35 plantstructure text

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

Plant Structure, Growth, and Development

• The plant body has a hierarchy of organs, tissues, and cells

• Plants, like multicellular animals

– Have organs composed of different tissues, which are in turn composed of cells


The Three Basic Plant Organs: Roots, Stems, and Leaves

• Three basic organs of plants: roots, stems, and leaves

• They are organized into a root system and a shoot system

Figure 35.2

Reproductive shoot (flower)

Terminal bud

Node

Internode

Terminalbud

Vegetativeshoot

BladePetiole

Stem

Leaf

Taproot

Lateral roots Rootsystem

Shootsystem

Axillarybud


Roots

• A root

– Is an organ that anchors the vascular plant

– Absorbs minerals and water

– Often stores organic nutrients

• In most plants

– The absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root


• Many plants have modified roots

Figure 35.4a–e

(a) Prop roots (b) Storage roots(c) “Strangling” aerialroots

(d) Buttress roots (e) Pneumatophores


Stems

• A stem is an organ consisting of

– An alternating system of nodes, the points at which leaves are attached

– Internodes, the stem segments between nodes

• An axillary bud

– Is a structure that has the potential to form a lateral shoot, or branch

• A terminal bud

– Is located near the shoot tip and causes elongation of a young shoot


• Many plants have modified stems

Figure 35.5a–d

Rhizomes. The edible base of this ginger plant is an example of a rhizome, a horizontal stem that grows just below the surface or emerges and grows along thesurface.

(d)

Tubers. Tubers, such as these red potatoes, are enlarged ends of rhizomes specializedfor storing food. The “eyes” arranged in a spiral pattern around a potato are clusters of axillary buds that markthe nodes.

(c)

Bulbs. Bulbs are vertical,underground shoots consistingmostly of the enlarged bases of leaves that store food. You can see the many layers of modified leaves attached to the short stem by slicing an onion bulb lengthwise.

(b)

Stolons. Shown here on a strawberry plant, stolons are horizontal stems that grow along the surface. These “runners”enable a plant to reproduce asexually, as plantlets form at nodes along each runner.

(a)

Storage leaves

Stem

Root Node

Rhizome

Root


Leaves

• The leaf

– Is the main photosynthetic organ of most vascular plants

• Leaves generally consist of

– A flattened blade and a stalk

– The petiole, which joins the leaf to a node of the stem


• Monocots and dicots

– Differ in the arrangement of veins, the vascular tissue of leaves

• Most monocots

– Have parallel veins

• Most dicots

– Have branching veins


• In classifying angiosperms

– Taxonomists may use leaf morphology as a criterion

Figure 35.6a–c

Petiole

(a) Simple leaf. A simple leafis a single, undivided blade.Some simple leaves are deeply lobed, as in anoak leaf.

(b) Compound leaf. In acompound leaf, theblade consists of multiple leaflets.Notice that a leaflethas no axillary budat its base.

(c) Doubly compound leaf. In a doubly compound leaf, each leaflet is divided into smaller leaflets.

Axillary bud

Leaflet

Petiole

Axillary bud

Axillary bud

LeafletPetiole


• Some plant species

– Have modified leaves that serve various functions

Figure 35.6a–e

(a) Tendrils. The tendrils by which thispea plant clings to a support are modified leaves. After it has “lassoed” a support, a tendril forms a coil that brings the plant closer to the support. Tendrils are typically modified leaves, but some tendrils are modified stems, as in grapevines.

(b) Spines. The spines of cacti, such as this prickly pear, are actually leaves, and photosynthesis is carried out mainly by the fleshy green stems.

(c) Storage leaves. Most succulents, such as this ice plant, have leaves modified for storing water.

(d) Bracts. Red parts of the poinsettia are often mistaken for petals but are actually modified leaves called bracts that surround a group of flowers. Such brightly colored leaves attract pollinators.

(e) Reproductive leaves. The leaves of some succulents, such as Kalanchoe daigremontiana, produce adventitious plantlets, which fall off the leaf and take root in the soil.


