Chapter 36

Resource Acquisition and Transport in Vascular Plants

Overview: Underground Plants

• The success of plants depends on their ability to

gather and conserve resources from their

environment

– Many plants have become highly proficient in

acquiring resources that are limited in their

environments

– The transport of these materials is also critical

to the integrated functioning of the whole plant

• Diffusion, active transport, and bulk flow

work together to transfer water, minerals,

and sugars

Concept 36.1: Land plants acquire resources both above and below ground

• The algal ancestors of land plants absorbed water, minerals, and CO2

directly from the surrounding water

– Transport in these algae was relatively simple because each cell was

close to the surface of the organism, and thus the water

– The evolution of xylem and phloem in land plants made possible the

long-distance transport of water, minerals, and products of

photosynthesis to support extensive root

and shoot systems

• Today, evolution has resulting in many

mechanisms for acquiring sunlight,

CO2, and water, while at the same

time minimizing evaporative loss

of water

Fig. 36-2-3

H2O

H2Oand minerals

CO2 O2

O2

CO2

Sugar

Light

Shoot Architecture and Light Capture

• In shoot systems, stems serve as conduits for water and

nutrients, and also act as supporting structures for leaves

– Shoot systems can vary in terms of:

• Their form and arrangement of leaves

– Leaf size (1.3 mm – 20 m)

• The largest leaves are usually found in tropical

rainforests, where water is plentiful and

evaporative loss is not a problem

– Phyllotaxy: the arrangement of leaves on a stem

• The outgrowth of axillary buds

• The relative growth of stem length and thickness

Phyllotaxy

• Phyllotaxy is determined by the shoot apical meristem and is specific to

each species:

– Alternate (spiral) phyllotaxy: one leaf per node

– Opposite phyllotaxy: two leaves per node

– Whorled phyllotaxy: many leaves per node

• Most angiosperms have alternate phyllotaxy, with their leaves arranged in

an ascending spiral around the stem

– This allows each leaf to get the maximum exposure to light and

reduces shading of lower leaves by those above

• In environments where intense

sunlight can harm leaves, the

greater shading provided by

oppositely-arranged leaves may

be more advantageous

• 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

– Leaf area index values of up to 7 are common for many mature plants

– There is little agricultural benefit to leaf area indexes higher than 7

• Adding more leaves increases shading of lower leaves to the point

that they respire more than photosynthesize

• Under these circumstances, nonreproductive leaves or branches

undergo programmed

cell death, known as

self-pruning

Fig. 36-4

Ground area

covered 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 also affects light absorption

– In low-light conditions, horizontal leaves capture sunlight

more effectively than vertical leaves

• In sunny regions, however, horizontal orientation may

expose upper leaves to intense light, resulting in injured

leaves and reduced photosythesis

– If leaves are vertical instead, light rays will essentially be

parallel to the leaf surfaces, so no leaf receives too much

light

• This arrangement also allows light to penetrate more

deeply to the lower leaves

• Other factors also contribute to the ecological success of plants

– Plants have a finite amount of energy they can devote to shoot

growth

• If they put most of their energy into branching, they have less

to devote towards growing tall and are thus at risk of being

shaded by taller plants

• If plants put all their energy into growing tall, they are not

optimally exploiting the resources above-ground

– Plant species also vary in stem thickness

• Most tall plants require thick stems that allow greater

vascular flow to leaves and more mechanical support

• In woody plants, stems become thicker through secondary

growth

Root Architecture and Acquisition of Water and Minerals

• Soil contains resources acquired by the root system

– The evolution of root branching allowed plants to obtain

water and minerals more effectively

• Taproot systems with numerous branches anchor plants

and are characteristic of most trees

• Most monocots do not reach tree-like heights because

their fibrous root systems do not anchor them as

strongly

– Evidence suggests that physiological mechanisms reduce

competition within the root system of a plant

• Ex) Cutting from the same plant develop fewer and

shorter roots in the presence of one another than they

did in the presence of cuttings from a different plant

Root Architecture and Acquisition of Water and Minerals

• The evolution of mutualistic associations between roots and fungi

called mycorrhizae was also a critical step in the successful

colonization of land by plants

– Mycorrhizal hyphae provide the fungus and plants roots with a

large surface area for absorbing water and minerals

• As much as 3 meters of

hyphae can extend from

each centimeter along a

root’s length, allowing

access to a greater

volume of soil than the

root alone could

penetrate

Fig. 36-5

2.5 mm

Concept 36.2: Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow

• Transport begins with the absorption of resources by plant cells

– The movement of substances into and out of cells is

regulated by selective permeability of the plasma

membrane

• Solutes tend to diffuse down their electrochemical gradient, the

combined effect of:

