Transcript section_09_04e_transpiration

Water Transport in
Vascular Plants
Key Concepts
• Concept.1: Physical forces drive the transport of
materials in plants over a range of distances
• Concept.2: Roots absorb water and minerals from
the soil
• Concept.3: Water and minerals ascend from roots
to shoots through xylem (木質部)
• Concept.4: Stomata help regulate the rate of
transpiration (蒸散作用)
• Non-vascular plants
Vs Vascular plants
– The evolutionary
journey onto land
involved the
differentiation of the
plant body into roots
and shoots
Moss are non-vascular plants
• Vascular tissue
– Transports nutrients throughout a plant; such
transport may occur over long distances
Figure 36.1
Anatomy of a woody stem
Water-conducting cells of xylem
• Concept 1: Physical forces drive the transport of
materials in plants over a range of distances
• Transport in vascular plants occurs on two scales
–
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
• A variety of physical processes
– are involved in the different types of transport
4
CO2
5
.
O2
Light
H2O
Sugar
3
6
2
.
7
1
O2
H2O
Minerals
Figure 36.2
CO2
• A variety of physical processes
– Are involved in the different types of transport
4 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.
CO2
O2
5 Sugars are produced by
photosynthesis in the leaves.
Light
H2O
Sugar
3 Transpiration, the loss of water
from leaves (mostly through
stomata), creates a force within
leaves that pulls xylem sap upward.
6 Sugars are transported as
phloem sap to roots and other
parts of the plant.
2 Water and minerals are
transported upward from
roots to shoots as xylem sap.
1 Roots absorb water
and dissolved minerals
from the soil.
Figure 36.2
O2
H2O
Minerals
CO2
7 Roots exchange gases
with the air spaces of soil,
taking in O2 and discharging
CO2. In cellular respiration,
O2 supports the breakdown
of sugars.
Selective Permeability of Membranes: A Review
• The selective permeability of a plant cell’s
plasma membrane
– Controls the movement of solutes into and
out of the cell
• Specific transport proteins
– Enable plant cells to maintain an internal
environment different from their
surroundings
Bulk Flow in Long-Distance Transport
• In bulk / mass flow (巨流)
– Movement of fluid (sap) in the xylem and
phloem is driven by pressure differences at
opposite ends of the xylem vessels and
phloem sieve tubes
Absorption of water and minerals from the soil
• Concept 2: Roots absorb water and
minerals from the soil
• Much of the absorption of water and minerals occurs
near root tips, where the epidermis is permeable
(without cuticle) to water and where root hairs are
located
• Root hairs account for much of the surface area of roots
Effects of Differences in Water Potential
• To survive
– Plants must balance water uptake and loss
• Osmosis
– Determines the net uptake or water loss by a
cell
• Water potential
– Is a measurement that combines the effects of
solute concentration and pressure
– Determines the direction of movement of
water
• Water
– Flows from regions of high water potential to
regions of low water potential
How Solutes and Pressure Affect Water Potential
• Both pressure and solute concentration
– Affect water potential
• The solute potential of a solution
– Is proportional to the number of dissolved
molecules
• Pressure potential
– Is the physical pressure on a solution
Quantitative Analysis of Water Potential
• The addition of solutes
– Reduces water potential
(a)
0.1 M
solution
Pure
water
H2O
 = 0 MPa
Figure 36.5a
P = 0
S = 0.23
 = 0.23 MPa
• Application of Positive physical pressure
– Increases water potential
(b)
(c)
H2O
H2O
 = 0 MPa
Figure 36.5b, c
P = 0.23
S = 0.23
 = 0 MPa
 = 0 MPa
P = 0.30
S = 0.23
 = 0.07 MPa
• Negative pressure
– Decreases water potential
(d)
H2O
P = 0.30
S = 0
 = 0.30 MPa
Figure 36.5d
P = 0
S = 0.23
 = 0.23 MPa
water vapour
A plant loses water
in the form of
water vapour
from the surface of
the plant into the
atmosphere
This process is
called
transpiration
water
Ascend from roots to shoots through the xylem
• Concept 3: Water and minerals ascend from
roots to shoots through the xylem
• Plants lose an enormous amount of water
through transpiration, the loss of water
vapor from leaves and other aerial parts of
the plant
• The transpired water must be replaced by
water transported up from the roots
The Ascent of Xylem Sap
• Xylem sap
Water-conducting cells
– Rises to
heights of
more than
100 m in
the tallest
plants
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 to the xylem from the root
cortex
– Generating root pressure
Guttation- a demonstration of root pressure
• Root pressure sometimes results in guttation, the
exudation of water droplets on tips of grass blades
or the leaf margins of some small, herbaceous
plants
Figure 36.11
Root pressure
• In most plants, root pressure is not the major
mechanism driving the ascent of xylem sap.