The Three Tissue Systems: Dermal, Vascular, and Ground

• Each plant organ

– Has dermal, vascular, and ground tissues

Figure 35.8

Dermaltissue

Groundtissue Vascular

tissue


• The dermal tissue system

– Consists of the epidermis and periderm

• The vascular tissue system

– Carries out long-distance transport of materials between roots and shoots

– Consists of two tissues, xylem and phloem

• Ground tissue

– Includes various cells specialized for functions such as storage, photosynthesis, and support


• Xylem

– Conveys water and dissolved minerals upward from roots into the shoots

• Phloem

– Transports organic nutrients from where they are made to where they are needed


• Some of the major types of plant cells include

– Parenchyma –metabolic function (photosynthesis). Most abundant type.

– Collenchyma – grouped in cylinders and support growing part of the plant.

– Sclerenchyma – found in areas where there is no active growth. Have tough cell walls. Supportive cells.

– Water-conducting cells of the xylem (Tracheids and Vessels)

– Sugar-conducting cells of the phloem (Sieve-tube elements and companion cells)


• Parenchyma, collenchyma, and sclerenchyma cells

Figure 35.9

Parenchyma cells 60 m

PARENCHYMA CELLS

80 m Cortical parenchyma cells

COLLENCHYMA CELLS

Collenchyma cells

SCLERENCHYMA CELLS

Cell wall

Sclereid cells in pear

25 m

Fiber cells

5 m


• Water-conducting cells of the xylem and sugar-conducting cells of the phloem

Figure. 35.9

WATER-CONDUCTING CELLS OF THE XYLEM

Vessel Tracheids 100 m

Tracheids and vessels

Vesselelement

Vessel elements withpartially perforated end walls

Pits

Tracheids

SUGAR-CONDUCTING CELLS OF THE PHLOEM

Companion cell

Sieve-tubemember

Sieve-tube members:longitudinal view

Sieveplate

Nucleus

CytoplasmCompanioncell

30 m

15 m


• Meristems generate cells for new organs

• Apical meristems

– Are located at the tips of roots and in the buds of shoots

– Elongate shoots and roots through primary growth

• Lateral meristems

– Add thickness to woody plants through secondary growth


• An overview of primary and secondary growth

Figure. 35.10

In woody plants, there are lateral meristems that add secondary

growth, increasing the girth of

roots and stems.

Apical meristemsadd primary growth,or growth in length.

Vascularcambium

Corkcambium

Lateralmeristems

Root apicalmeristems

Primary growth in stems

Epidermis

Cortex

Primary phloem

Primary xylem

Pith

Secondary growth in stems

PeridermCorkcambium

CortexPrimary phloem

Secondaryphloem

Vascular cambium

Secondaryxylem

Primaryxylem

Pith

Shoot apicalmeristems(in buds)

The corkcambium addssecondarydermal tissue.

The vascularcambium addssecondaryxylem andphloem.


• In woody plants

– Primary and secondary growth occur simultaneously but in different locations

Figure 35.11

This year’s growth(one year old)

Last year’s growth(two years old)

Growth of twoyears ago (threeyears old)

One-year-old sidebranch formedfrom axillary budnear shoot apex

Scars left by terminalbud scales of previouswinters

Leaf scar

Leaf scar

Stem

Leaf scar

Bud scale

Axillary buds

Internode

Node

Terminal bud


• Primary growth lengthens roots and shoots

• Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems

• The root tip is covered by a root cap, which protects the delicate apical meristem as the root pushes through soil during primary growth


Primary Growth of Roots

Figure 35.12

Dermal

Ground

Vascular

Key

Cortex Vascular cylinder

Epidermis

Root hair

Zone ofmaturation

Zone ofelongation

Zone of celldivision

Apicalmeristem

Root cap

100 m


• The primary growth of roots

– Produces the epidermis, ground tissue, and vascular tissue


• Organization of primary tissues in young roots

Figure 35.13a, b

Cortex

Vascularcylinder

Endodermis

Pericycle

Core ofparenchymacells

Xylem

50 m

Endodermis

Pericycle

Xylem

Phloem

Key

100 m

Vascular

GroundDermal

Phloem

Transverse section of a root with parenchymain the center. The stele of many monocot roots is a vascular cylinder with a core of parenchymasurrounded by a ring of alternating xylem and phloem.

