Transport in Plant - transportinfloweringplants

Transcription

Hwa Chong Institution
Sec3 (SMTP) Biology
Sec3 SMTP
Prepared by Mr Tan Kaiyuan
CAA 110409
Name: _____________________________________
Class: 3SMTP____
Date: ______________________
INTERNET RESOURCES
Grey with Anatomy? Check these sites out:
1. http://iweb.tntech.edu/mcaprio/stems.htm
2. http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantsStructure/MatureRoot/
MatureRoot.htm
3. http://sols.unlv.edu/Schulte/Anatomy/
The above three sites show some light microscope photographs of various plant structures.
4. http://www.biology.iastate.edu/Courses/212L/New%20Site/26Leaf&Ps/%20LeavesandPs.htm
Forgot about the earlier chapter? Here’s a good site for revision.
For the Intermediate Learner
5. http://leavingbio.net/TRANSPORT%20OF%20MATERIALS%20IN%20A%20FLOWERING%2
0PLANT.htm
A comprehensive website on plant transport with clear diagrams.
6. http://croptechnology.unl.edu/animationOut.cgi?anim_name=transpiration.swf
Very interactive animation that allows you to adjust parameters of transport in plants. Must see!
7. http://www.vcbio.science.ru.nl/public/pdf/leaves_eng.pdf
Here’s a website to tell you more about leaf adaptations to varying water availability.
CHAPTER MAP & OVERVIEW
Transport in Plant
6.1 Stem & Root
Structure & Function
6.1.1 Vascular Bundle
Structure &
Function
6.1.2 Stem Internal
Structure
6.1.3 Root Internal
Structure
6.1.4 Leaf Internal
Structure in
Relation to
Vascular Bundle
Arrangements
6.2 Transport of Water
& Minerals
6.2.1
6.2.2
6.2.3
6.2.4
6.3 Transports in
Phloem
Water Potential
Overview of Water Movement
Water Movement in Roots
Mechanism of Water & Mineral
Transport in Stems
i) Root Pressure
ii) Capillary Action
iii) Transpiration & Transpiration
Pull
6.2.5 Water Movement in Leaves
6.2.6 Factors Influencing Water
Movement and Water Loss
6.2.7 Wilting
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6.4 Water Relations &
Leaf Adaptation
6.3.1 Pressure Flow
Hypothesis
6.3.2 Evidence for
Sucrose
Translocation
6.4.1 Hydrophytic
Leaves
6.4.2 Xerophytic
Leaves
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STEM AND ROOT – STRUCTURE AND FUNCTION
Section Learning Objectives
At the end of this Section, you should be able to:
(1) Identify the positions and explain the functions of xylem vessels, phloem (sieve tube
elements and companion cells), in transverse and longitudinal sections of
unthickened, herbaceous dicotyledonous root and stem, under the light microscope.
(2) *Recognize xylem vessels, sieve tubes and companion cells in transverse and
longitudinal sections of monocotyledonous root and stem
(3) Draw plan diagrams of tissues (including a transverse section of a dicotyledonous
leaf)
(4) Describe adaptations of xylem and phloem tissue in transport
(5) *Identify different zones in a longitudinal section of the root: root cap, zones of
division, elongation and maturation / differentiation where root hairs are found
(6) Identify and state the function of various parts in a transverse section of the root:
piliferous layer, cortex, stele (vascular cylinder)
(7) Relate the structure and functions of root hairs to their surface area, and to water
and ion uptake
INTRODUCTION
In the previous chapter, we examine the
key biological process that drives most
part of life on Earth – photosynthesis. In
this chapter we shall look at how the raw
materials
and
products
of
plant
metabolism may be distributed throughout
the plant body. Delivery of these materials
to their target organ requires that the plant
body possesses an efficient transport
system. Figure 6.1 on the right provides a
general overview of the transport system
in a typical dicotyledonous plant. It
illustrates
the
two
vessel
types
responsible for movement of materials,
known as xylem and phloem. These
vessel types each possess a specific
function, and together they form the
vascular bundle. In the first part of this
chapter, we shall examine the structural
adaptations and anatomical features of
the vascular bundle tissues in details.
Following that, we shall look at how these
Figure 6.1 Overview of vascular system in a typical dicotyledonous
plant. Image from
http://www.southtexascollege.edu/nilsson/4_GB_Lecture_figs_f/4_G
B_22_Plantae_Fig_f/Vascular_Bundles.GIF
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vascular bundles are arranged in the roots, stem and leaves, and compare these
arrangements between dicots and monocots.
Transport of materials in plants also depends on the universal solvent – water. In the second
part of this chapter, we shall discuss the mechanisms involve in transport in plants. Plants,
unlike most animals, lack an actively pumping heart. We shall see how the mechanics of
water movement may replace the critical function of the heart. At the end of the chapter we will
also discuss how leaves may be modified to conserve water in harsh environments.
6.1.1 Vascular Bundle – Structure & Function
• The vascular bundles serve the critical function of transporting metabolites between
different organs. Before we examine how these vascular bundles are arranged
anatomically in the roots, stems and leaves, let us first look at the structural
adaptations of the xylem and phloem vessels
• Recall in the previous chapter that vascular bundles contain complex tissues derived
from the vascular tissue system.
• There are two types of complex tissues in vascular bundles, namely, xylem tissue
and phloem tissue.
1) Xylem Vessels
• Comprises long hollow tubes of xylem vessels that were formed from individual
vessel elements and tracheids.