– At most, root pressure can force water
upward only a few meters, and many plants
generate no root pressure at all.
• For the most part, xylem sap is not pushed
from below by root pressure but pulled
upward by the leaves themselves.
Pulling Xylem Sap: The Transpiration-CohesionTension Mechanism
• Water is pulled upward by negative pressure
in the xylem
Transpirational Pull
starts with water vapor in the airspaces of a leaf
diffuses down its water potential gradient and exits
the leaf via stomata
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Water Movement from the Leaf to the Atmosphere
Transpiration = the evaporation of
water from leaf surfaces
• Transpiration produces negative pressure (tension) in
the leaf
– Which exerts a pulling force on water in the xylem, pulling
water into the leaf
Hydrogen Bond between water molecules
Water is a polar molecule, In water, the negative regions on
one molecule are attracted to the positive regions on
another, and the molecules form hydrogen
bonds.
The Process of Transpiration
Movement of Water Up Xylem Vessels
When water enters the roots, hydrogen bonds link each
water molecule to the next so the molecules of water are
pulled up the thin xylem vessels like beads on a string. The
water moves up the plant, enters the leaves, moves into air
spaces in the leaf, and then evaporates (transpires) through
the stomata (singular, stoma).
Xylem Sap Ascent by Bulk Flow: A Review
• The mechanism of transpiration depends on the
generation of negative pressure (tension) in the leaf
due to unique physical properties of water.
– As water transpires from the leaf, water coating
the mesophyll cells replaces water lost from the
air spaces..
– Adhesion to the wall and surface tension causes
the surface of the water film to form a meniscus,
“pulling on” the water by adhesive and
cohesive forces.
adhesion and surface tension lowers the water potential
• The tension generated by adhesion and surface tension
lowers the water potential, drawing water from where its
potential is higher to where it is lower.
– Mesophyll cells will lose water to the surface film
lining the air spaces, which in turn loses water by
transpiration.
– The water lost via the stomata is replaced by water
pulled out of the leaf xylem.
Transpiration pull on xylem sap is transmitted all the way…
• The transpirational pull on xylem sap is transmitted all
the way from the leaves to the root tips and even into
the soil solution.
– Cohesion of water due to hydrogen bonding makes it
possible to pull a column of sap from above without
the water separating.
– Helping to fight gravity is the strong adhesion of
water molecules to the hydrophilic walls of the
xylem cells.
– The very small diameter of the tracheids and vessel
elements exposes a large proportion of the water to
the hydrophilic walls.
tension within the xylem….
• The upward pull on the cohesive sap creates tension
within the xylem
– This tension can actually cause a decrease in the
diameter of a tree on a warm day.
– Transpiration puts the xylem under tension all the
way down to the root tips, lowering the water
potential in the root xylem and pulling water from
the soil.