(b)Transverse section of a typical root. In theroots of typical gymnosperms and eudicots, aswell as some monocots, the stele is a vascularcylinder consisting of a lobed core of xylemwith phloem between the lobes.

(a)

100 m

Epidermis


• Lateral roots

– Arise from within the pericycle, the outermost cell layer in the vascular cylinder

Figure 35.14

Cortex

Vascularcylinder

Epidermis

Lateral root

100 m

1 2

3 4

Emerginglateralroot


Primary Growth of Shoots

• A shoot apical meristem

– Is a dome-shaped mass of dividing cells at the tip of the terminal bud

– Gives rise to a repetition of internodes and leaf-bearing nodes

Figure. 35.15

Apical meristem Leaf primordia

Developingvascularstrand

Axillary budmeristems

0.25 mm


Tissue Organization of Stems

• In gymnosperms and most eudicots

– The vascular tissue consists of vascular bundles arranged in a ring

Figure 35.16a

XylemPhloem

Sclerenchyma(fiber cells)

Ground tissueconnecting pith to cortex

Pith

Epidermis

Vascularbundle

Cortex

Key

Dermal

Ground

Vascular1 mm

(a) A eudicot stem. A eudicot stem (sunflower), withvascular bundles forming a ring. Ground tissue towardthe inside is called pith, and ground tissue toward theoutside is called cortex. (LM of transverse section)


Groundtissue

Epidermis

Vascularbundles

1 mm

(b) A monocot stem. A monocot stem (maize) with vascularbundles scattered throughout the ground tissue. In such anarrangement, ground tissue is not partitioned into pith andcortex. (LM of transverse section)

Figure 35.16b

• In most monocot stems

– The vascular bundles are scattered throughout the ground tissue, rather than forming a ring


Tissue Organization of Leaves

• The epidermal barrier in leaves

– Is interrupted by stomata, which allow CO2 exchange between the surrounding air and the photosynthetic cells within a leaf

• The ground tissue in a leaf

– Is sandwiched between the upper and lower epidermis

• The vascular tissue of each leaf

– Is continuous with the vascular tissue of the stem


Keyto labels

DermalGround

Vascular

Guardcells

Stomatal pore

Epidermalcell

50 µm

Surface view of a spiderwort(Tradescantia) leaf (LM)

(b)Cuticle

Sclerenchymafibers

Stoma

Upperepidermis

Palisademesophyll

Spongymesophyll

Lowerepidermis

Cuticle

Vein

Guard cells

Xylem

Phloem

Guard cells

Bundle-sheathcell

Cutaway drawing of leaf tissues(a)

Vein Air spaces Guard cells

100 µmTransverse section of a lilac(Syringa) leaf (LM)

(c)Figure 35.17a–c

• Leaf anatomy


• Secondary growth adds girth to stems and roots in woody plants

• Secondary growth

– Occurs in stems and roots of woody plants but rarely in leaves

• The secondary plant body

– Consists of the tissues produced by the vascular cambium and cork cambium


The Vascular Cambium and Secondary Vascular Tissue

• The vascular cambium

– Is a cylinder of meristematic cells one cell thick

– Develops from parenchyma cells

– Produces secondary xylem


Vascular cambium

Pith

Primary xylem

Secondary xylem

Vascular cambium

Secondary phloem

Primary phloem

Periderm(mainly cork cambiaand cork)

Pith

Primary xylem

Vascular cambium

Primary phloem

Cortex

Epidermis

Vascular cambium

4 First cork cambium

Secondary xylem (twoyears ofproduction)

PithPrimary xylemVascular cambium

Primary phloem

2

1

6

Growth

Primary xylem

Secondary xylem

Secondary phloem

Primary phloem Cork

Phloem ray3

Xylem ray

Growth

Bark

8 Layers of periderm

7 Cork5 Most recentcork cambium

CortexEpidermis

9

In the youngest part of the stem, you can see the primary plant body, as formed by the apical meristem during primary growth. The vascular cambium is beginning to develop.