• Vessel elements and tracheids are derived from dead cells.
• Contain deposits of lignin on the inner walls of the vessels, which makes them
rigid and mechanically strong.
A
B
E
C
D
F
Figure 6.2 (A) Light microscope image of tracheids in a softwood
gymnosperm, and (B) false colour image scanning of tracheids in the white pine;
(E) and (F) Lignified xylems in longitudinal sections of (Helianthus annuus);
Diagrammatic representation of (E) tracheids and (F) vessel elements. (A) taken
from http://www.steve.gb.com/science/plant_growth.html; (B) taken from
http://www.britannica.com/EBchecked/topic-art/647253/55376/Tracheidsfrom-white-pine-shown-in-a-false-colour-scanning; Images (E) and (F) from
http://plantscienceimages.org.uk/pages/image.aspx?sectionId=2&subsectionId=
35&imageId=174. Images (E) and (F) taken from Introductory Plant Biology, 10th
ed. (Stern, 2006).
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• Lignin = a complex macromolecule that usually makes up most of the secondary
cell wall in plant cells.
• Lignin has high tensile strength, thus allowing the xylem vessels to sustain
large mechanical forces without collapsing.
• Lignin is also hydrophobic and impermeable to water. Thus preventing water
from migrating out of the xylem vessels where they are lignified.
• Xylem vessels also possess pits. These are small unlignified parts along a
vessel element of trachied that facilitate the lateral movement of water and
solutes should the path within a particular vessel become obstructed.
2) Phloem Vessels
• Comprises individual sieve tube element and companion cells.
• Sieve tube elements join end to end, separated by a perforated cross-wall,
known as a sieve plate.
• Each sieve tube element has degenerate protoplasm (ie. lacks nucleus,
vacuole and most of the organelles), and has a thin cytoplasm that is continuous
from one sieve tube element to another through the sieve plates.
• Companion cells which possess numerous mitochondria provide source of
energy and maintains metabolic activities for sieve tube elements.
A
B
Figure 6.3 (A) Longitudinal section of part of the phloem of a
black locust tree (Robinia pseudo-acacia) (B) Sieve plate.
Images (A) taken from Introductory Plant Biology, 10th ed.
(Stern, 2006); Image (C) taken from www.biologie.unihamburg.de
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The table below compares the structure and function of xylem and phloem tissues:
Tissue
Cell Types
•
Vessel
elements
•
Tracheids
Xylem
Characteristics
•
Dead cells which form a long,
narrow hollow tube.
1.
Narrowness increases capillarity in
the xylem vessels.
•
No protoplasm or cross-walls in
the lumen of the hollow tube.
2.
Reduce resistance to water flow in
xylem so that it may be an
uninterrupted, continuous stream.
•
Presence of lignin deposited in
rings or spirals along the inner
walls of xylem vessels.
Lignin has high tensile strength.
Lignin is hydrophobic and
impermeable.
3.
Prevents collapse of vessels.
4.
Provides mechanical support.
5.
Prevents escape of water en route.
6.
In events where a single vessel may
be capacitated (ie. air bubble
trapped within), lateral transfer of
water may occur between xylem
vessels through pits.
•
•
•
Sieve tube
members
•
Presence of pits along xylem
vessel elements and tracheids.
•
Cross-wall between consecutive
sieve tube members has
numerous minute pores and
resembles a sieve. Hence, known
as sieve plate.
Mature cells possess thin layer of
cytoplasm which is continuous
from one sieve tube member to
the next, passing through the
pores in the sieve plates.
1.
Holes in sieve plates allow rapid
flow of manufactured food through
sieve tubes.
•
Sieve plates may also secrete
callose, when damaged, plugging
the pores of the sieve tube.
2.
Protective function, to minimise loss
of photosynthates nutrients and
other organic compounds.
•
Degenerate protoplasm – ie. Lost
most organelles, central vacuole
and nucleus, hence is dependent
on companion cells.
3.
Reduce resistance to phloem sap
Narrow, thin-walled cell with
numerous mitochondria,
cytoplasm.
4.
Presence of mitochondria provides
energy needed for companion cells
to help load sugars from mesophyll
cells into sieve tube elements by
active transport.
Posses a prominent nucleus.
5.
Nucleus to coordinate cellular
activities, especially in the sieve
tube element
•
Phloem
•
Function
Companion •
cells
•
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6.1.2 Stem Internal Structure
• In stems, xylem and phloem vessels are organised such that they form closely
associated bundles, known as vascular bundles.
• Vascular bundles are organised differently in dicots and monocots. Figure 6.4 and 6.5
illustrate the differences.
A
B
Ground
tissue
C
Figure 6.4
Light microscope cross-section of a monocot,
corn (Zea mays) stem, showing the scattering of vascular
bundles in the ground tissue (A); Diagrammatic
representation (B). Close up of one individual vascular
bundle (C). Images (A) and (C) taken from Biology 7th ed.
(Solomon, Berg and Martin, 2005). Image (B) taken from
http://www.uic.edu/classes/bios/bios100/labs/corna3.gif
Vascular
Bundles
Epidermis
Xylem
Cambium
Phloem
Pith
Ground Tissue
Epidermis
A
B
C
Figure 6.5
Cross section of a dicot stem, showing the ring arrangement of vascular bundles in the
ground tissue (A); Diagrammatic representation of a dicot stem (B); Close up of an individual vascular
bundles (C). Notice that the xylem vessels are directed towards the pith, while the phloem is nearer to the
epidermis. Images (A) and (C) taken from Biology 7th ed. (Solomon, Berg and Martin, 2005). Image (C) taken
from Biology Matters (Lam and Lam, 2007).