– The solute’s concentration gradient

– The voltage (charge difference) across the membrane

Concept 36.2: Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow

• Diffusion across a membrane is passive, while the pumping of

solutes across a membrane against their electrochemical

gradients is active and requires energy

– Whether or not the process requires energy, most solutes

pass through transport proteins embedded in the cell

membrane

• In some cases, transport proteins selectively bind a

solute on one side and the change shape, releasing the

solute on the opposite side

• Other transport proteins provide selective channels

across the membrane, some of which (gated channels)

open or close in response to stimuli

• The most important transport protein for active transport in plant cells is the

proton pump

– Proton pumps use energy from ATP to pump protons (H+) out of the

cell

– This movement results in an H+ gradient, with a higher H+

concentration outside the cell than inside

• This potential (stored) energy can be harnessed to do work as H+

flows back into the cell

– Movement of H+ out of the cell also makes the inside of the cell

negative in charge relative to the outside

• This charge separation across the membrane contributes to a

voltage called membrane potential

– Membrane potential is

another form of potential

energy that can be

harnessed to do

cellular work

Fig. 36-6

CYTOPLASM EXTRACELLULAR FLUID

ATP

H+

H+

H+

H+

H+

H+

H+

H+

H+

Proton pump

generates mem-

brane potential

and gradient.

+

+

+

+

+

_

_

_

_

_

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

to drive the transport of many different solutes

– Ex) The membrane potential

generated by proton pumps

contributes to the absorption

of K+ by root cells

– Ex) In a form of cotransport, a

transport protein couples the

diffusion of one solute (H+)

with active transport of

another (NO3- or sucrose)

Fig. 36-7

CYTOPLASM EXTRACELLULAR FLUID

K+

K+

K+

K+

K+

K+

K+

Transport protein

_

_

_

_

+

_

+

+

+

+

(a) Membrane potential and cation uptake

NO 3?

NO3?

NO 3? NO

3?

NO3?

NO3

?

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

(b) Cotransport of an anion with H+

H+

_

_

_

_

_

_

+

+

+

+

+

+

H+ H+H+

H+

H+H+

H+

H+

H+

H+

H+ H+

_

_

_

_

_

_

+

+

+

+

+

+

(c) Cotransport of a neutral solute with H+

S

SS

S

S

S

Diffusion of Water (Osmosis)

• To survive, plants must balance water uptake and loss

– The net uptake of water by a cell is determined by the diffusion of water

across the membrane, a process called osmosis

• In animal cells, water will move from the solution with lower solute

concentration to higher solute concentration if the plasma

membrane is impermeable to the solute

• Because a plant cell has almost rigid cell walls, however, the

physical pressure of that wall pushing back against the cell adds

another factor that affects osmosis

– Water potential is a measurement that combines the effects of solute

concentration and pressure, which together determine the direction of

movement of water

• The term water potential refers to water’s potential energy (capacity

to perform work) when it moves from a region of higher water

potential to a region of lower water potential

• Water potential is abbreviated as Ψ (psi) and

measured in units of pressure called megapascals

(MPa)

• Ψ = 0 MPa for pure water at sea level (in a

container open to the atmosphere) and room

temperature

• One Mpa is equal to about 10 atmospheres of

pressure (~ 1 kg pressure/cm2)

• The internal pressure of a plant cell is ~0.5

MPa

How Solutes and Pressure Affect Water Potential

• Both pressure and solute concentration affect water potential:

Ψ = ΨS + ΨP

• The solute potential (ΨS) of a solution is proportional to the number of

dissolved molecules (molarity)

• Solute potential is also called osmotic potential because solutes

affect the direction of osmosis

• By definition, ΨS = 0 for pure water, since there are no dissolved

molecules

• As solutes are added, they bind to water molecules, reducing the

number of free water molecules

• This lowers the capacity of water to move and do work, meaning

that:

• Adding solutes always lowers water potential

• ΨS for a solution is always negative

How Solutes and Pressure Affect Water Potential

• Both pressure and solute concentration affect water potential:

Ψ = ΨS + ΨP

• Pressure potential (ΨP) is the physical pressure on a solution

• Unlike ΨS, ΨP can be positive or negative relative to atmospheric

pressure

• Turgor pressure is the pressure exerted by the plasma membrane

against the cell wall, and the cell wall against the protoplast (the living

part of the cell, including nucleus, cytoplasm, and plasma membrane)

• The contents of the living cell are usually under positive pressure

Measuring Water Potential

• Consider a U-shaped tube where the two arms are separated by a

membrane permeable only to water

• Keep in mind: water moves in the direction from higher water potential

to lower water potential

• The right arm contains 0.1M solution (ΨS = -0.23 MPa)

• The left arm contains pure water (ΨS = 0)

• There is no physical pressure because it is an

open system (ΨP = 0)

• Thus, Ψ = ΨS + ΨP = ΨS + 0 = ΨS =

-0.23 MPa for the right arm

• For the left arm, Ψ = ΨS + ΨP = 0 + 0 =

0 MPa

• Because water moves from regions of lower water

potential, the net water movement will be from the

left arm to the right arm

ψ = ?0.23 MPa

Fig. 36-8a

(a)

0.1 M

solution

Purewater

H2O

ψP = 0

ψS = 0ψP = 0

ψS = ?0.23

ψ = 0 MPa

• Physical pressure increases water potential

– Applying a positive physical pressure of

+0.23 MPa to the solution in the right arm raises

its water potential from a negative value to 0 MPa

Ψ = ΨS + ΨP = -0.23 + 0.23 = 0 MPa

– There is now no net flow of water between

this pressurized solution and the

compartment of pure water

• If we increase ΨP even further to +0.3 MPa in the

right arm, then the solution has a water potential of

+0.07 MPa

Ψ = ΨS + ΨP = -0.23 + 0.3 = +0.7 MPa

– This solution will actually lose water to a

compartment containing pure water

Fig. 36-8b

(b)Positive

pressure

H2O

ψP = 0.23

ψS = ?0.23ψP = 0

ψS = 0

ψ = 0 MPa ψ = 0 MPaFig. 36-8c

ψP =

ψS = ?0.23

(c)

Increasedpositivepressure

H2O

ψ = 0.07 MPa

ψP = 0

ψS = 0

ψ = 0 MPa

0.30

• Negative pressure (tension) decreases water potential

– A negative pressure potential of –0.30 MPa reduces the water potential

of the pure water compartment enough so that water is drawn from the

solution on the right side

Ψ = ΨS + ΨP = 0 + -0.30 = -0.30 MPa

Fig. 36-8d

(d)

Negativepressure(tension)

H2O

ψP = ?0.30

ψS =

ψP =

ψS = ?0.23

ψ = ?0.30 MPa ψ = ?0.23 MPa

0

0

• Water potential also affects uptake and loss of water by plant cells

– If a flaccid (limp) cell (ΨP = 0) is placed in an environment with a

higher solute concentration, the cell will lose water and undergo

plasmolysis (the protoplast shrinks and pulls away from the cell

wall)

Ψ = ΨS + ΨP = -0.9 + 0 = -0.9 MPa (sucrose solution)

Ψ = ΨS + ΨP = -0.7 + 0 = -0.7 MPa (initial flaccid cell)

Video: Plasmolysis

Fig. 36-9a

(a) Initial conditions: cellular ψ > environmental ψ

ψP = 0 ψS = ?0.9

ψP = 0 ψS = ?0.9

ψP = 0ψS = ?0.7

ψ = ?0.9 MPa

ψ = ?0.9 MPa

ψ = ?0.7 MPa0.4 M sucrose solution:

Plasmolyzed cell

Initial flaccid cell:

• If the same flaccid cell is placed in pure water (Ψ = 0), the

contents of the cell becomes turgid, swelling as water enters it

Ψ = ΨS + ΨP = 0 + 0 = 0 MPa (pure water)

Ψ = ΨS + ΨP = -0.7 + 0 = -0.7 MPa (initial flaccid cell)

Video: Turgid Elodea

Fig. 36-9b

ψP = 0ψS = ?0.7

Initial flaccid cell:

Pure water:ψP = 0ψS = 0

ψ = 0 MPa

ψ = ?0.7 MPa

ψP = 0.7ψS = ?0.7

ψ = 0 MPa

Turgid cell

(b) Initial conditions: cellular ψ < environmental ψ

• Turgor loss in plants causes wilting, during which

the leaves and stems droop as a result of water

loss

– Wilting can be reversed when the plant is

watered Fig. 36-10

Aquaporins: Facilitating Diffusion of Water

• Although water molecules are small enough to diffuse

across the phospholipid bilayer, their movement is too

rapid to be explained by passive transport

– Aquaporins are transport proteins in the cell

membrane that facilitate the passage of water

• These selective channels affect the rate at which

water diffuses down its water potential gradient

– The rate of water movement is likely regulated by

phosphorylation of these aquaporin proteins

• Phosphorylation can be induced by increases in

cytoplasmic calcium ions or decreases in

cytoplasmic pH

Three Major Pathways of Transport

• Transport is also regulated by the compartmental structure of plant cells

– The cell wall, cytosol, and vacuole are the 3 main compartments of

most mature plant cells

• Transport proteins in the plasma membrane regulate traffic of

molecules between the cytosol and the cell wall

• Transport proteins in

the vacuolar

membrane regulate

traffic of molecules

between the cytosol

and the vacuole

Fig. 36-11

Cell wall

Cytosol

Vacuole

Plasmodesma Vacuolar membranePlasma membrane

(a) Cell compartments Key

Transmembrane routeApoplast

SymplastApoplast

Symplast

Apoplast

Symplastic route

Apoplastic route

(b) Transport routes between cells

Three Major Pathways of Transport

• Ions can diffuse across tissues entirely through the continuum, known as the

apoplast, formed by cell walls, extracellular spaces, and the dead interiors

of tracheids and vessels

– The plasma membrane directly controls the traffic of molecules into and

out of the protoplast

• It is a barrier between two of the major compartments, the cell wall

and the cytosol

– The continuum formed

from the cytosol of cells

is collectively referred

to as the symplast

– Cytoplasmic channels

called plasmodesmata

connect the cytoplasm

of neighboring cells

Fig. 36-11

Cell wall

Cytosol

Vacuole

Plasmodesma Vacuolar membranePlasma membrane

(a) Cell compartments Key

Transmembrane routeApoplast

SymplastApoplast

Symplast

Apoplast

Symplastic route

Apoplastic route

(b) Transport routes between cells

• The compartmental structure of plant cells provides 3 routes for short-

distance transport within a plant tissue or organ: the apoplastic, symplastic,

and transmembrane routes

– In the apoplastic route, water and solutes move along the continuum of

cell walls and extracellular spaces

– In the symplastic route, water and solutes move along the continuum of

cytosol within a plant tissue

• This route requires only one crossing of a plasma membrane

• After entering one cell, substances can move from cell to cell via

plasmodesmata

– In the transmembrane route, water and solutes move out of one cell,

across the cell wall, and into the neighboring cell

• This route requires

repeated crossings of

plasma membranes as

water and solutes exit one

cell and enter the next

Fig. 36-11

Cell wall

Cytosol

Vacuole

Plasmodesma Vacuolar membranePlasma membrane

(a) Cell compartments Key

Transmembrane routeApoplast

SymplastApoplast

Symplast

Apoplast

Symplastic route

Apoplastic route

(b) Transport routes between cells

Bulk Flow in Long-Distance Transport

• Efficient long distance (roots to stems and leaves) 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

• Bulk flow is also enhanced by the perforation plates at the ends of

vessel elements and the porous sieve plates connecting sieve-tube

elements

• Diffusion, active transport, and bulk flow act together to transport resources

throughout the whole plant

– Ex) Bulk flow due to a pressure difference is how long-distance

transport of sugars occurs from the phloem, but active transport of

sugar at the cellular level maintains this pressure difference

Concept 36.3: Water and minerals are transported from roots to shoots

• Even though they have no pumping mechanism, plants can move a large

volume of water from their roots to shoots

• Most water and mineral absorption occurs near root tips

• Here, the epidermal cell are permeable to water

• In addition, many of these epidermal cells are differentiated into

root hairs that account for much of the absorption of water by roots

• When soil solution (water and dissolved minerals) enters the roots, it first

flows into the hydrophilic walls of the epidermal cells and passes freely

along the extracellular spaces into the root cortex

• The exposure of these cortical cells to soil solution provides a greater

membrane surface area for absorption than the surface area of the

epidermal cells alone

• Active transport then allows roots to concentrate essential

minerals up to 100s of times higher than in the soil

Animation: Transport in Roots

• Before water and minerals can be transported to the rest of the plant, they

must enter the xylem of the vascular cylinder (stele)

