Resource Acquisition and Transport in Vascular Plants

Chapter 36

Overview: Underground Plants

• Stone plants (Lithops) are adapted to life in the desert

– Two succulent leaf tips are exposed above ground; the rest of the plant lives below ground

Figure 36.1

• The success of plants depends on their ability to gather and conserve resources from their environment

• The transport of materials is central to the integrated functioning of the whole plant

Adaptations for acquiring resources were key steps in the evolution of vascular plants

• The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water

• Early nonvascular land plants lived in shallow water and had aerial shoots

• Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport

• The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis

• Xylem transports water and minerals from roots to shoots

• Phloem transports photosynthetic products from sources to sinks

Figure 36.2-3

H2Oand minerals

O2

CO2

CO2O2

H2O

Light

Sugar

• Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss

Shoot Architecture and Light Capture

• Stems serve as conduits for water and nutrients and as supporting structures for leaves

• There is generally a positive correlation between water availability and leaf size

• Phyllotaxy, the arrangement of leaves on a stem, is specific to each species

• Most angiosperms have alternate phyllotaxy with leaves arranged in a spiral

• The angle between leaves is 137.5 and likely minimizes shading of lower leaves

Figure 36.3

Buds

1 mm

Shootapicalmeristem

42 29

3421

16

8

1326

18

1031

23 15

2820

12

25

17

22

14

27

40

19

32

113

6

1

9

4

7

2

5

24

• Light absorption is affected by the leaf area index, the ratio of total upper leaf surface of a plant divided by the surface area of land on which it grows

• Self-pruning is the shedding of lower shaded leaves when they respire more than photosynthesize

Figure 36.4

Ground areacovered by plant

Plant ALeaf area 40%of ground area

(leaf area index 0.4)

Plant BLeaf area 80%of ground area

(leaf area index 0.8)

• Leaf orientation affects light absorption• In low-light conditions, horizontal leaves capture

more sunlight• In sunny conditions, vertical leaves are less

damaged by sun and allow light to reach lower leaves

• Shoot height and branching pattern also affect light capture

• There is a trade-off between growing tall and branching

Root Architecture and Acquisition of Water and Minerals

• Soil is a resource mined by the root system• Taproot systems anchor plants and are

characteristic of gymnosperms and eudicots• Root growth can adjust to local conditions

– For example, roots branch more in a pocket of high nitrate than low nitrate

• Roots are less competitive with other roots from the same plant than with roots from different plants

• Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae

• Mutualisms with fungi helped plants colonize land• Mycorrhizal fungi increase the surface area for

absorbing water and minerals, especially phosphate

Roots

Fungus

Figure 36.5

Different mechanisms transport substances over short or long distances

• There are two major pathways through plants– The apoplast

– The symplast

The Apoplast and Symplast: Transport Continuums

• The apoplast consists of everything external to the plasma membrane

• It includes cell walls, extracellular spaces, and the interior of vessel elements and tracheids

• The symplast consists of the cytosol of the living cells in a plant, as well as the plasmodesmata

• Three transport routes for water and solutes are– The apoplastic route, through cell walls and

extracellular spaces

– The symplastic route, through the cytosol

– The transmembrane route, across cell walls

Figure 36.6

Cell wall

Cytosol

Plasmodesma

Plasma membrane

Apoplastic route

Symplastic route

Transmembrane route

Key

Apoplast

Symplast

Short-Distance Transport of Solutes Across Plasma Membranes

• Plasma membrane permeability controls short-distance movement of substances

• Both active and passive transport occur in plants• In plants, membrane potential is established

through pumping H by proton pumps• In animals, membrane potential is established

through pumping Na by sodium-potassium pumps

Figure 36.7CYTOPLASM EXTRACELLULAR FLUID

ATP

Proton pump

Hydrogen ion

H+

(a) H+ and membrane potential

H+

H+

H+

H+

H+

H+

H+

+

+

+

+

+

(b) H+ and cotransport of neutralsolutes

H+/sucrosecotransporter

Sucrose(neutral solute)

