cells (Part 2)
35.3 How Do Stomata Control the Loss of Water
and the Uptake of CO2?
Farmers and gardeners would like to
reduce the amount of transpiration from
crops, to reduce need for irrigation.
An antitranspirant: a compound to
reduce transpiration without limiting CO2
uptake.
Abscisic acid is too expensive for largescale trials.
35.3 How Do Stomata Control the Loss of Water
and the Uptake of CO2?
Transgenic plants with a mutant allele for
the era gene are very sensitive to
abscisic acid and thus resistant to
wilting in droughts.
Some compounds form a polymer film
around leaves, and seal the stomata.
They are mostly used for transplants of
nursery stock.
Table 35.1 Mechanisms of Sap Flow in Plant Vascular Tissues
XYLEM
Check out
35.2 Recap, page 773
35.3 Recap, page 774
35.2 Chapter summary, page 778
35.3 Chapter summary, page 778
Self Quiz
Page 778-779: Chapter 35, questions 6-8
For Discussion
Page 779: Chapter 35, Question 4
XYLEM
Key terms:
anti-transpirant, abscisic acid (ABA), cohesion, guard
cell, guttation, hydrogen bonding, proton gradient,
stomata, tension, tracheary element, tracheid,
transpiration, turgor pressure, xylem, vessel
element