As primary growth continues to elongate the stem, the portion of the stem formed earlier the same year has already started its secondary growth. This portion increases in girth as fusiform initials of the vascular cambium form secondary xylem to theinside and secondary phloem to the outside.

The ray initials of the vascular cambium give rise to the xylem and phloem rays.

As the diameter of the vascular cambium increases, the secondary phloem and other tissues external to the cambium cannot keep pace with the expansion because the cells no longer divide. As a result, these tissues, including the epidermis, rupture. A second lateral meristem, the cork cambium, develops from parenchyma cells in the cortex. The cork cambium produces cork cells, which replace the epidermis.

In year 2 of secondary growth, the vascular cambium adds to the secondary xylem and phloem, and the cork cambium produces cork.

As the diameter of the stem continues to increase, the outermost tissues exterior to the cork cambium rupture and slough off from the stem.

Cork cambium re-forms in progressively deeper layers of thecortex. When none of the original cortex is left, the cork cambium develops from parenchyma cells in the secondary phloem.

Each cork cambium and the tissues it produces form a layer of periderm.

Bark consists of all tissues exterior to the vascular cambium.

1

2

3

4

5

6

7

8

9

Secondary phloem

(a) Primary and secondary growthin a two-year-old stem

• Primary and secondary growth of a stem

Figure 35.18a


Secondary phloemVascular cambiumLate wood

Early woodSecondaryxylem

Corkcambium

CorkPeriderm

(b) Transverse sectionof a three-year-old stem (LM)

Xylem ray

Bark

0.5 mm0.5 mmFigure 35.18b


• As a tree or woody shrub ages

– The older layers of secondary xylem, the heartwood, no longer transport water and minerals

• The outer layers, known as sapwood

– Still transport materials through the xylem


Growth ring

Vascularray

Heartwood

Sapwood

Vascular cambium

Secondary phloem

Layers of periderm

Secondaryxylem

Bark

Figure 35.20


Cork Cambia and the Production of Periderm

• The cork cambium

– Gives rise to the secondary plant body’s protective covering, or periderm

• Periderm

– Consists of the cork cambium plus the layers of cork cells it produces

• Bark

– Consists of all the tissues external to the vascular cambium, including secondary phloem and periderm


Morphogenesis and Pattern Formation

• Pattern formation

– Is the development of specific structures in specific locations

– Is determined by positional information in the form of signals that indicate to each cell its location


• Polarity

– Is one type of positional information

• In the gnom mutant of Arabidopsis

– The establishment of polarity is defective

Figure 35.26


• Morphogenesis in plants, as in other multicellular organisms

– Is often under the control of homeotic genes

Figure 35.27


Gene Expression and Control of Cellular Differentiation

• In cellular differentiation

– Cells of a developing organism synthesize different proteins and diverge in structure and function even though they have a common genome


• Cellular differentiation

– To a large extent depends on positional information

– Is affected by homeotic genes

Figure 35.28

When epidermal cells border a single corticalcell, the homeotic gene GLABRA-2 is selectivelyexpressed, and these cells will remain hairless.(The blue color in this light micrograph indi-cates cells in which GLABRA-2 is expressed.)

Here an epidermal cell borders twocortical cells. GLABRA-2 is not expressed,and the cell will develop a root hair.

The ring of cells external to the epi-dermal layer is composed of rootcap cells that will be sloughed off asthe root hairs start to differentiate.

