transport in plant -...

Prepared by Mr Tan Kaiyuan Hwa Chong Institution CAA 110409 Sec3 SMTP

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Name: _____________________________________ Date: ______________________ Class: 3SMTP____ INTERNET RESOURCES

1.

Grey with Anatomy? Check these sites out:

2. http://iweb.tntech.edu/mcaprio/stems.htm

3.

http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantsStructure/MatureRoot/MatureRoot.htm

The above three sites show some light microscope photographs of various plant structures. http://sols.unlv.edu/Schulte/Anatomy/

4. Forgot about the earlier chapter? Here’s a good site for revision. http://www.biology.iastate.edu/Courses/212L/New%20Site/26Leaf&Ps/%20LeavesandPs.htm

5. For the Intermediate Learner

A comprehensive website on plant transport with clear diagrams.

http://leavingbio.net/TRANSPORT%20OF%20MATERIALS%20IN%20A%20FLOWERING%20PLANT.htm

6. Very interactive animation that allows you to adjust parameters of transport in plants. Must see! http://croptechnology.unl.edu/animationOut.cgi?anim_name=transpiration.swf

7. Here’s a website to tell you more about leaf adaptations to varying water availability. http://www.vcbio.science.ru.nl/public/pdf/leaves_eng.pdf

CHAPTER MAP & OVERVIEW

Transport in Plant

6.1 Stem & Root Structure & Function

6.2 Transport of Water & Minerals

6.3 Transports in Phloem

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

6.2.1 Water Potential 6.2.2 Overview of Water Movement 6.2.3 Water Movement in Roots 6.2.4 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

6.4.1 Hydrophytic Leaves

6.4.2 Xerophytic Leaves

6.4 Water Relations & Leaf Adaptation

6.3.1 Pressure Flow Hypothesis

6.3.2 Evidence for Sucrose Translocation

Hwa Chong Institution Sec3 (SMTP) Biology

http://iweb.tntech.edu/mcaprio/stems.htm�

http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantsStructure/MatureRoot/MatureRoot.htm�

http://www.cartage.org.lb/en/themes/Sciences/BotanicalSciences/PlantsStructure/MatureRoot/MatureRoot.htm�

http://sols.unlv.edu/Schulte/Anatomy/�

http://www.biology.iastate.edu/Courses/212L/New%20Site/26Leaf&Ps/%20LeavesandPs.htm�

http://www.vcbio.science.ru.nl/public/pdf/leaves_eng.pdf�


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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_GB_22_Plantae_Fig_f/Vascular_Bundles.GIF

6.1 STEM AND ROOT – STRUCTURE AND FUNCTION

(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.

Section Learning Objectives At the end of this Section, you should be able to:

(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


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A

B

E F D C 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/Tracheids-from-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).

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.

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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.uni-hamburg.de

A

B

• 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.


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The table below compares the structure and function of xylem and phloem tissues:

Tissue Cell Types Characteristics Function

Xylem

• Vessel elements

• Tracheids

• 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.

• Presence of pits along xylem vessel elements and tracheids.

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.

Phloem

• Sieve tube members

• 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

• Companion cells

• 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


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C

A

B Ground tissue

Vascular Bundles

Epidermis

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

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.

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).

C A B

Phloem

Cambium

Xylem

Pith Ground Tissue

Epidermis


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Figure 6.6 Longitudinal section of a typical dicot root tip. Image taken from Biology 7th ed. (Solomon, Berg and Martin, 2005)

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:

• A thimble-shaped mass of Root Cap

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.

• Comprises an apical meristem, Zone of Cell Division

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.

• Cells become several times their original length in this region. Zone of Elongation

• 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.

• Epidermal cells develop protuberances known as root hairs, which absorb water and minerals, and adhere tightly to soil particles.

Zone of Maturation (aka Zone of Differentiation or Zone of Root Hairs)

• Root hairs greatly increase the surface area to volume ration for efficient absorption of water and minerals.


