Water transport in vascular plants
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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 )

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Water Transport in Vascular Plants

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Water transport in vascular plants

Water Transport in Vascular Plants


Key concepts

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)


Water transport in vascular plants

  • 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


Water transport in vascular plants

Figure 36.1

  • Vascular tissue

    • Transports nutrients throughout a plant; such transport may occur over long distances


Anatomy of a woody stem

Anatomy of a woody stem


Water conducting cells of xylem

Water-conducting cells of xylem


Water transport in vascular plants

  • 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


Water transport in vascular plants

1

2

3

4

.

6

5

7

.

  • A variety of physical processes

    • are involved in the different types of transport

CO2

O2

Light

H2O

Sugar

O2

H2O

CO2

Minerals

Figure 36.2


Water transport in vascular plants

A variety of physical processes

Are involved in the different types of transport

1

2

4

3

Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for

photosynthesis. Some O2produced by photosynthesis is used in cellular respiration.

Sugars are produced by

photosynthesis in the leaves.

Transpiration, the loss of water

from leaves (mostly through

stomata), creates a force within

leaves that pulls xylem sap upward.

6

5

7

Water and minerals are

transported upward from

roots to shoots as xylem sap.

Roots absorb water

and dissolved minerals

from the soil.

Roots exchange gases

with the air spaces of soil,

taking in O2 and discharging

CO2. In cellular respiration,

O2 supports the breakdown

of sugars.

CO2

O2

Light

H2O

Sugar

Sugars are transported as

phloem sap to roots and other

parts of the plant.

O2

H2O

CO2

Minerals

Figure 36.2


Selective permeability of membranes a review

Selective Permeability of Membranes: A Review

  • The selective permeability of a plant cell’s plasma membrane

    • Controls the movementofsolutes into and out of the cell

  • Specific transport proteins

    • Enable plant cells to maintain an internal environmentdifferent from their surroundings


Bulk flow in long distance transport

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

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

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 transport in vascular plants

Water potential

Is a measurement that combines the effects of solute concentration and pressure

Determines the directionofmovement of water

Water

Flows from regions of high water potential to regions of low water potential


How solutes and pressure affect 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

Quantitative Analysis of Water Potential

The addition of solutes

Reduces water potential

(a)

0.1 M

solution

Pure

water

H2O

P= 0

S= 0.23

= 0.23 MPa

= 0 MPa

Figure 36.5a


Water transport in vascular plants

Application of Positive physical pressure

Increases water potential

(b)

(c)

H2O

H2O

P= 0.23

S= 0.23

= 0 MPa

P= 0.30

S= 0.23

= 0.07 MPa

= 0 MPa

= 0 MPa

Figure 36.5b, c


Water transport in vascular plants

Negative pressure

Decreases water potential

(d)

H2O

P= 0.30

S= 0

= 0.30 MPa

P= 0

S= 0.23

= 0.23 MPa

Figure 36.5d


Water transport in vascular plants

water vapour

water

A plant loses water

in the form ofwater vapour

from the surface of the plant into the atmosphere

This process is called transpiration


Ascend from roots to shoots through the xylem

Ascend from roots to shoots through the xylem

  • Concept 3: Water and mineralsascend 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

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

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, loweringthe water potential

  • Waterflows in to the xylem from the root cortex

    • Generating root pressure


Guttation a demonstration of root pressure

Figure 36.11

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


Root pressure

Root pressure

  • In most plants, root pressure is notthe 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 butpulledupward by the leaves themselves.


Pulling xylem sap the transpiration cohesion tension mechanism

Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism

  • Water is pulled upward by negativepressure 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


Water movement from the leaf to the atmosphere

Water Movement from the Leaf to the Atmosphere

Transpiration = the evaporation of water from leaf surfaces


Water transport in vascular plants

  • Transpiration produces negative pressure (tension) in the leaf

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


Water transport in vascular plants

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.                    


Water transport in vascular plants

The Process of Transpiration


Water transport in vascular plants

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

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

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

Transpiration pull on xylem sap is transmitted all the way….