Corticalcells

20 µm


Location and a Cell’s Developmental Fate

• A cell’s position in a developing organ

– Determines its pathway of differentiation

• Plants pass through developmental phases, called phase changes

– Developing from a juvenile phase to an adult vegetative phase to an adult reproductive phase


• The most obvious morphological changes

– Typically occur in leaf size and shape

Leaves produced by adult phaseof apical meristem

Leaves produced by juvenile phaseof apical meristem

Figure 35.29


Genetic Control of Flowering

• Flower formation

– Involves a phase change from vegetative growth to reproductive growth

– Is triggered by a combination of environmental cues and internal signals

• The transition from vegetative growth to flowering

– Is associated with the switching-on of floral meristem identity genes


• Plant biologists have identified several organ identity genes

– That regulate the development of floral pattern

Figure 35.30a, b

(a) Normal Arabidopsis flower. Arabidopsisnormally has four whorls of flower parts: sepals(Se), petals (Pe), stamens (St), and carpels (Ca).

(b) Abnormal Arabidopsis flower. Reseachers haveidentified several mutations of organ identity genes that cause abnormal flowers to develop.This flower has an extra set of petals in place of stamens and an internal flower where normal plants have carpels.

Ca

St

Pe

Se

Pe

Pe

Se

Pe

Se


Transport in Vascular Plants

• 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


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


The Central Role of Proton Pumps

• Proton pumps in plant cells

– Create a hydrogen ion gradient that is a form of potential energy that can be harnessed to do work

– Contribute to a voltage known as a membrane potential

Figure 36.3

CYTOPLASM EXTRACELLULAR FLUID

ATP

H+

H+ H+

H+

H+

H+H+

H+

Proton pump generates membrane potentialand H+ gradient.

–

––

–

– +

+

+

+

+


• Plant cells use energy stored in the proton gradient and membrane potential

– To drive the transport of many different solutes+

CYTOPLASM EXTRACELLULAR FLUID

Cations ( , for example) are driven into the cell by themembrane potential.

Transport protein

K+

K+

K+

K+

K+ K+

K+

K+

–

–

– +

+

(a) Membrane potential and cation uptake

–

–

+

+

Figure 36.4a


• In the mechanism called cotransport

– A transport protein couples the passage of one solute to the passage of another

Figure 36.4b

H+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

NO3–

NO 3 –

NO3

–

NO 3

–

NO3

–

NO3 –

–

–

– +

+

+

–

–

– +

+

+

NO3–

(b) Cotransport of anions

H+of through a

cotransporter.

Cell accumulates anions ( , for example) by coupling their transport to theinward diffusion


H+

H+

H+

H+

H+H+

H+

H+ H+

H+

SS

S

S

SPlant cells canalso accumulate a neutral solute,such as sucrose

( ), bycotransporting

down the

steep protongradient.

S

H+

–

–

–

+

+

+

–

–

++–

Figure 36.4c

H+ H+S+

–(c) Contransport of a neutral solute

• The “coattail” effect of cotransport

– Is also responsible for the uptake of the sugar sucrose by plant cells


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

– Is affected by solute concentration and pressure


• 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

• 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 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 potential

– Affects uptake and loss of water by plant cells

• If a flaccid cell is placed in an environment with a higher solute concentration

– The cell will lose water and become plasmolyzed

Figure 36.6a

0.4 M sucrose solution:

Initial flaccid cell:

Plasmolyzed cellat osmotic equilibriumwith its surroundings

P = 0

S = 0.7

P = 0

S = 0.9

P = 0

S = 0.9

= 0.9 MPa

= 0.7 MPa

= 0.9 MPa


• If the same flaccid cell is placed in a solution with a lower solute concentration

– The cell will gain water and become turgid

Distilled water:

Initial flaccid cell:

Turgid cellat osmotic equilibriumwith its surroundings

P = 0

S = 0.7

P = 0

S = 0

P = 0.7

S = 0.7

Figure 36.6b

= 0.7 MPa

= 0 MPa

= 0 MPa


• Turgor loss in plants causes wilting

– Which can be reversed when the plant is watered

Figure 36.7


Aquaporin Proteins and Water Transport

• Aquaporins

– Are transport proteins in the cell membrane that allow the passage of water

– Do not affect water potential


Bulk Flow in Long-Distance Transport

• In bulk flow

– Movement of fluid in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and sieve tubes

• Roots absorb water and minerals from the soil

• Water and mineral salts from the soil

– Enter the plant through the epidermis of roots and ultimately flow to the shoot system


• Lateral transport of minerals and water in roots

Figure 36.9

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 crossthe plasma membranes of roothairs enter the symplast.