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Piliferous layer

Xylem Arms

Pericycle

Cortex

Endodermis Phloem

C

A B

D

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. (Solomon, Berg and Martin, 2005). Image (C) taken from http://www.soilandhealth.org/01aglibrary/010139fieldcroproots/010139ch2.html; (D) from Biology Matters (Lam and Lam, 2007).

(2) • The xylem and phloem vessels in a typical dicot root are grouped into a central

vascular cylinder or stele.

Radial Arrangement (From centre axis to epidermis)

• 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.

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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).

• 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.


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• The table below summarises the various root structures and their functions:

Structure Characteristics Function

Root Cap

• 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

Piliferous Layer (Epidermis)

• 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.

Cortex

• Parenchyma cells of cortex contains abundance of amyloplasts (starch storing organelles)

1. Storage of starch produced from photosynthesis.

Stele

Endodermis

• 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.

Pericycle • A single-layer of parenchyma

cells. 1. Will give rise to lateral roots by

cell division and penetrate through the endodermis, cortex and epidermis.

Phloem

• Consists of sieve tube elements that possess degenerate protoplasm.

• Companion cell with mitochondria and nucleus.

1. Involves in the bidirectional transport of metabolites.

2. Degenerate protoplasm reduces resistance to material flow.

3. Companion cell provides energy for cellular activities of the sieve tube elements.

Xylem

• Consists of hollow vessels formed by dead cells known as tracheids or vessel elements.

• Possess spirals or rings of lignin on inner walls

1. Hollow tube reduces resistance to water and dissolved mineral flow upwards.

2. Provide mechanical support.


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A

B

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/image-gallery/show/labels/print/PL0130/.

Legend: 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)

6.1.4 Leaf Internal Structure (in relation to Vascular Bundle Arrangements)


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6.2 TRANSPORT OF WATER & MINERALS

At the end of this Section, you should be able to: Section Learning Objectives

(8) 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.)

(9) State that most mineral salts are absorbed via active transport by the root hair cells

(10) *Outline the three pathways of movement of water through plant cells: apoplast, symplast and vacuolar pathways

(11) Outline the pathway by which water is transported from the roots to the leaves through the xylem vessels

(12) Describe capillarity and root pressure in the transport of water up a plant (13) 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

(14) Explain the movement of water through the stem in terms of transpiration pull (15) Describe the effects of variation of temperature, humidity, wind speed and light

intensity on transpiration rate: (16) Describe how water vapour loss is related to cell surfaces, air spaces and stomata (17) 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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Figure 6.10 (A) Transverse section of a monocot (Zea mays) leaf. Image taken from Biology 7th ed. (Solomon, Berg and Martin, 2005).

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.


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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

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)

• 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.

Apoplast pathway (cell walls)


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Figure 6.12 3 Pathways of water movement in the root. Images taken from Advanced Biology (Michael Kent, 2000).

(2)

(cytoplasm via Symplast pathway

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.

(3)

(vacuole to vacuole) Vacuolar pathway

• 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.


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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).

A

B

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 is a process involving

Transpiration and Transpiration Pull

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 pressure

low hydrostatic

water from the xylem vessels into the in the leaves, and draws more

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 pressure

high hydrostatic

pressure, thus forces water to move . The difference in hydrostatic

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 of water

continuously flowing stream

tallest parts of a plant, known as from the deepest roots to the

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


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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 is first established when root epidermal cells actively transport mineral ions

into their cytoplasm.

Root Pressure

• 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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Figure 6.14 (A) – Capillarity in narrow tubes. The smaller the tube diameter, the greater the rise in fluid. Image (A) taken from Biology 7th ed. (Solomon, Berg and Martin, 2005).

(3) • Water tends to move up inside

Capillary action (Capillarity)

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 owing to the narrowness of the xylem tubes.

• In small plants, the role of capillary 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 in plants occurs first in the roots by active transport, and subsequently

by diffusion in the rest of the plant tissues.

Mineral Transport

• 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.


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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).

6.2.5 Water Movement in Leaves

• 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 Influence Transpiration Rate

increased if decreased if

External

Light Stomata open in the presence of light and closes in the dark.

Higher light intensity

Lower light intensity

Humidity Affects diffusion gradient between the air spaces in the leaf and the atmosphere.