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

S

S

S S

S

S

+

+

+

+

+

+

(c) H+ and cotransport of ions

Nitrate

H+NO3

cotransporter

H+

H+H+

H+

H+

H+

H+

H+

H+

H+ H+

NO 3

NO 3

NO3

NO3

NO3

NO3

++

+

+

+

+

(d) Ion channelsPotassium ion

Ion channel

K+

K+

K+

K+

K+

K+

K+

+

+

+

+

+

Figure 36.7a

CYTOPLASM EXTRACELLULAR FLUID

ATP

Proton pump

Hydrogen ion

H+

(a) H+ and membrane potential

H+

H+

H+

H+

H+

H+

H+

+

+

+

+

+

• Plant cells use the energy of H gradients to cotransport other solutes by active transport

Figure 36.7b

(b) H+ and cotransport of neutral solutes

H+/sucrosecotransporter

Sucrose(neutral solute)

H+

H+

H+

H+

H+H+

H+

H+

H+

H+

H+

S

S

S S

S

S

+

+

+

+

+

+

Figure 36.7c

(c) H+ and cotransport of ions

Nitrate

H+NO3

cotransporter

H+

H+H+

H+

H+

H+

H+

H+

H+

H+ H+

NO 3

NO 3

NO3

NO3

NO3

NO3

+

+

+

+

+

+

• Plant cell membranes have ion channels that allow only certain ions to pass

Figure 36.7d

(d) Ion channels

Potassium ion

Ion channel

K+

K+

K+

K+

K+

K+

K+

+

+

+

+

+

Short-Distance Transport of Water Across Plasma Membranes

• To survive, plants must balance water uptake and loss

• Osmosis determines the net uptake or water loss by a cell and is affected by solute concentration and pressure

• Water potential is a measurement that combines the effects of solute concentration and pressure

• Water potential determines the direction of movement of water

• Water flows from regions of higher water potential to regions of lower water potential

• Potential refers to water’s capacity to perform work

• Consider a U-shaped tube where the two arms are separated by a membrane permeable only to water

• Water moves in the direction from higher water potential to lower water potential

Figure 36.8

Solutes have a negative effect on by bindingwater molecules.

Pure water at equilibrium

H2O

Adding solutes to theright arm makes lowerthere, resulting in netmovement of water tothe right arm:

H2O

Pure water

Membrane Solutes

Positive pressure has a positive effect on by pushing water.

Pure water at equilibrium

H2O

H2O

Positivepressure

Applying positivepressure to the right armmakes higher there,resulting in net movementof water to the left arm:

Solutes and positivepressure have opposingeffects on watermovement.

Pure water at equilibrium

H2O

H2O

Positivepressure

Solutes

In this example, the effectof adding solutes isoffset by positivepressure, resulting in nonet movement of water:

Negative pressure(tension) has a negativeeffect on by pullingwater.

Pure water at equilibrium

H2O

H2O

Negativepressure

Applying negativepressure to the right armmakes lower there,resulting in net movementof water to the right arm:

• 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 undergo plasmolysis

• Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall

Water Movement Across Plant Cell Membranes

Turgorpressureand plasmolysis

• If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid

• Turgor loss in plants causes wilting, which can be reversed when the plant is watered

Aquaporins: Facilitating Diffusion of Water

• Aquaporins are transport proteins in the cell membrane that allow the passage of water

• These affect the rate of water movement across the membrane

Long-Distance Transport: The Role of Bulk Flow

• Efficient long distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure

• Water and solutes move together through tracheids and vessel elements of xylem, and sieve-tube elements of phloem

• Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm

Transpiration drives the transport of water and minerals from roots to shoots via the xylem

• Plants can move a large volume of water from their roots to shoots

Absorption of Water and Minerals by Root Cells

• Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water

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

• After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals

• The concentration of essential minerals is greater in the roots than soil because of active transport

• The endodermis is the innermost layer of cells in the root cortex

• It surrounds the vascular cylinder and is the last checkpoint for selective passage of minerals from the cortex into the vascular tissue

Transport of Water and Minerals into the Xylem

• 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 in the apoplast must cross the plasma membrane of an endodermal cell to enter the vascular cylinder

Pathway alongapoplast

Casparian stripEndodermal

cell

Pathwaythroughsymplast

Plasmamembrane Casparian strip

Apoplasticroute

Symplasticroute

Roothair

Epidermis Endodermis

Vessels(xylem)

Vascular cylinder(stele)

Cortex

Figure 36.10

• The endodermis regulates and transports needed minerals from the soil into the xylem

• Water and minerals move from the protoplasts of endodermal cells into their cell walls

• Diffusion and active transport are involved in this movement from symplast to apoplast

• Water and minerals now enter the tracheids and vessel elements

Bulk Flow Transport via the Xylem

• Xylem sap, water and dissolved minerals, is transported from roots to leaves by bulk flow

• The transport of xylem sap involves transpiration, the evaporation of water from a plant’s surface

• Transpired water is replaced as water travels up from the roots

• Is sap pushed up from the roots, or pulled up by the leaves?