Cohesion and Adhesion in the Ascent of Xylem Sap
• The transpiration
pull on xylem sap
Xylem
sap
Outside air Y
= –100.0 MPa
Mesophyll
cells
Stoma
– Is facilitated
by cohesion
and adhesion
Water
molecule
Leaf Y (air spaces)
= –7.0 MPa
Leaf Y (cell walls)
= –1.0 MPa
Trunk xylem Y
= – 0.8 MPa
Transpiration
Atmosphere
Water potential gradient
– Is transmitted
all the way
from the
leaves to the
root tips and
even into the
soil solution
Xylem
cells
Adhesion
Cohesion
and adhesion
in the xylem
Cell
wall
Cohesion,
by
hydrogen
bonding
Water
molecule
Root xylem Y
= – 0.6 MPa
Root
hair
Soil Y
= – 0.3 MPa
Soil
particle
Figure 36.13
Water uptake
from soil
Water
The Plant – Soil – Atmosphere Continuum
Movement of water from soil through plant to
atmosphere involves different mechanisms of
transport:
In the vapor phase, water moves by diffusion until it reaches outside
air (and convection, a form of bulk flow, becomes dominant)
In xylem, water moves by bulk flow in response to a pressure
gradient (ΔΨp)
For water transport across membranes, water potential difference
across membrane is driving force (osmosis, e.g. when cells absorb
water and roots transport water from soil to xylem)
In all cases: water moves toward regions of low water potential (or
free energy)
Maple tree
Transpiration:
200 liters/day
75 cm/min
Stomata: Major Pathways for Water Loss
• About 90% of the water a plant loses
– Escapes through stomata
Stomata
Stomata from sedge (Carex)
Cytosol
and vacuole
Pore
Heavily
thickened
guard cell
wall
Guard cells
Stoma from a grass
Subsidiary cells
Epidermal cell
Stomata
Stomata from onion epidermis
Outside surface of the leaf with
stomatal pore inserted into the
cuticle
Guard cells facing the stomatal
cavity, toward the inside of the leaf
Stomatal pore
Guard cell
40
The cell walls of guard cells have specialized features
Two main types of guard cells
- kidney-shaped stomata: for dicots, monocots, mosses,
ferns, gymnosperms
- grass-like stomata: grasses and a few other monocots
(e.g. palms)
Grass-like stomata
Kidney-shaped stomata
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An increase in guard cell turgor pressure opens the stomata
- guard cells function as multisensory hydraulic valves
- guard cells sense changes in the environment: light intensity, light quality,
temperature, leaf water status, intracellular CO2
- this process requires ion uptake and other metabolic changes
in the guard cells (discussed later)
- due to this ion uptake → Ψs decreases → Ψw decreases → water moves
into guard cells → Ψp (turgor pressure) increases → cell volume increases
→ opening of stomata (due to differential thickening of guard cell walls)
The cell walls of guard cells have specialized features
Portions of
the guard
cell wall are
substantially
thickened
(up to 5 um
across)
Atmosphere
Pore
Nucleus
Vacuole
Substomatal cavity
Inner cell wall
Plastid
Changes in turgor pressure that open and close stomata
– Result primarily from the reversible uptake and
loss of potassium ions by the guard cells
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.
H2O
K+
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
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
Turgor and wilting
• Turgor loss in plants causes wilting
– Which can be reversed when the plant is
watered
Figure 36.7
Importance of transpiration
absorption of water
transport of water and minerals
Evaporative cooling?
• 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
– But recent findings cast doubt on the actual
significance of the cooling effects
Environmental factors affecting
the rate of transpiration
Humidity
 concentration gradient
of water vapour between
the inside of a leaf and
the atmosphere
 more water vapour
diffuses out
rate of transpiration
1. Relative humidity
relative humidity (%)
2. Temperature
Temperature
 rate of evaporation
of water
rate of transpiration
Environmental factors affecting
the rate of transpiration
temperature (oC)
3. Air movement
Wind moves the
water vapour away
from leaf.
rate of transpiration
Environmental factors affecting
the rate of transpiration
wind velocity (km/h)
Environmental factors affecting
the rate of transpiration
Stomata open wider
as light intensity
increases. Wider
stomata allow more
water vapour to
diffuse out.
rate of transpiration
4. Light intensity
light intensity
Environmental Factors affecting transpiration
– Light: stimulates the stomata to open allowing gas exchange
for photosynthesis → transpiration increases (plants may lose
water during the day and wilt)
– Temperature: High temperature increases the rate of
evaporation of water from the spongy cells, and reduces air
humidity, so transpiration increases.