• The endodermis (the innermost layer of cells in the root cortex)

surrounds the stele and acts as a last checkpoint for selective passage

of minerals from the cortex into the vascular tissue

• Water and minerals can reach the endodermis of the cortex in one of 2

ways:

• Through the symplast

(cytosolic continuum)

• Through the apoplast

(continuum of cell walls

and extracellular spaces)

Transport of Water and Minerals into the Xylem

Fig. 36-12a

Casparian strip

Plasma

membrane

Apoplastic

route

Symplastic

routeRoot

hair

Epidermis

Cortex

Endodermis

Vessels

(xylem)

Stele

(vascular

cylinder)

• Minerals already in the symplast when they reach the endodermis move

through plasmodesmata of the endodermal cells and into the stele

• Minerals that reach the endodermis via the apoblast, however, meet a

barrier called the Casparian strip, located in the walls of endodermal

cells

• It is made of suberin, a waxy material impervious to water and

dissolved minerals

• The strip forces water

and minerals in the

apoblast to cross the

plasma membrane of

the endodermal cells

and enter the stele via

the symplast

Transport of Water and Minerals into the Xylem

Fig. 36-12

Pathway along

apoplast

Casparian strip

Endodermal cell

Pathway

through

symplast

Casparian strip

Plasma

membrane

Apoplastic

route

Symplastic

routeRoot

hair

Epidermis

Cortex

Endodermis

Vessels

(xylem)

Stele

(vascular

cylinder)

• The endodermis, along with its Casparian strip, ensures that no minerals

can reach the root’s vascular tissue without crossing a selectively permeable

plasma membrane

• It also prevents solutes that have accumulated in the xylem of the stele

from leaking back into the soil solution

• Once in the xylem, endodermal cells discharge water and minerals into their

walls (apoblast), where it can pass into the xylem’s tracheids and vessel

elements

• The xylem vessels then

transport the water and

dissolved minerals upward

to the shoot system by

bulk flow

Fig. 36-12a

Casparian strip

Plasma

membrane

Apoplastic

route

Symplastic

routeRoot

hair

Epidermis

Cortex

Endodermis

Vessels

(xylem)

Stele

(vascular

cylinder)

Bulk Flow Driven by Negative Pressure in the Xylem

• Plants lose a large volume of water from

transpiration, the evaporation of water from a plant’s

surface

– Ex) A single maize plant transpires 60 L of water

during one growing season

• This water is replaced by the bulk flow of water

and minerals, called xylem sap, from the steles

of roots to the stems and leaves

– Is sap mainly pushed up from the roots, or pulled

up by the leaves?

Pushing Xylem Sap: Root Pressure

• At night, when transpiration is very low, root cells continue pumping mineral

ions into the xylem of the stele

– The accumulation of minerals lowers the water potential within the stele

• Because water flow towards areas of lower water potential, water

flows in from the root cortex and generates root pressure, a push

of xylem sap

– Root pressure sometimes results in guttation, the exudation of water

(not dew) droplets on tips or edges of leaves

• In most plants, root pressure

is relatively weak and

therefore only a minor

mechanism driving

xylem sap ascent

Fig. 36-13

Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism

• For the most part, xylem sap is pulled upward

by negative pressure in the xylem of leaves

– Transpiration provides the pull

– Cohesion of water due to hydrogen bonding

transmits this pull along the entire length of

the xylem to the roots

• Transpiration produces negative pressure (tension) in the leaf, which exerts

a pulling force on water in the xylem, pulling water into the leaf

• 1) Water vapor (blue dots) diffuses from moist air spaces of the leaf to

drier air outside via stomata

• This is due to the tendency of water to move toward areas of lower

water potential

• 2) Water vapor lost

by transpiration is

replaced by

evaporation from

the water film that

coats mesophyll

cells

Fig. 36-14

Cuticle Xylem

Upper

epidermis

Mesophyll

Lower

epidermis

Cuticle

Airspace

Microfibrils incell wall of

mesophyll cell

Stoma

Microfibril(cross section)