As soil solution moves alongthe apoplast, some water andminerals are transported intothe protoplasts of cells of theepidermis and cortex and thenmove 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 thepassage 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.

Endodermal cells and also parenchyma cells within thevascular cylinder discharge water and minerals into theirwalls (apoplast). The xylem vessels transport the waterand minerals upward into the shoot system.

Casparian strip

Pathway alongapoplast

Pathwaythroughsymplast

Plasmamembrane

Apoplasticroute

Symplasticroute

Root hair

Epidermis Cortex Endodermis Vascular cylinder

Vessels(xylem)

Casparian strip

Endodermal cell

4 5

2

1


The Roles of Root Hairs, Mycorrhizae, and Cortical Cells

• Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located

• Root hairs account for much of the surface area of roots


• Most plants form mutually beneficial relationships with fungi, which facilitate the absorption of water and minerals from the soil

• Roots and fungi form mycorrhizae, symbiotic structures consisting of plant roots united with fungal hyphae

Figure 36.10

2.5 mm


• Once soil solution enters the roots

– The extensive surface area of cortical cell membranes enhances uptake of water and selected minerals


The Endodermis: A Selective Sentry

• The endodermis

– Is the innermost layer of cells in the root cortex

– Surrounds the vascular cylinder and functions as the last checkpoint for the selective passage of minerals from the cortex into the vascular tissue


• Water can cross the cortex

– Via the symplast or apoplast

• The waxy Casparian strip of the endodermal wall

– Blocks apoplastic transfer of minerals from the cortex to the vascular cylinder


• 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


Factors Affecting the Ascent of Xylem Sap

• Xylem sap

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

• 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


Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism

• Water is pulled upward by negative pressure in the xylem

• Water vapor in the airspaces of a leaf

– Diffuses down its water potential gradient and exits the leaf via stomata


• 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.

CuticleUpperepidermis

Mesophyll

Lowerepidermis

CuticleWater vaporCO2 O2 Xylem CO2 O2

Water vaporStoma

Evaporation

At first, the water vapor lost bytranspiration 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 theleaf to the drier air outside via stomata.

Airspace

Cytoplasm

Cell wall

VacuoleEvaporationWater film

Low rate oftranspiration

High rate oftranspiration

Air-waterinterface

Cell wall

Airspace

= –0.15 MPa = –10.00 MPa

3

1 2

Figure 36.12

Air-space


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


• Ascent of xylem sap

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

po

ten

tia

l g

rad

ien

t

Root xylem = – 0.6 MPa

Soil = – 0.3 MPa

MesophyllcellsStoma

Watermolecule

Atmosphere

Transpiration

Xylemcells Adhesion Cell

wall

Cohesion,byhydrogenbonding

Watermolecule

Roothair

Soilparticle

Water

Cohesion and adhesionin the xylem

Water uptakefrom soil Figure 36.13


Xylem Sap Ascent by Bulk Flow: A Review

• The movement of xylem sap against gravity

– Is maintained by the transpiration-cohesion-tension mechanism


• Stomata help regulate the rate of transpiration

• Leaves generally have broad surface areas

– And high surface-to-volume ratios


• Both of these characteristics

– Increase photosynthesis

– Increase water loss through stomata

20 µm

Figure 36.14


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


• 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


• Each stoma is flanked by guard cells

– Which control the diameter of the stoma by changing shape

Cells flaccid/Stoma closedCells turgid/Stoma open

Radially oriented cellulose microfibrils

Cellwall

VacuoleGuard cell

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 increasein length more than width when turgor increases. The two guard cells are attached at their tips, so the increase in length causes buckling.

(a)

Figure 36.15a


• 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+

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.