– Humidity: High humidity means a higher water potential in the
air, so a lower water potential gradient between the leaf and the
air, so less evaporation.
– Wind: Blows away saturated air from around stomata,
replacing it with drier air, so increasing the water potential
gradient and increasing transpiration.
Water and Plants – Summary
- water is the essential medium of life
- plants gain energy from sunlight and need to
have open pathway for CO2
- plants have large surface area that is not
differentially permeable to CO2 vs. water vapor
→ conflict: need for water conservation and
need for CO2 assimilation
- the need to resolve this conflict determines
structure of plants: ?
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Factors affecting transpiration
Plant factors
Leaf structure
Leaf area
Shoot-Root ratio
Adaptations to dry habitats
Plants in different habitats are adapted to cope with different problems of
water availability.
Mesophytes plants adapted to a habitat with adequate water
Xerophytes
plants adapted to a dry habitat
Halophytes
plants adapted to a salty habitat
Hydrophytes plants adapted to a freshwater habitat
The stomata of xerophytes
Are concentrated on the lower leaf surface
Are often located in depressions (sunken) that
shelter the pores from the dry wind
Upper
epidermal
tissue
Cuticle
Figure 36.16
Trichomes
100 m
Lower
(“hairs”)
epidermal
Stomata
tissue
Marram grass
Marram grass –a xerophyte
Rolled leaves of marram grass
Internal Factors affecting transpiration
Some adaptations of xerophytes are:
Adaptation
How it works
Example
stops uncontrolled evaporation most dicots
through leaf cells
less area for evaporation
conifer needles, cactus
spines
more humid air on lower
surface, so less evaporation
most dicots
reduce water loss at certain
times of year
deciduous plants
maintains humid air around
stomata
marram grass, pine
maintains humid air around
stomata
marram grass, couch
grass
maintains humid air around
stomata
marram grass,
stores water
cacti
maximize water uptake
cacti
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Internal Factors affecting transpiration
Some adaptations of xerophytes are:
Adaptation
How it works
Example
thick cuticle
stops uncontrolled evaporation most dicots
through leaf cells
small leaf surface area
less area for evaporation
conifer needles, cactus
spines
stomata on lower
surface of leaf only
more humid air on lower
surface, so less evaporation
most dicots
shedding leaves in
dry/cold season
reduce water loss at certain
times of year
deciduous plants
sunken stomata
maintains humid air around
stomata
marram grass, pine
stomatal hairs
maintains humid air around
stomata
marram grass, couch
grass
folded leaves
maintains humid air around
stomata
marram grass,
succulent leaves and
stem
stores water
cacti
extensive roots
maximize water uptake
cacti
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Stomatal Control -- Water conservation vs Leaf Photosynthesis
Problem:
• Plants have to take up CO2 from atmosphere, but simultaneously need to
limit water loss – transpiration is a necessary evil
• Cuticle protects from desiccation
• However, plants cannot prevent outward diffusion of water without
simultaneously excluding CO2 from leaf and concentration gradient for
CO2 uptake is much smaller than concentration gradient that drives water
loss
Solution:
Stomatal Control -- Water conservation vs Leaf Photosynthesis
Problem:
• Plants have to take up CO2 from atmosphere, but simultaneously need to
limit water loss – transpiration is a necessary evil
• Cuticle protects from desiccation
• However, plants cannot prevent outward diffusion of water without
simultaneously excluding CO2 from leaf and concentration gradient for
CO2 uptake is much smaller than concentration gradient that drives water
loss
Solution: Temporal regulation of stomatal apertures
• Closed at night: no photosynthesis → no demand for CO2 → stomatal
aperture kept small → preventing unnecessary loss of water
• Open during day: sunny morning; water abundant, light favors
photosynthesis → large demand for CO2 → stomata wide open →
decreased stomatal resistance to CO2 diffusion → water loss by
transpiration also substantial, but water supply is plentiful, i.e. plant trades
water for the product of photosynthesis needed for growth