Waterfilm

Air-waterinterface

• 3) The evaporation of this water film causes the air-water interface to retreat

farther in to the cell wall and to become more curved

• This curvature increases the surface tension and the rate of

transpiration, pulling water from surrounding cells and air spaces

• 4) Water from the xylem is pulled into the surrounding cells and air spaces to

replace the water

that was lost

Fig. 36-14

Cuticle Xylem

Upper

epidermis

Mesophyll

Lower

epidermis

Cuticle

Airspace

Microfibrils incell wall of

mesophyll cell

Stoma

Microfibril(cross section)

Waterfilm

Air-waterinterface

• The transpirational pull on xylem sap is transmitted all the way from the

leaves to the root tips and even into the soil solution

• This pull is also facilitated by cohesion of water molecules to each other

and adhesion of water molecules to cell walls

• Cohesion of water due to H-bonding allows xylem sap to be pulled

from above without water

molecules separating

• Adhesion of water molecules

to the hydrophilic walls of

xylem cells, again due to

H-bonding, also helps

offset the downward force

of gravity

Animation: Transpiration

Animation: Water Transport

Fig. 36-15

Outside air ψ

= ?100.0 Mpa

Leaf ψ (air spaces)

= ?7.0 Mpa

Leaf ψ (cell walls)

= ?1.0 Mpa

Trunk xylem ψ

= ?0.8 Mpa

Trunk xylem ψ

= ?0.6 Mpa

Soil ψ

= ?0.3 Mpa

Xylemsap

Mesophyllcells

StomaStoma

Watermolecule

TranspirationAtmosphere

Adhesionby hydrogenbonding

Cellwall

Xylemcells

Cohesion andadhesion inthe xylem

Cohesionby hydrogenbonding

Watermolecule

Roothair

Soilparticle

WaterWater uptakefrom soil

Wate

r p

ote

nti

al g

rad

ien

t

• The formation of a water vapor pocket, called cavitation, can break the chain

of water molecules and hinder transpirational pull

• Cavitation occurs most often during drought or freezing

• Air bubbles resulting from cavitation expand and block water

channels of the xylem

• Small plants can refill their blocked vessels using root pressure in the

spring

• In trees, however, where root pressure is very weak (due to height),

a water vapor pocket usually can’t function in xylem sap transport

• Water can instead detour around the water vapor pocket

through pits between adjacent tracheids or vessels

• Also, as secondary growth adds a new layer of xylem each

year, the older layer of secondary xylem ceases to transport

water anyways

Xylem Sap Ascent by Bulk Flow: A Review

• The movement of xylem sap against gravity is

maintained by a combination of forces: transpiration,

cohesion, and tension

– Transpiration lowers water potential in leaves, and

this generates negative pressure (tension) that

pulls water up through the xylem

– There is no energy cost to bulk flow of xylem sap

• Absorption of sunlight drives most transpiration

by causing water to evaporate from the moist

cell walls of mesophyll cells

Concept 36.4: Stomata help regulate the rate of transpiration

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

ratios

– These characteristics increase photosynthesis (more light, CO2) but

also increase water loss through stomata (via transpiration)

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

– The waxy cuticle limits water loss through the remaining leaf

surface

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

diameter of the stoma by

changing shape

– By opening and closing

stomata, guard cells

help balance the plant’s

environmental

requirements for

photosynthesis with

conservation of water

Fig. 36-16

Stomata: Major Pathways for Water Loss

• The amount of water lost by a leaf depends largely

on the number of stomata and the average size of

their pores

– Desert plants tend to have lower stomatal

densities than marsh plants

– Environmental condition can also influence

stomatal density

• Ex) High light exposure and low CO2 levels

during leaf development lead to increased

stomatal density in many species

Mechanisms of Stomatal Opening and Closing

• Changes in turgor pressure open and close stomata

– When guard cells are turgid as a result of uptake of water

via osmosis from nearby cells, stoma are open

– When the guard cells lose water and become flaccid, they

become less bowed and the pore closes

Fig. 36-17a

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

Radially orientedcellulose microfibrils

Cellwall

VacuoleGuard cell

(a) Changes in guard cell shape and stomatal opening andclosing (surface view)