Pushing Xylem Sap: Root Pressure

• At night 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

• Root pressure sometimes results in guttation, the exudation of water droplets on tips or edges of leaves

Figure 36.11

• Positive root pressure is relatively weak and is a minor mechanism of xylem bulk flow

Pulling Xylem Sap: The Cohesion-Tension Hypothesis

• According to the cohesion-tension hypothesis, transpiration and water cohesion pull water from shoots to roots

• Xylem sap is normally under negative pressure, or tension

Transpirational Pull

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

• As water evaporates, the air-water interface retreats further into the mesophyll cell walls

• The surface tension of water creates a negative pressure potential

• This negative pressure pulls water in the xylem into the leaf

• The transpirational pull on xylem sap is transmitted from leaves to roots

© 2011 Pearson Education, Inc.

Figure 36.12

Cuticle

Upper epidermis

Mesophyll

Lower epidermis

Cuticle

Xylem

Airspace

Microfibrils incell wall of

mesophyll cell

Microfibril(cross section)

Waterfilm

Air-waterinterface

Stoma

Outside air 100.0 MPa

7.0 MPa

1.0 MPa

0.8 MPa

0.6 MPa

0.3 MPa

Leaf (air spaces)

Leaf (cell walls)

Trunk xylem

Trunk xylem

Soil

Wa

ter

po

ten

tia

l g

rad

ien

t

Xylem sap

Mesophyll cells

Stoma

Water moleculeAtmosphereTranspiration

Xylemcells

Adhesion byhydrogen bonding

Cell wall

Cohesion andadhesion inthe xylem

Cohesion byhydrogen bonding

Water molecule

Root hair

Soil particle

WaterWater uptakefrom soil

Figure 36.13

Figure 36.13a

Water molecule

Root hair

Soil particle

WaterWater uptakefrom soil

Figure 36.13b

Xylemcells

Adhesion byhydrogen bonding

Cell wall

Cohesion andadhesion inthe xylem

Cohesion byhydrogen bonding

Figure 36.13c

Xylem sap

Mesophyll cells

Stoma

Water molecule

AtmosphereTranspiration

Adhesion and Cohesion in the Ascent of Xylem Sap

• Water molecules are attracted to cellulose in xylem cell walls through adhesion

• Adhesion of water molecules to xylem cell walls helps offset the force of gravity

• Water molecules are attracted to each other through cohesion

• Cohesion makes it possible to pull a column of xylem sap

• Thick secondary walls prevent vessel elements and tracheids from collapsing under negative pressure

• Drought stress or freezing can cause cavitation, the formation of a water vapor pocket by a break in the chain of water molecules

Xylem Sap Ascent by Bulk Flow: A Review

• The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism

• Bulk flow is driven by a water potential difference at opposite ends of xylem tissue

• Bulk flow is driven by evaporation and does not require energy from the plant; like photosynthesis it is solar powered

• Bulk flow differs from diffusion– It is driven by differences in pressure potential,

not solute potential

– It occurs in hollow dead cells, not across the membranes of living cells

– It moves the entire solution, not just water or solutes

– It is much faster

The rate of transpiration is regulated by stomata

• Leaves generally have broad surface areas and high surface-to-volume ratios

• These characteristics increase photosynthesis and increase water loss through stomata

• Guard cells help balance water conservation with gas exchange for photosynthesis

Figure 36.14

Stomata: Major Pathways for Water Loss

• About 95% of the water a plant loses escapes through stomata

• Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape

• Stomatal density is under genetic and environmental control

Mechanisms of Stomatal Opening and Closing

• Changes in turgor pressure open and close stomata

– When turgid, guard cells bow outward and the pore between them opens

– When flaccid, guard cells become less bowed and the pore closes

Figure 36.15

Radially orientedcellulose microfibrils

Guard cells turgid/Stoma open

Guard cells flaccid/Stoma closed

Cellwall

VacuoleGuard cell

(a) Changes in guard cell shape and stomatal openingand closing (surface view)