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

Internal

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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Figure 6.16 Diffusion of water from mesophyll cells out of the leaf. Image from Advanced Biology (Michael Kent, 2000).

(1) • The intercellular air spaces

Humidity of the air

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 blows away the layer

Wind or air movement

of stationary air (i.e. diffusion shell) – that contains accumulated water vapour – outside the stomata (Figure 6.15).

• This maintains the water vapour concentration 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) • 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.

Temperature of the air

(4) • 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.

Light


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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).

DISADVANTAGE ADVANTAGE

Figure 6.18 Flow chart describing the process of wilting and comparing the advantages and disadvantages of wiling.

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 of transpiration exceeds

rate

the rate of absorptionwater by the roots, the

of

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.


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Figure 6.19 Phloem loading. Images taken from Biology 7th ed. (Solomon, Berg and Martin, 2005).

6.3 TRANSPORTS IN PHLOEM

(5) *Describe the pressure flow hypothesis in translocation (transport of sucrose in the phloem tissue).

Section Learning Objectives At the end of this Section, you should be able to:

(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: regions of a plant that Sugar Sources

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


24

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).

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: regions of a plant that is consuming or storing sugar. Sugar Sinks

• 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.

• 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.

Pressure Gradient and Bulk Flow

• 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.

• The current data owes its comprehensiveness to the method of using aphids to probe the content of phloem sap without severing the phloem.

EVIDENCE 1: Using Aphids to Probe Phloem Sap Content


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Figure 6.21 Using carbon radioisotopes in translocation studies. Image from Biology Matters (Lam and Lam, 2007).

• 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.

• Carbon-14 or C-14 is a radioactive isotope of carbon, and may be detected on a photographic plate or film.

EVIDENCE 2: Using Carbon Radioisotopes

• 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.


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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.


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Figure 6.23 Bark ringing or girdling experiments. Experimental set-up (A). Diagram (B) showing the girdle when the ring of bark was freshly removed (left) and after a couple of days later (right). The top portion of the girdle begins swelling as materials are translocated downwards. The region of the stem below the girdle begins to die after some time. Image (A) taken from Biology Matters (Lam and Lam, 2007).

(A)

(B)

EVIDENCE 3: Using Bark Ringing Experiments • In the cross-section of 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 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.

• The portion just beneathe the girdle is thus cut-off from the suply of manufactured food, and will wither after some time.

• By repeating this 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.


28

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/

6.4 WATER RELATIONS & LEAF ADAPTATION

Section Learning Objectives At the end of this Section, you should be able to: (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.

http://sols.unlv.edu/Schulte/Anatomy/Leaves/Leaves.html�




29

Feature Characteristics Function

Cuticle Thin or absent Allow diffusion of dissolved gases

Stomata Permanently opened or may be completely absent.

Allow entry of water or gas exchange function is replaced by direct diffusion into

mesophyll cells

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

Large flat leaves on surface of water

For floatation Intercellular Air

spaces Abundance of air sacs or canals

Roots Number of Stomata

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.


30

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.uk; Image (C) from http://schulen.eduhi.at/kultfor/eee/parks/scotland/np_eng/rolledleaves.gif

A B

C

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.

http://www.dkimages.com/�

http://www.seftoncoast.org.uk/�

http://www.seftoncoast.org.uk/�


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Function Feature Characteristics

Limiting water loss

Cuticle Thick to reduce cuticular transpiration

Stomata

Few

Waxy

Sunken

Opens at night (CAM photosynthesis)

Trichomes (hairs on leaves)

Numerous to reduced air movement just above diffusion shell

AND To reflect off intense radiation that may cause

damaging photo-oxidation to the plants

Leaf morphology Curled OR

Reduced to scales or spines

Water storage

Leaves

Succulent and fleshy Stems

Tubers

Water acquisition

Leaves Numerous for increasing surface area to volume ratio for efficient absorption of water

Root system

Long, deep to tap water below the water table AND/OR

Numerous widespread shallow roots to quickly capture short periods of precipitation

~end~

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