Mechanisms of Stomatal Opening and Closing

• Changes in turgor pressure result primarily from the reversible uptake and loss of potassium ions by the guard cells

– Stomata open when guard cells actively transport K+ ions from neighboring epidermal cells

• Energy for transport comes from cotransport of H+ ions (proton pump)

• As the guard cells accumulate K+ ions, this lowers water potential inside them and causes them to take up water and become turgid

– Stomatal closing results from loss of K+ from guard cells to neighboring cells

• This leads to an osmotic loss of water due to increased water potential (less solutes)

Fig. 36-17b

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

(b) Role of potassium in stomatal opening and closing

H2O

H2OH2O

H2OH2O

H2O H2O

H2O

H2O

H2O

K+

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 at least 3 cues:

• Light – stimulates guard cells to accumulate K+ and become turgid

– Triggered by the illumination of blue-light receptors in the plasma

membranes of guard cells

• This stimulates the activity of proton pumps (and in turn,

promotes K+ absorption)

• CO2 depletion as a result of photosynthesis – concentrations of CO2

decrease throughout the day

– This causes stomata to progressively increase in diameter (if

sufficient water is available)

• An internal “clock” in guard cells – ensures that stomata continue their

daily rhythm of opening and closing, even if a plant is kept in the dark

– All eukaryotic organisms have internal clocks; circadian rhythms

are 24-hour cycles

• Environmental stresses (ex: drought) can cause

stomata to close during the daytime

– If plants have a water deficiency guard cells may

lose turgor and close stomata

• In addition, a hormone produced in roots and

leaves in response to drought called abscisic

acid signals guard cell to close stomata

– This response reduces wilting but also

restricts CO2 absorption, thereby slowing

photosynthesis

• As a result, growth ceases, since turgor is

necessary for cell elongation

Effects of Transpiration on Wilting and Leaf Temperature

• Plants lose a large amount of water by transpiration

– Transpiration is greatest on days that are warm, sunny, dry,

and windy because these factors increase evaporation

• If the lost water is not replaced by sufficient transport of

water, the plant will lose water and wilt

– Transpiration can be beneficial on warm days, however,

since it also results in evaporative cooling, which can lower

the temperature of a leaf

• This cooling prevents denaturation of various enzymes

involved in photosynthesis and other metabolic

processes

Adaptations That Reduce Evaporative Water Loss

• Xerophytes are plants adapted to arid climates, like hot and dry deserts or

frozen regions where access to liquid water is problematic

– Many species of desert plants avoid drying out by completing their

short life cycles during the brief rainy seasons

• Longer-lived species have physiological or morphological

adaptations that allow them to withstand harsh desert conditions

– Cacti have highly reduced leaves that resist excessive water

loss, carrying out photosynthesis mainly in their stems

• The stems of these plants are fleshy because they store

water for use during prolonged dry periods

– Some plants use a specialized form of photosynthesis called

crassulacean acid metabolism (CAM) where stomatal gas

exchange occurs at night

• Because stomata remain closed during the day,

evaporative stresses are reduced

Concept 36.5: Sugars are transported from leaves and other sources to sites of use or storage

• Transpiration cannot meet all the long-distance

transport needs of plants

– Transport of sugars from leaves to roots (the

opposite direction of water and mineral

transport) requires another tissue called the

phloem

• The phloem transports these products of

photosynthesis the process of translocation

• In angiosperms, translocation occurs through specialized cells called sieve-

tube elements

– These cells are arranged end-to-end, forming long sieve tubes through

which sap can flow

• Phloem sap consists mainly of sugar (sucrose), along with smaller

amounts of amino acids, hormones, and minerals

– Phloem sap 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 roots, buds, stems, and fruits

– A storage organ, such as a tuber or bulb, can be both a sugar

sink in summer and sugar source in winter

– Sugar sinks usually receive sugar from the nearest sugar sources

• The upper leaves on a branch may export sugar to growing shoot

tips, whereas lower leaves may export sugar to roots

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

exposed to sinks

– Depending on the species, sugar may move by symplastic

(through plasmodesmata) or both symplastic and apoplastic

pathways

• In some plants, the walls of modified companion cells of

sieve-tube elements, known as transfer cells, have

ingrowths that enhance solute transfer between the apoblast

and symplast

Fig. 36-19a

Key

Apoplast

Symplast

Mesophyll cell

Cell walls (apoplast)