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6.1.3 Root Internal Structure and Functions
• Figure 6.7 depicts the internal structure of a typical dicot root. For the purpose of our
discussion, we shall not be examining in details the internal structure of monocot roots. A
light microscope photograph of a monocot root is provided in “Chapter 5: Plant Nutrition”
Figure 5.8 (A) for reference and comparison.
(1) Longitudinal Zoning of a Dicot Root Tip
Four distinct zones may be identified
on a typical dicot root tip:
Root Cap
• A
thimble-shaped
mass
of
parenchyma cells that surrounds the
delicate tissues involve in active cell
division.
• Serves to protect the apical
meristem.
• Also sloughs off to form a
mucilaginous
lubricant
which
facilitates the root tips movement
through the soil.
• The lubricant also attracts beneficial
bacteria involve in supplying nitrates
to the plant.
Figure 6.6 Longitudinal section of a typical dicot root
Zone of Cell Division
tip. Image taken from Biology 7th ed. (Solomon,
Berg and Martin, 2005)
• Comprises an apical meristem,
responsible for active cell division in
this region. Daughter cells however, do not expand and elongate until they mature
into the zone of elongation.
• Depends on the zone of elongation for the force to push through the substrate.
Zone of Elongation
• Cells become several times their original length in this region.
• Elongation in length pushes the root cap and apical meristem through the soil.
• Only the apical meristem and root cap are actually pushing through the soil; once
cells in the zone of elongation mature, they remain stationary for the rest of the
plant’s life.
• Tiny vacuoles of each cell begin merging to form one central vacuole.
Zone of Maturation (aka Zone of Differentiation or Zone of Root Hairs)
• Epidermal cells develop protuberances known as root hairs, which absorb water
and minerals, and adhere tightly to soil particles.
• Root hairs greatly increase the surface area to volume ration for efficient absorption
of water and minerals.
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Radial Arrangement (From centre axis to epidermis)
• The xylem and phloem vessels in a typical dicot root are grouped into a central
vascular cylinder or stele.
• The xylem vessels are arranged in the centre and may form extensions known as
“xylem arms”. Between the xylem arms, are phloem tissues.
• The xylem and phloem tissues are surrounded by a single layer of parenchyma cells
known as the pericycle, which are responsible for producing lateral roots by cell
division.
B
A
Piliferous layer
Cortex
Endodermis
Phloem
Xylem Arms
Pericycle
D
C
Figure 6.7 Cross-sections of the roots of a dicot. (A) – Buttercup (Ranunculus sp.)
root. (B) – Close up of the stele in the Buttercup root. Xylem elements with the
“xylem arms” are visible. Phloem cells are localised in patches between the “xylem
arms”. (C) and (D) – Diagrammatic representation of dicot root in cross-section
(C) and longitudinal section (D). Images (A) and (B) taken from Biology 7th ed.
taken
from
(Solomon,
Berg
and
Martin,
2005).
Image
(C)
http://www.soilandhealth.org/01aglibrary/010139fieldcroproots/010139ch2.html
; (D) from Biology Matters (Lam and Lam, 2007).
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• Surrounding the pericycle is another single layer of cells, known as the endodermis.
The endodermis is structurally unique from all other cells in that they possess a band
of fatty material known as the Casparian strip. The presence of the Casparian strip
is the key to regulating the ions which enter the xylem.
• Between the endodermis, to the epidermis, is a large region of ground tissue
consisting primarily of loosely packed parenchyma cells, known as the cortex. These
cells contain abundance of starch granules. There are also numerous intercellular
spaces between cortex cells.
• The epidermis also known as the piliferous layer consists of epidermal cells with
protuberances known as root hairs. As described above, root hairs aid in water
absorption.
Figure 6.8 Diagram representing the endodermis and position of the Casparian strip. Water movements are
represented by broad arrows. Image taken from Biology 7th ed. (Solomon, Berg and Martin, 2005).
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• The table below summarises the various root structures and their functions:
Structure
Root Cap
Characteristics
•
Thimble-shaped structure
surrounding root apical meristem.
•
Consists of large parenchyma
cells.
1.
Protects the delicate root apical
meristem as root pushes
through soil.
2.
Lubricates root for movement.
3.
Attracts beneficial bacterial.
4.
Involved in gravitropism
•
Consists of epidermal cells with
long protuberances known as root
hairs.
1.
Increases the root surface area
to volume ratio for effective
absorption of water and
minerals.
•
Parenchyma cells of cortex
contains abundance of
amyloplasts (starch storing
organelles)
1.
Storage of starch produced
from photosynthesis.
•
Possess Casparian strip, which
consists a band of fatty material
(known as suberin) deposited
along the lateral and radial cell
walls of the endodermis cells.
1.
Regulates the flow of ions and
water into the stele.
•
A single-layer of parenchyma
cells.
1.
Will give rise to lateral roots by
cell division and penetrate
through the endodermis, cortex
and epidermis.
•
Consists of sieve tube elements
that possess degenerate
protoplasm.
1.
Involves in the bidirectional
transport of metabolites.
2.
Companion cell with mitochondria
and nucleus.
Degenerate protoplasm reduces
resistance to material flow.
3.