  • The transpirational pull on xylemsap 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 hydrophilicwalls 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

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

Xylem

sap

Outside air Y

= –100.0 MPa

Mesophyll

cells

Stoma

Water

molecule

Leaf Y (air spaces)

= –7.0 MPa

Transpiration

Atmosphere

Leaf Y (cell walls)

= –1.0 MPa

Xylem

cells

Adhesion

Cell

wall

Water potential gradient

Trunk xylem Y

= – 0.8 MPa

Cohesion,

by

hydrogen

bonding

Cohesion

and adhesion

in the xylem

Water

molecule

Root xylem Y

= – 0.6 MPa

Root

hair

Soil Y

= – 0.3 MPa

Soil

particle

Water

Water uptake

from soil

Figure 36.13

Cohesion and Adhesion in the Ascent of Xylem Sap

  • The transpiration pull on xylem sap

    • Is transmittedall the way from the leaves to the root tips and even into the soil solution

    • Is facilitated by cohesion and adhesion


The plant soil atmosphere continuum

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 bybulk 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)


Water transport in vascular plants

Maple treeTranspiration: 200 liters/day

75 cm/min


Stomata major pathways for water loss

Stomata: Major Pathways for Water Loss

About 90% of the water a plant loses

Escapes through stomata


Stomata

Stomata

Stomata from sedge (Carex)

Cytosol

and vacuole

Pore

Heavily thickened

guard cell wall

Guard cells

Subsidiary cells

Stoma from a grass

Epidermal cell


Stomata1

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


The cell walls of guard cells have specialized features

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


An increase in guard cell turgor pressure opens the stomata

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 features1

The cell walls of guard cells have specialized features

Atmosphere

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

Pore

Nucleus

Plastid

Vacuole

Substomatal cavity

Inner cell wall


Changes in turgor pressure that open and close stomata

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

H2O

H2O

H2O

H2O

K+

H2O

H2O

H2O

H2O

H2O


Effects of transpiration on wilting and leaf temperature

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 and wilting

  • Turgor loss in plants causes wilting

    • Which can be reversed when the plant is watered

Figure 36.7


Water transport in vascular plants

Importance of transpiration

  • absorption of water

  • transport of water and minerals


Evaporative cooling

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


Water transport in vascular plants

rate of transpiration

relative humidity (%)

Environmental factors affecting the rate of transpiration

1. Relative humidity

Humidity

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

     more water vapour diffuses out


Water transport in vascular plants

rate of transpiration

temperature (oC)

Environmental factors affecting the rate of transpiration

2.Temperature

Temperature

 rate of evaporation of water


Water transport in vascular plants

rate of transpiration

wind velocity (km/h)

Environmental factors affecting the rate of transpiration

3. Air movement

Wind moves the water vapour away from leaf.


Water transport in vascular plants

rate of transpiration

light intensity

Environmental factors affecting the rate of transpiration

4. Light intensity

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


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.

Environmental Factors affecting transpiration


Water transport in vascular plants

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: ?


Factors affecting transpiration

Plant factors

Leaf structure

Leaf area

Shoot-Root ratio

Factors affecting transpiration

Adaptations to dry habitats

Plants in different habitats are adapted to cope with different problems of water availability.

Mesophytesplants adapted to a habitat with adequate water

Xerophytesplants adapted to a dry habitat

Halophytesplants adapted to a salty habitat

Hydrophytesplants adapted to a freshwater habitat


The stomata of xerophytes

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

Trichomes

(“hairs”)

Lower

epidermal

tissue

100 m

Stomata

Figure 36.16


Marram grass

Marram grass


Marram grass a xerophyte

Marram grass –a xerophyte


Rolled leaves of marram grass

Rolled leaves of marram grass


Internal factors affecting transpiration

Internal Factors affecting transpiration

Some adaptations of xerophytes are:

60


Internal factors affecting transpiration1

Internal Factors affecting transpiration

Some adaptations of xerophytes are:

61


Stomatal control water conservation vs leaf photosynthesis

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 photosynthesis1

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


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