Water Transport in Water Transport in Vascular PlantsVascular 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

MineralsH2O CO2

O2

CO2 O2

H2O Sugar

Light

• A variety of physical processes

– are involved in the different types of transport.5

6

4

3

.2

1

Figure 36.2

7

MineralsH2O CO2

O2

CO2 O2

H2O Sugar

Light

• A variety of physical processes

– Are involved in the different types of transport Sugars are produced byphotosynthesis in the leaves.5

Sugars are transported asphloem sap to roots and otherparts of the plant.

6

Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon forphotosynthesis. Some O2 produced by photosynthesis is used in cellular respiration.

4

Transpiration, the loss of waterfrom leaves (mostly through

stomata), creates a force withinleaves that pulls xylem sap upward.

3

Water and minerals aretransported upward from

roots to shoots as xylem sap.

2

Roots absorb waterand dissolved minerals

from the soil.

1

Figure 36.2

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

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

Figure 36.5a

0.1 Msolution

H2O

Purewater

P = 0

S = 0.23

= 0.23 MPa = 0 MPa

(a)

• Application of Positive physical pressure

– Increases water potential

H2O

P = 0.23

S = 0.23

= 0 MPa = 0 MPa

(b)

H2O

P = 0.30

S = 0.23

= 0.07 MPa = 0 MPa

(c)

Figure 36.5b, c

• Negative pressure

– Decreases water potential

H2O

P = 0

S = 0.23

= 0.23 MPa

(d)

P = 0.30

S = 0

= 0.30 MPa

Figure 36.5d

water

water vapourA plant loses water

This process is called

transpiration

This process is called

transpiration

from the surface of the plant into the

atmosphere

in the form of water vapour

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

– Rises to heights of more than 100 m in the tallest plants

Water-conducting cells

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

• 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.

Root pressure

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism

• Water is pulled upward by negative pressure in the xylem

starts with water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata

Transpirational Pull

Transpiration = the evaporation of water from leaf surfaces

Water Movement from the Leaf to the Atmosphere

• Transpiration produces negative pressure (tension) in the leaf

– Which exerts a pulling force on water in the xylem, pulling water into the leaf

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. hydrogen bonds.                                        

Hydrogen Bond between water molecules

                                                                                                

The Process of Transpiration

        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).

Movement of Water Up Xylem Vessels

• 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.

Xylem Sap Ascent by Bulk Flow: A Review

• 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.

adhesion and surface tension lowers the water potential

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

– Is transmitted all the way from the leaves to the root tips and even into the soil solution

– Is facilitated by cohesion and adhesion

Xylemsap

Outside air = –100.0 MPa

Leaf (air spaces) = –7.0 MPa

Leaf (cell walls) = –1.0 MPa

Trunk xylem = – 0.8 MPa

Wat

er p

ote

nti

al g

rad

ien

tRoot xylem = – 0.6 MPa

Soil = – 0.3 MPa

Mesophyllcells

Stoma

Watermolecule

Atmosphere

Transpiration

Xylemcells Adhesion Cell

wall

Cohesion,byhydrogenbonding

Watermolecule

Roothair

Soilparticle

Water

Cohesion and adhesionin the xylem

Water uptakefrom soil Figure 36.13

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)

The Plant – Soil – Atmosphere Continuum

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

Cytosol and vacuole

Pore

Heavily thickened guard cell wall

Stoma from a grass

Stomata from sedge (Carex)

Guard cells

Subsidiary cells

Epidermal cell

Stomata

40

Stomata from onion epidermis

Guard cell

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

Stomata

41

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) Kidney-shaped stomata Grass-like stomata

The cell walls of guard cells have specialized features

- 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)

An increase in guard cell turgor pressure opens the stomata

Portions of the guard cell wall are substantially thickened (up to 5 um across)

Atmosphere

Pore

Vacuole

PlastidNucleus

Substomatal cavity

Inner cell wall

The cell walls of guard cells have specialized features

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

H2OH2O

H2O

K+

H2O H2O

H2O

H2OH2O

Role of potassium in stomatal opening and closing.

The transport of K+ (potassium ions, symbolized here as red dots) across the plasma membrane andvacuolar membrane causes the turgor changes of guard cells.

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 transpirationImportance of transpiration

absorption of waterabsorption of water

transport of water and mineralstransport 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 Environmental factors affecting the rate of transpirationthe rate of transpiration

Humidity Humidity

concentration gradient concentration gradient of water vapour between of water vapour between the inside of a leaf and the inside of a leaf and the atmospherethe atmosphere

more water vapour more water vapour diffuses outdiffuses out

1. Relative humidityRelative humidity

relative humidity (%)

rate

of

tran

spir

atio

n

Temperature

rate of evaporation of water

rate

of

tran

spir

atio

n

temperature (oC)

2. Temperature

Environmental factors affecting Environmental factors affecting the rate of transpirationthe rate of transpiration

3. 3. Air movementAir movement

Wind moves the Wind moves the water vapour away water vapour away from leaf.from leaf. ra

te o

f tr

ansp

irat

ion

wind velocity (km/h)

Environmental factors affecting Environmental factors affecting the rate of transpirationthe rate of transpiration

4. 4. Light intensityLight intensity

rate

of

tran

spir

atio

n

light intensity

Stomata open wider Stomata open wider as light intensity as light intensity increases. Wider increases. Wider stomata allow more stomata allow more water vapour to water vapour to diffuse out.diffuse out.

Environmental factors affecting Environmental factors affecting the rate of transpirationthe rate of 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.

Environmental Factors affecting transpiration

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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: ?

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

Factors affecting transpiration

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

Figure 36.16

Lower epidermaltissue

Trichomes(“hairs”)

Cuticle

Upper epidermal tissue

Stomata100 m

Marram grass

Marram grass –a xerophyte

Rolled leaves of marram grass

60

Some adaptations of xerophytes are:

Adaptation How it works Example

stops uncontrolled evaporation through leaf cells

most dicots

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

Internal Factors affecting transpiration

61

Some adaptations of xerophytes are:

Adaptation How it works Example

thick cuticle stops uncontrolled evaporation through leaf cells

most dicots

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

Internal Factors affecting transpiration

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

Stomatal Control -- Water conservation vs Leaf Photosynthesis