Companion cell provides energy
for cellular activities of the sieve
tube elements.
Piliferous Layer
(Epidermis)
Cortex
Endodermis
Pericycle
Stele
Phloem
Function
•
•
Consists of hollow vessels formed
by dead cells known as tracheids
or vessel elements.
1.
Hollow tube reduces resistance
to water and dissolved mineral
flow upwards.
•
Possess spirals or rings of lignin
on inner walls
2.
Provide mechanical support.
Xylem
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6.1.4 Leaf Internal Structure (in relation to Vascular Bundle Arrangements)
A
B
Legend:
Figure 6.9
(A) Transverse section of
a monocot corn (Zea mays) leaf and a
dicot lilac (Syringa vulgaris) leaf,
showing the arrangement of the vascular
bundles with respect to the mesophyll
layer. Image (A) taken from Biology 7th
ed. (Solomon, Berg and Martin, 2005);
(B) from
http://www.vcbio.science.ru.nl/en/imagegallery/show/labels/print/PL0130/.
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1 upper epidermis
3 spongy parenchyma
4 air cavity
5 lower epidermis
7 trichome
8 major vein
9 xylem
2 palisade parenchyma
6 stomata
10 phloem
11 supporting tissue (sclerenchyma)
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TRANSPORT OF WATER & MINERALS
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
(17)
Explain the movement of water between plant cells, and between them and the
environment in terms of water potential (Calculations on water potential is not
required.)
State that most mineral salts are absorbed via active transport by the root hair
cells
*Outline the three pathways of movement of water through plant cells: apoplast,
symplast and vacuolar pathways
Outline the pathway by which water is transported from the roots to the leaves
through the xylem vessels
Describe capillarity and root pressure in the transport of water up a plant
Define the term transpiration and explain that transpiration is a consequence of
gaseous exchange in plants, in which the loss of water vapour from the stomata is
inevitable
Explain the movement of water through the stem in terms of transpiration pull
Describe the effects of variation of temperature, humidity, wind speed and light
intensity on transpiration rate:
Describe how water vapour loss is related to cell surfaces, air spaces and stomata
Describe how wilting occurs.
6.2.1 Water Potential, Ψ
• Water moves into plants, in the case of terrestrial plants mainly from the soil; and water
moves out of plants, mainly into the atmosphere. There is also much movement of water
within plants. Movement implies the involvement of energy.
• Water movement, too, is driven by energy levels. Water will move from a system or area of
higher free energy, to a system of lower free energy. In order to predict the direction of
movement of water into/put of plants, plants cells or tissues, we therefore need a measure
for the free energy of water. This measure is known as the water potential.
Osmosis is the movement of water molecules from a region of less negative
water potential to a region of more negative water potential, across a partially
permeable membrane.
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6.2.2 Overview of Water Movements
• Diagram below provides an overview of the movement of water from the roots, through the
stem and eventually to the leaves, before exiting through stomata.
Figure 6.10
(A) Transverse section of a monocot (Zea mays) leaf. Image taken from Biology 7th ed.
(Solomon, Berg and Martin, 2005).
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6.2.3 Water Movement in Plant Root Cells
(occurs in all other tissues except endodermal cells)
• There are 3 routes through which water moves in the roots (and other cells except
endodermal cells):
(1)
Apoplast pathway (cell walls)
• This pathway consists of the interconnected permeable cellulose cell walls of
adjacent cells forming a continuous system. Through this pathway, water passes
freely through cellulose cell walls from one cell to another. Movement of water
through this route could be an entirely passive process resulting from the tension
created by transpiration. As water is pulled up the xylem, the cohesive forces
between water molecules would ensure that water is drawn across adjacent cell
walls.
Figure 6.11
Pathways of water and dissolved mineral salts. Image taken from Biology 7th ed. (Solomon,
Berg and Martin, 2005) and http://www.bio.miami.edu/dana/pix/water_pathways.jpg
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(2)
•
•
•
(3)
•
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Symplast pathway
(cytoplasm via
plasmodesmata)
In the symplast pathway,
water moves by osmosis
down a water potential
gradient
through
the
cytoplasm of adjacent
cells.
The
cytoplasm
of
adjacent
cells
is
interconnected
by
cytoplasmic
strands
called plasmodesmata
which pass through the
pores in the cellulose cell
walls.
The plasmodesmata and
cytoplasm
form
a
continuous pathway for
water movement.
Vacuolar pathway
(vacuole to vacuole)
In this pathway, water
moves through the same
water potential gradient
as in the symplast
pathway, but through the
vacuoles as well as the
cytoplasm.
• Whatever route it takes to get
there, once water reaches the
endodermis, it is forced to go
through the living parts of the
cell.
The
impermeable
Casparian strip (Figure 6.8)
prevents
water
and
its
dissolved mineral salts from
entering the xylem via the
cellulose cell walls.
Figure 6.12
3 Pathways of water movement in the root.
Images taken from Advanced Biology (Michael Kent, 2000).
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6.2.4 Mechanism of Water and Mineral Transport
• Three key processes are involved with the movement of water and mineral salts up the
xylem. These are namely: transpiration, capillary action and root pressure.
(1) Transpiration and Transpiration Pull
• Transpiration is a process involving
the loss of water vapour from the
aerial parts (ie. parts above the
ground) of a plant, especially through
the stomata.
• Water vapour evaporates from the thin
film of water on the surface of mesophyll
cells into the intercellular air spaces. This
then diffuses into the atmosphere via the
stomata.