(b) H2O H2O

H2O

H2OH2O

Figure 36.15b


Xerophyte Adaptations That Reduce Transpiration

• Xerophytes

– Are plants adapted to arid climates

– Have various leaf modifications that reduce the rate of transpiration


• 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 epidermaltissue

Trichomes(“hairs”)

Cuticle Upper epidermal tissue

Stomata 100 m

Figure 36.16


• Organic nutrients are translocated through the phloem

• Translocation

– Is the transport of organic nutrients in the plant


Movement from Sugar Sources to Sugar Sinks

• Phloem sap

– Is an aqueous solution that is mostly sucrose

– Travels from a sugar source to a sugar sink


• A sugar source

– Is a plant organ that is a net producer of sugar, such as mature leaves

• A sugar sink

– Is an organ that is a net consumer or storer of sugar, such as a tuber or bulb


Figure 36.17a

Mesophyll cellCell walls (apoplast)

Plasma membranePlasmodesmata

Companion(transfer) cell

Sieve-tubemember

Mesophyll cell

Phloem parenchyma cell

Bundle-sheath cell

Sucrose manufactured in mesophyll cells can travel via the symplast (blue arrows) to sieve-tube members. In some species, sucrose exits the symplast (red arrow) near sieve tubes and is actively accumulated from the apoplast by sieve-tube members and their companion cells.

(a)

• Sugar must be loaded into sieve-tube members before being exposed to sinks

• In many plant species, sugar moves by symplastic and apoplastic pathways


A chemiosmotic mechanism is responsible forthe active transport of sucrose into companion cells and sieve-tube members. Proton pumps generate an H+ gradient, which drives sucrose accumulation with the help of a cotransport protein that couples sucrose transport to the diffusion of H+ back into the cell.

(b)

High H+ concentration Cotransporter

Protonpump

ATPKey

SucroseApoplast

Symplast

H+ H+

Low H+ concentration

H+

S

S

Figure 36.17b

• In many plants

– Phloem loading requires active transport

• Proton pumping and cotransport of sucrose and H+

– Enable the cells to accumulate sucrose


Vessel(xylem)

H2O

H2O

Sieve tube(phloem)

Source cell(leaf)

Sucrose

H2O

Sink cell(storageroot)

1

Sucrose

Loading of sugar (green dots) into the sieve tube at the source reduces water potential inside the sieve-tube members. This causes the tube to take up water by osmosis.

2

4 3

1

2 This uptake of water generates a positive pressure that forces the sap to flow along the tube.

The pressure is relieved by the unloading of sugar and the consequent loss of water from the tubeat the sink.

3

4 In the case of leaf-to-roottranslocation, xylem recycles water from sinkto source.

Tra

ns

pir

atio

n s

tre

am

Pre

ssu

re f

low

Figure 36.18

Pressure Flow: The Mechanism of Translocation in Angiosperms

• In studying angiosperms

– Researchers have concluded that sap moves through a sieve tube by bulk flow driven by positive pressure


• The pressure flow hypothesis explains why phloem sap always flows from source to sink

• Experiments have built a strong case for pressure flow as the mechanism of translocation in angiosperms

Aphid feeding Stylet in sieve-tubemember

Severed styletexuding sap

Sieve-Tubemember

EXPERIMENT

RESULTS

CONCLUSION

Sap dropletStylet

Sapdroplet

25 m

Sieve-tubemember

To test the pressure flow hypothesis,researchers used aphids that feed on phloem sap. An aphid probes with a hypodermic-like mouthpart called a stylet that penetrates a sieve-tube member. As sieve-tube pressure force-feeds aphids, they can be severed from their stylets, which serve as taps exuding sap for hours. Researchers measured the flow and sugar concentration of sap from stylets at different points between a source and sink.

The closer the stylet was to a sugar source, the faster the sap flowed and the higher was its sugar concentration.

The results of such experiments support the pressure flow hypothesis.Figure 36.19

35 plantstructure text

Documents

Transcript of 35 plantstructure text