Plasma membrane

Plasmodesmata

Companion(transfer) cell

Sieve-tubeelement

Mesophyll cellBundle-sheath cell

Phloemparenchyma cell

• In many plants, phloem loading requires active transport because sucrose is

more concentrated in sieve-tube elements and companion cells than in

mesophyll

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

accumulate sucrose

• Once they reach the sink, sugar molecules diffuse down their concentration

gradient from the phloem to sink tissues

• The concentration of sugar is always lower in the sink because

unloaded is consumed rapidly

during growth and

metabolism or converted to

other polymers like starch

(storage plants)

Fig. 36-19b

High H+ concentration Cotransporter

Protonpump

Low H+ concentration

Sucrose

H+

H+ H+ATP

S

S

Bulk Flow by Positive Pressure: 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, known as pressure flow

– The building of pressure at the source end and reduction

of that pressure at the sink cause water to flow from

source to sink, carrying the sugar along

Animation: Translocation of Phloem Sap in Summer

Animation: Translocation of Phloem Sap in Spring

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

• 1) Loading of sugar (green dots) into the sieve tube at the source reduces

water potential in these cells

– This causes the tube to take up water by osmosis

• 2) The uptake of water by sieve tubes generates a positive pressure that

forces sap to flow along the tube

• 3) This pressure is relieved by the

unloading of sugar, which in turn

causes loss of water at the sink via

osmosis

• 4) The xylem then recycles water from

sink to source

4

Fig. 36-20

3

2

1

1

2

34

Vessel(xylem)

Sieve tube(phloem)

Source cell(leaf) Loading of sugar

Uptake of water

Unloading of sugar

Water recycled

Sink cell(storageroot)

Sucrose

H2O

H2O

Bu

lk f

low

by n

eg

ati

ve p

res

su

re

H2O

Sucrose

Bu

lk f

low

by p

osit

ive p

ressu

re

Concept 36.6: The symplast is highly dynamic

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

plant transport processes

– Plasmodesmata are dynamic in that they can change in permeability

and number

• They can open or close in response to changes in turgor pressure,

cytoplasmic calcium levels, or cytoplasmic pH

• Loss of function is also common during differentiation

– As a leaf matures from a sink (growing leaf) to a source

(mature leaf), its plasmodesmata either close or are eliminated

and phloem loading ceases

– In addition, plant viruses can cause plasmodesmata to dilate

• Plant viruses produce viral movement proteins that cause widening

of these pores to allow viral RNA to pass between cells

Electrical Signaling in the Phloem

• Rapid, long-distance electrical signaling through the phloem is

another dynamic feature of the symplast

– Studies have revealed that a stimulus in one part of a plant

can trigger an electrical signal in the phloem that affects

another part

• This can elicit changes in gene transcription,

respiration, photosynthesis, phloem unloading, or

hormonal levels

– This is similar to the function of nerves in that swift

electrical communication can occur between widely

separated organs

Phloem: An Information Superhighway

• The phloem is also a “superhighway” for systemic

transport of macromolecules and viruses

– Systemic changes are those that spread throughout

the body and affect many or all of the body’s systems

or organs

• In systemic transport, proteins and RNAs enter

sieve tubes through plasmodesmata, helping to

integrate the functions of the whole plant

– Ex) A defensive response to localized infection

in a plant organ sends signals through the

phloem that activate defense genes in

noninfected tissues

You should now be able to:

1. Describe how proton pumps function in

transport of materials across membranes

2. Define the following terms: osmosis, water

potential, flaccid, turgor pressure, turgid

3. Explain how aquaporins affect the rate of

water transport across membranes

4. Describe three routes available for short-

distance transport in plants

5. Relate structure to function in sieve-tube cells,

vessel cells, and tracheid cells

6. Explain how the endodermis functions as a

selective barrier between the root cortex and

vascular cylinder

7. Define and explain guttation

8. Explain this statement: “The ascent of xylem

sap is ultimately solar powered”

9. Describe the role of stomata and discuss

factors that might affect their density and

behavior

10. Trace the path of phloem sap from sugar

source to sugar sink; describe sugar loading

and unloading