• Water
is
also
consumed
by
photosynthesis in the mesophyll cells.
• This results in a low hydrostatic
pressure in the leaves, and draws more
water from the xylem vessels into the
leaves, through pits in the vessel
elements and tracheids.
• Water thus migrates by osmosis down a
water potential gradient from the xylem
vessels into these mesophyll cells.
• This occurs through the three pathways
of water movement in plant tissues
(apoplast,
symplast
and
vacuolar
pathways).
• In the roots, water is drawn into the
xylem, creating a high hydrostatic
pressure. The difference in hydrostatic
pressure, thus forces water to move
upwards along the stem.
• The movement of water in the xylem
occurs by bulk flow or mass flow, which
carries the entire mass of water and its
solutes upwards into the leaves.
• This forms a continuously flowing stream
of water from the deepest roots to the
tallest parts of a plant, known as
transpiration stream.
• The transpiration stream is critical in
maintaining bulk flow as it transmits the
suction force produced by the differing
hydrostatic pressure between the roots
A
B
Figure 6.13
(A) – A representation of
transpiration stream in a plant. (B) – Experimental
set-up used to measure the rate of transpiration
using a photometer. Images taken from Biology 7th
ed. (Solomon, Berg and Martin, 2005). Image (B)
taken from Biology Matters (Lam and Lam, 2007).
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•
•
•
•
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and the leaves through the stream.
Intermolecular forces of attraction between water molecules in the xylem, known as
cohesive forces (the phenomenon being known as cohesion), are responsible for
transmitting this suction force from one water molecule to the next.
In situations when the flow of water in the xylem is disrupted by the presence of an air
bubble (a phenomenon known as cavitation), cohesive forces cannot act on water streams
separated by the air bubble. Consequently, water may not be drawn up effectively.
In addition, cohesion of water molecules also generates tensile strength or tension
within the water column (analogous to a spring, only that the tension of water also acts
inwards, towards the main axis of the column). This adds on to the upward pulling forces
acting on the transpiration stream.
The cohesive and tensional forces are so strong that xylem vessels must be mechanically
rigid enough to prevent inward collapse.
Driven by the differences in hydrostatic pressure between the leaves and the roots, and
maintain by cohesive and tensional forces in the transpiration stream, this main mechanism
responsible for the transport of water and its solutes in the phloem is known as
transpiration pull.
Importance of transpiration
1. Transpiration pull draws water and dissolved mineral salts from the roots to the stems
and leaves.
2. Evaporation of water from the cells in the leaves removes latent heat of vaporisation
and cools plant from the intense heat of the Sun.
3. Water transported to the leaves can be used in photosynthesis; to keep cells turgid;
and to replace water lost by the cells. Turgid cells keep the leaves spread out widely to
trap sunlight for photosynthesis.
(2) Root Pressure
• Root pressure is first established when root epidermal cells actively transport mineral ions
into their cytoplasm.
• This results in a more negative water potential in the root cells, and creates a water
potential gradient between the substrate and the cytoplasm.
• Water molecules are then drawn into the root cells from the substrate by osmosis, and
migrate into the xylem as it passes through the root cells via the three pathways (apoplast,
symplast and vacuolar pathways).
• The drawing of water into the roots by the above mechanism creates a root pressure that
contributes to the upward movement of water in terrestrial plants.
• Root pressure complements transpiration pull in moving water against gravity and up the
stems. It alone is insufficient to drive the movement of water towards the leaves.
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(3) Capillary action (Capillarity)
• Water tends to move up inside
very narrow tubes (capillary tubes)
due to the attractive interactions
between
water
molecules
(cohesion) and the surfaces of the
tubes (adhesion).
• The combine influence of cohesion
and adhesion produces an effect
known as capillary action.
• Capillary action in xylem vessels
aid in drawing water upwards
Figure 6.14
(A) – Capillarity in narrow tubes.
owing to the narrowness of the
The smaller the tube diameter, the greater the rise
xylem tubes.
in fluid. Image (A) taken from Biology 7th ed.
• In small plants, the role of capillary
(Solomon, Berg and Martin, 2005).
action may be significant.
• Like root pressure, it alone cannot
account for water rising up a tall tree, and complements transpiration pull in driving water
upwards.
The transport of minerals occurs primarily by two processes: (i) active transport and (ii)
diffusion.
(4) Mineral Transport
• Mineral transport in plants occurs first in the roots by active transport, and subsequently
by diffusion in the rest of the plant tissues.
• Active transport occurs across the cell membranes of the root epidermal cells, moving
minerals in the soil into the cytoplasm of the root epidermal cells.
• This acts against a concentration gradient of dissolve minerals, where the concentration of
dissolved minerals is higher in the cytoplasm and lower in the substrate.
• The dissolved minerals then pass into the stele of the roots, via the parenchyma cells in the
root cortex, through the endodermis and pericycle. This is facilitated by a series of ion
channels.
• Once in the xylem, dissolved minerals move by bulk flow or mass flow with the
transpiration stream into the other plant tissues.
• Upon reaching the other plant tissues, diffusion of these mineral ions from the xylem into
the plant cells occur down a concentration gradient between the higher concentration of
minerals in the xylem and the lower concentration of mineral ions in the cytoplasm.
18
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6.2.5 Water Movement in Leaves
Figure 6.15
Diagram representing the flow of water in the cross-section of a leaf. Image taken from
Biology Matters (Lam and Lam, 2007).