(b) Role of potassium in stomatal opening and closing

K

H2OH2O

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O

Figure 36.15a

Radially orientedcellulose microfibrils

Guard cells turgid/Stoma open

Guard cells flaccid/Stoma closed

Cellwall

VacuoleGuard cell

(a) Changes in guard cell shape and stomatal openingand closing (surface view)

• This results primarily from the reversible uptake and loss of potassium ions (K) by the guard cells

Figure 36.15b

(b) Role of potassium in stomatal opening and closing

K

H2O

H2O

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2O

Guard cells turgid/Stoma open

Guard cells flaccid/Stoma closed

Stimuli for Stomatal Opening and Closing

• Generally, stomata open during the day and close at night to minimize water loss

• Stomatal opening at dawn is triggered by– Light

– CO2 depletion

– An internal “clock” in guard cells

• All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles

• Drought, high temperature, and wind can cause stomata to close during the daytime

• The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata

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 sufficient transport of water, the plant will lose water and wilt

• Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes

Adaptations That Reduce Evaporative Water Loss

• Xerophytes are plants adapted to arid climates

Ocotillo(leafless)

Ocotillo afterheavy rain

Oleander leaf cross section

Oleanderflowers

Ocotillo leaves

Old man cactus

Cuticle Upper epidermal tissue

Trichomes(“hairs”)

Crypt Stoma Lower epidermaltissue

10

0

m

Figure 36.16

• Some desert plants complete their life cycle during the rainy season

• Others have leaf modifications that reduce the rate of transpiration

• Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night

Sugars are transported from sources to sinks via the phloem

• The products of photosynthesis are transported through phloem by the process of translocation

Movement from Sugar Sources to Sugar Sinks

• In angiosperms, sieve-tube elements are the conduits for translocation

• Phloem sap is an aqueous solution that is high in sucrose

• It travels from a sugar source to a sugar sink• A sugar source is an 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

• A storage organ can be both a sugar sink in summer and sugar source in winter

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

• Depending on the species, sugar may move by symplastic or both symplastic and apoplastic pathways

• Companion cells enhance solute movement between the apoplast and symplast

Figure 36.17

Mesophyll cell

Key

Apoplast

Symplast

Cell walls (apoplast)

Plasma membrane

Plasmodesmata

Mesophyll cellBundle-sheath cell

Phloemparenchyma cell

Companion(transfer) cell

Sieve-tubeelement

High H concentration Cotransporter

Protonpump

Sucrose

Low H concentration

H

H H

S

SATP

(a) (b)

• In many plants, phloem loading requires active transport

• Proton pumping and cotransport of sucrose and H+ enable the cells to accumulate sucrose

• At the sink, sugar molecules diffuse from the phloem to sink tissues and are followed by water

Bulk Flow by Positive Pressure: The Mechanism of Translocation in Angiosperms

• Phloem sap moves through a sieve tube by bulk flow driven by positive pressure called pressure flow

Figure 36.18

Loading of sugar

Uptake of water

Unloading of sugar

Water recycled

Source cell(leaf)Vessel

(xylem)

Sieve tube

(phloem)

SucroseH2O

H2O

H2OSucrose

Sink cell(storageroot)

Bu

lk f

low

by

ne

ga

tiv

e p

res

su

re

Bu

lk f

low

by

po

sit

ive

pre

ss

ure

2

1

3

4

2

1

34

• 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

• Self-thinning is the dropping of sugar sinks such as flowers, seeds, or fruits

The symplast is highly dynamic

• The symplast is a living tissue and is responsible for dynamic changes in plant transport processes

Changes in Plasmodesmata

• Plasmodesmata can change in permeability in response to turgor pressure, cytoplasmic calcium levels, or cytoplasmic pH

• Plant viruses can cause plasmodesmata to dilate so viral RNA can pass between cells

Figure 36.20

Plasmodesma Virus particles

Cell wall

100 nm

Phloem: An Information Superhighway

• Phloem is a “superhighway” for systemic transport of macromolecules and viruses

• Systemic communication helps integrate functions of the whole plant

Electrical Signaling in the Phloem

• The phloem allows for rapid electrical communication between widely separated organs

– For example, rapid leaf movements in the sensitive plant (Mimosa pudica)

Figure 36.UN01

Turgid

Figure 36.UN02

Wilted

Figure 36.UN03

MineralsH2O

CO2

CO2

H2O

O2

O2

Figure 36.UN05