• As transpiration occurs mainly through the stomata, it is linked to gaseous exchange
between the plant and the environment. In daylight, stomata open to allow carbon dioxide
to diffuse into the leaf for photosynthesis. Oxygen and water vapour are more concentrated
in the intercellular air spaces, so they diffuse out of the leaf through the stomata. Therefore,
the plant will necessarily lose water. In other words, transpiration is an essential part of
photosynthesis.
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6.2.6 Factors Influencing Water Movement and Water Loss
• Transpiration is affected by evaporation. Therefore, any factor that affects the rate of
evaporation of water will affect the rate of transpiration. External factors that influence
the rate of transpiration are humidity, air, movement, temperature and light.
• The table below summarises these factors and their influence on transpiration.
• We shall examine 4 of the external factors in the following page.
Type
Factor
Light
Humidity
External
Internal
Influence
Transpiration Rate
Stomata open in the presence of
light and closes in the dark.
Affects diffusion gradient between
the air spaces in the leaf and the
atmosphere.
increased if
decreased if
Higher light
intensity
Lower light
intensity
Lower humidity
Higher humidity
Wind Speed
Changes the diffusion gradient by
altering the rate at which most air is
removed from around the leaf.
Higher wind
speed
Lower wind
speed
Temperature
Affects the kinetic energy of the
water molecules and the humidity
of the air.
Higher
temperature
Lower
temperature
Water
Availability
Influences water potential gradient
between soil and the leaf.
Wetter soil
Drier soil
Leaf Area
Some water is lost over the surface
area of the leaf.
Larger leaf area
Smaller leaf area
Cuticle
Forms a waterproofing layer on the
surface of the leaf.
Thinner cuticle
Thicker cuticle
Number of
Stomata
Most water is lost by evaporation
through the stomata.
More stomata
Less stomata
Distribution
of Stomata
Upper surface is more exposed to
the environmental factors that
increases the rate of transpiration
(eg. greater transpiration rate on
leaves with stomata distributed
mainly on upper leaf surface than
lower leaf surface)
More stomata on
upper surface
More stomata on
lower surface
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(1) Humidity of the air
• The intercellular air spaces
in the leaf are normally
saturated with water vapour.
• There is a water vapour
concentration
gradient
between the leaf and the
atmosphere.
• The drier or less humid the
air outside the leaf, the
steeper is this concentration
gradient, thus the rate of
transpiration will be faster.
• Increasing the humidity of
the air outside will decrease
the
water
vapour
concentration
gradient
between the leaf and the
atmosphere.
• Thus the rate of transpiration
will decrease.
(2) Wind or air movement
• Wind blows away the layer
of stationary air (i.e. diffusion
shell) – that contains
accumulated water vapour –
outside the stomata (Figure
6.15).
Figure 6.16 Diffusion of water from mesophyll cells out
• This maintains the water
of the leaf. Image from Advanced Biology (Michael Kent,
vapour
concentration
2000).
gradient between the leaf
and the atmosphere.
• Thus, the stronger the wind, the faster is the rate of transpiration. In still air, the water
vapour that diffuses out of the leaf makes the air around the leaf more humid. This
decreases the rate of transpiration.
(3) Temperature of the air
• Assuming that other factors remain constant, a rise in the temperature of the surroundings
increases the rate of evaporation. Thus the rate of transpiration is greater at higher
temperatures.
(4) Light
• Light affects the size of the stomata on the leaf and consequently, the rate of transpiration.
Specifically, the stomata open and become wider in sunlight, increasing the rate of
transpiration. Conversely, in darkness, the stomata close and less water is lost from the
leaf.
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6.2.7 Wilting
• The turgor pressure in
the leaf mesophyll cells
helps to support the leaf
and keep it firm and
spread out widely to
absorb
sunlight
for
photosynthesis.
• However,
in
strong
sunlight, when the rate
of transpiration exceeds
the rate of absorption of
water by the roots, the
cells lose their turgor.
They become flaccid
and the plant wilts
(Figure 6.17).
• Temporary wilting is
common and happens
for
various
reasons
(refer to factors that
affect
the
rate
of
transpiration). Prolonged
wilting, however, can
lead
to
permanent
damage to the plant.
• The flow chart below
illustrates the advantage
and disadvantage of
wilting, in relation to the
physiological responses.
Sec3 SMTP
Figure 6.17 Diagrams showing the same parts of a plant
at different times of the day. Taken from Biology Matters
(Lam and Lam, 2007).
ADVANTAGE
DISADVANTAGE
Figure 6.18
Flow chart describing the process of wilting and
comparing the advantages and disadvantages of wiling.
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6.3
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TRANSPORTS IN PHLOEM
(5) *Describe the pressure flow hypothesis in translocation (transport of sucrose in the
phloem tissue).
(6) Describe three lines of evidence that phloem tissue is involved in translocation: using
aphids, radioisotopes and ringing experiment
• In the previous chapter we learnt that the
products of photosynthesis may be used
locally by the cell which produces them, or
be transported to other organs in the form of
sucrose.
• Translocation of sucrose occurs in the
phloem vessels along with small amounts of
amino acids, organic acids, proteins, plant
growth regulators, certain minerals and
sometimes disease-causing viruses.
• Translocation in the phloem is bidirectional
and occurs in a much slower rate than
water movement in the xylem.
• Let us now examine the possible mechanism
behind sucrose translocation.
6.3.1 Pressure-Flow Hypothesis
• Currently,
experimental
evidence
supports the Pressure-flow Hypothesis
in explaining the movement of
substances in the phloem. This theory
was first proposed by German plant
physiologist, Ernst Münch, in 1930.
Sugar Sources
• Sugar sources: regions of a plant that
are producing sugar (eg. leaves
undergoing
photosynthesis)
or
exporting sugar (eg. storage organs
such as tubers, breaking down starch).
• At sugar sources: Phloem vessels are
actively loaded with metabolites by the
companion cells. These metabolites
are produced by neighbouring leaf
mesophyll cells. This causes the water
potential of the sieve tube element to
23
Figure 6.19
Phloem loading. Images
taken from Biology 7th ed. (Solomon,
Berg and Martin, 2005).
CAA 110409
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become more negative (at least two to three times that of surrounding cells).
• Water from xylem vessels (which are arranged adjacent to phloem vessels) then
enters the loaded phloem by osmosis, causing the turgor (or hydrostatic) pressure
in the phloem sieve tube element to increase.
Sugar Sinks
• Sugar sinks: regions of a plant that is consuming or storing sugar.
• At sugar sinks: Storage cells have high concentrations of sugars and metabolites,
and consequently a very negative water potential. Sucrose and metabolites from the
phloem vessels which originate from sugar sources are then transported (both
actively and passively) from the phloem into these storage cells, thus making the
water potential of the phloem vessels less negative.
• Water from phloem then migrates into the neighbouring xylem vessels by osmosis.
Pressure Gradient and Bulk Flow
• Thus, at the sugar sources, there is high turgor pressure, while at the sugar sinks, the
turgor pressure is low. A pressure gradient is thus established.
• This pressure gradient drives the movement of water from sugar sources to sugar
sinks, carrying with it the dissolved sucrose and metabolites, by bulk flow or mass
flow.
• This process is passive and does not require energy input.
6.3.2 Evidence for Sucrose Translocation
• The processes involve in phloem transport is very complex and difficult to elucidate.
The pressure-flow hypothesis has been adequate in explaining data known up to this
point in time, but much details remain unresolved.
• It was not until recently, that data collection of phloem sap content became much
easier. This was because severing the phloem to extract its contents inevitably
affects it turgor pressure.
EVIDENCE 1: Using Aphids to Probe Phloem Sap Content
• The current data owes its comprehensiveness to the method of using aphids to probe
the content of phloem sap without severing the phloem.
Figure 6.20
Aphid feeding on fluid extracted from phloem of a plant. Light microscope
image of an aphid mouthpart lodged into the sieve tube element in the host plant. Images
taken from Biology 7th ed. (Solomon, Berg and Martin, 2005).
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• Aphids use a long mouth part known as the proboscis to feed on plant sap.
• The needle-like proboscis penetrates through the leaves or stems, and is inserted
into the phloem. Pressure in the phloem drives the phloem sap through the proboscis
and into the aphids digestive system.
• By anaesthesizing the aphids with carbon dioxide, a laser beam may then be used to
sever the body of the aphid from its mouthpart.
• The phloem sap will continue to flow through the proboscis much like a hose. The
rate of flow is proportional to the pressure in the phloem, and this may be measured.
• Hence, the effect of different environmental conditions on the pressure within the
phloem may be investigated.
• The content of the phloem sap may also be determined using this technique. Studies
have shown that most of the sugars in the phoem are transported in the form of
sucrose.
EVIDENCE 2: Using Carbon Radioisotopes
• Carbon-14 or C-14 is a radioactive isotope of carbon, and may be detected on a
photographic plate or film.
• It may also be incorporated into carbon dioxide as radioactive 14CO2.
• During photosynthesis, plants exposed to 14CO2 will produce sugars containing the
radioactive carbon isotope.
• By cuttng the stem of the plant, and exposing it to a photographic film, we may obtain
an imprint of where C-14-containing sugars may be found in the stem cross-section.
• It can be shown that the sugars containing radioactive C-14 are concentrated in the
positions that coindice with the positions of the phloem vessels.
Figure 6.21
Using carbon radioisotopes in
translocation studies. Image from Biology
Matters (Lam and Lam, 2007).
25
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Figure 6.22 Photosynthates from the leaves appears in the sieve elements of phloem in the
stem. 14CO2 was supplied to a source leaf of morning glory (Ipomea nil). 14C was incorporated
into sugars synthesized in the photosynthetic process, which were then transported to other
parts of the plant. The location of the label is revealed in the tissue cross sections by the
presence of dark grains on the film. (A) shows a low magnification of the cross section of the
stem, revealing dark spots resulting from the silver grains in the film, shown in higher
magnification in (B). The label is confined almost entirely to the sieve elements of the phloem.
Image from http://4e.plantphys.net/image.php?id=139.
26
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EVIDENCE 3: Using Bark Ringing Experiments
• In the cross-section of a
(A)
dicot stem, the phloem
vessels form a ring just
beneathe the epidermis
while the xylem vessels
are nearer to the pith.
• By removing a complete
“ring” (or “girdle”) of the
bark of a woody dicot
stem, we may remove the
phloem vessels but leave
the xylem vessels intact
and exposed to the
external environment.
• It may be noted that the
portion of the bark above
the girdle will begin
(B)
swelling after a couple of
days, while the portion
beneathe
the
girdle
remain unswollen, and
begins to wither after a
longer period of time.
• Photosynthates from the
leaves are translocated
downwards along the
stem, but are interrupted
at the position where the
girdle was made. This
resulted in the swelling of
the region just above the
girdle.
Figure 6.23
Bark ringing or girdling experiments.
Experimental
set-up
(A). Diagram (B) showing the girdle
• The portion just beneathe
when the ring of bark was freshly removed (left) and after
the girdle is thus cut-off
a couple of days later (right). The top portion of the girdle
from
the
suply
of
begins swelling as materials are translocated downwards.
manufactured food, and
The region of the stem below the girdle begins to die after
will wither after some time.
some time. Image (A) taken from Biology Matters (Lam
• By
repeating
this
and Lam, 2007).
experiment but having the
girdle submerged in a
beaker of water, it may be
noted that the swelling that was observed in the upper portion of the stem no longer
occurs.
• This is because the photosynthates that are translocated downwards may now be
drawn into the solution.
27
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6.4
Sec3 SMTP
WATER RELATIONS & LEAF ADAPTATION
(7) *Describe adaptations of leaf structure to the availability of water (hydrophytic and
xerophytic leaves)
• Water availability greatly influence the life of plants. Where water is in abundance,
plants may evolve strategies to control osmotic balance in their intenal environment.
Conversely, in dry and arid environments, plants may develop adaptations to
conserve water.
• We shall examine two types of leaves related to water availability, namely,
hydrophytic leaves and xerophytic leaves.
• Hydrophytes are plants adapted to living in aquatic environments.
• Xerophytes are plants that are capable of surviving in very arid and dry
environments, where water availability is extremely low and evapotranspiration far
exceeds the amount of precipitation for most of their growing seasons.
• Halophytes are plants that have adapted to environments where there is high salinity,
which may affect their osmotic balance. This include many mangrove and coastal
plants. We will not be examining these plants.
6.4.1 Hydrophytic Leaves
• The image below is the transverse section of an example of a hydrophytic leaf. The
table in the following page lists some charactersitics of a typical hydrophyte.
Figure 6.24 Transverse
section of a leaf of Castalia
sp. Notice the abundance of
large air sacs. Image taken
from
http://sols.unlv.edu/Schulte/
28
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Feature
Cuticle
Stomata
Sec3 SMTP
Characteristics
Function
Thin or absent
Allow diffusion of dissolved gases
Allow entry of water or gas exchange
function is replaced by direct diffusion into
mesophyll cells
Permanently opened or may be
completely absent.
Distribution of
stomata
Usually distributed on top surfaces
of leaves.
For gaseous exchange with the atmosphere
Lignified
structures
Much lesser than terrestrial plants
Plants supported by water pressure
Leaf
morphology
Intercellular Air
spaces
Roots
Number of
Stomata
Large flat leaves on surface of
water
For floatation
Abundance of air sacs or canals
Reduced
Water may diffuse directly into leaves
Feathery
Traps air and holds plants up
Modified to pick up oxygen
• Hydrophytic leaves usually possess a single epidermal layer, a thin cuticle, often protruding
guard cells, and reduced supporting tissue.











Plants with floating leaves
Buoyancy to large air cavities in the sponge parenchyma.
No lack water but exposure to intense sun radiation and therefore often show a
protective cuticle.
Stomata in floating leaves are present at the top only to facilite gas exchange.
Well developed palisade tissue due to water availability and high irradiance.
Large air canals (aerenchyma) allow oxygen to diffuse from the leaf to the stem and the
root.
Completely submerged water plants
No need stomata; exchange CO2 and O2 directly via the thin, air-permeable cuticle.
The epidermis is under-developed or even absent.
Leaves are very thin or thread-like to increase surafe area to volume ration for dissusion.
Despite the reduction in supporting tissues, submerged plants can grow upright toward
light thanks to large air cavities in the leaf, which provide buoyancy.
Xylem, strongly reduced, as water is plenty available.
Palisade parenchyma absent, since the intensity of the sunlight is relatively low under
water.
29
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6.4.2 Xerophytic Leaves
• The image below is the transverse section of an example of a xerophytic leaf. The
table in the following page lists some charactersitics of a xerophyte.
A
B
C
Figure 6.25
Transverse section of
a leaf a xerophytes,
the Marram grass
(Ammophila arenaria)
in
(A)
and
(B).
Diagrammatic
representation in (C).
Image (A) taken from
www.dkimages.com;
Image
(B)
from
www.seftoncoast.org.u
k; Image (C) from
http://schulen.eduhi.at/
kultfor/eee/parks/scotl
and/np_eng/rolledleav
es.gif
30
CAA 110409
Function
Sec3 SMTP
Feature
Characteristics
Cuticle
Thick to reduce cuticular transpiration
Few
Waxy
Stomata
Sunken
Opens at night (CAM photosynthesis)
Limiting
water loss
Numerous to reduced air movement just above
diffusion shell
AND
Trichomes (hairs on leaves)
To reflect off intense radiation that may cause
damaging photo-oxidation to the plants
Curled OR
Leaf morphology
Reduced to scales or spines
Leaves
Water
storage
Succulent and fleshy
Stems
Tubers
Leaves
Water
acquisition
Numerous for increasing surface area to volume
ratio for efficient absorption of water
Long, deep to tap water below the water table
AND/OR
Root system
Numerous widespread shallow roots to quickly
capture short periods of precipitation
~end~
31

Transport in Plant - transportinfloweringplants

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