Lecture 5 Notes: Secondary Growth and Transport

Secondary Growth

• Vascular Cambium

  • Produces secondary xylem (wood) to the inside and secondary phloem to the outside, increasing stem and root diameter.

  • Also produces rays for lateral transport of water and nutrients.

  • Activity varies seasonally, producing growth rings in woody plants, which can be used to estimate age and environmental conditions.

• Cork Cambium (Phellogen)

  • Arises from the cortex or phloem and produces the periderm, which replaces the epidermis.

  • The periderm consists of:

  • Phelloderm: A thin layer of parenchyma cells to the inside.

  • Cork Cambium: The meristem itself.

  • Cork (Phellem): Suberized, dead cells that protect the woody plant from water loss, damage, and pathogens.

Transport

• Three Pathways

  • Apoplast: The continuum of cell walls and extracellular spaces, allowing water and solutes to move without crossing a plasma membrane.

  • Symplast: The continuum of cytoplasm connected by plasmodesmata, allowing substances to move from cell to cell.

  • Transmembrane: Involves crossing plasma membranes, providing the greatest control over what enters and exits cells.

• Water Potential ($\Psi$)

  • A measure of the potential energy of water, determining the direction of water movement.

  • Measured in megapascals (MPa).

  • Influenced by:

  • Solute Potential ($\Psi$s): The effect of dissolved solutes on water potential (always negative).

  • Pressure Potential ($\Psi$p): The physical pressure on a solution (can be positive or negative).

  • Water moves from areas of high water potential to areas of low water potential.

• Xylem - Water enters root

  • Water and minerals are absorbed by root hairs and move through the cortex to the endodermis.

  • The endodermis has a Casparian strip, which forces water and minerals to enter the symplast, allowing the plant to control uptake.

  • Ascent of sap: mechanism

  • Transpiration: Water evaporates from the leaves, creating a negative pressure (tension) in the xylem.

  • Cohesion: Water molecules stick together due to hydrogen bonds.

  • Tension: The negative pressure pulls water up the xylem from the roots.

  • Stomate control

  • Stomata open and close to regulate transpiration and gas exchange.

  • Guard cells control the opening and closing of stomata in response to:

    • Light

    • CO2 concentration

    • Water availability

    • Abscisic acid (ABA) a plant hormone that signals stomatal closure during water stress.

• Phloem

  • Transports sugars and other organic compounds from sources (e.g., leaves) to sinks (e.g., roots, fruits).

  • Translocation: The process of transporting phloem sap.

  • Pressure flow hypothesis: Sugars are actively loaded into the phloem, increasing the solute concentration and drawing water in by osmosis, which creates a positive pressure that drives the sap towards the sink.

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Types & Location of Meristems

• Apical Meristems (AM) - Root AM and shoot AM

  • Located at the tips of roots and shoots.

  • Responsible for primary growth (increase in length).

  • Give rise to primary tissues: epidermis, ground tissue, and vascular tissue.

  • All vascular plants

• Lateral Meristems - Vascular cambium & cork cambium

  • Responsible for secondary growth (increase in thickness).

  • Vascular cambium produces secondary xylem (wood) and secondary phloem (inner bark).

  • Cork cambium produces the periderm (outer bark).

  • Only in conifers & woody eudicots

  • Make wood & bark

Secondary Growth

• Produces

  • Wood: Secondary xylem, providing structural support and water transport.

  • Bark: All tissues outside the vascular cambium, protecting the stem from damage and water loss.

• Occurs in

  • Conifers

  • Woody eudicots (e.g., oak)

Primary and Secondary Growth in Stems

• Role of apical and lateral meristems

  • Secondary growth:

  • All woody species:

    • Conifers

    • Eudicots (woody species only)

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• Primary growth:

  • All vascular plants:

    • Ferns

    • Seed plants

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Secondary Growth in Stems

• Periderm: Protective outer layer, replacing the epidermis in older stems.

• Pith: Central core of parenchyma cells in young stems.

• Cork cambium adds secondary dermal tissue.

• Cortex: Outer layer of ground tissue in young stems.

• Primary phloem: Original phloem produced by the apical meristem.

• Secondary phloem: Phloem produced by the vascular cambium.

• Vascular cambium adds secondary xylem and phloem.

• Primary xylem: Original xylem produced by the apical meristem.

• Secondary xylem: Xylem produced by the vascular cambium (wood).

Vascular Cambium

• Vascular cambium is a meristem!

• VC produces

  1. Secondary xylem to inside (wood!)

  2. Secondary phloem to outside

  3. More VC (to increase in circumference)

  4. Rays—parenchyma for lateral transport

Vascular Cambium Details

• Vascular cambium is a meristem!

• A cambial initial (C) can divide to form two cambial initials, increasing the circumference of the vascular cambium.

• A cambial initial can also divide to form an initial and either a secondary xylem cell (X) or secondary phloem cell (P).

• Cambial initials usually produce much more xylem than phloem.

• Vascular cambium also makes rays.

Vascular Cambium (Tangential View)

• Ray initials make rays (in both xylem & phloem)

• Fusiform initials make

  • Tracheids and vessel elements (xylem)

  • Sieve elements (phloem)

Wood

• Wood: oak

• Xylem rays: Parenchyma cells that transport water and nutrients laterally within the wood.

• Growth rings: why?

  • Formed due to seasonal variations in vascular cambium activity.

  • Early wood (spring wood): Larger cells, less dense.

  • Late wood (summer wood): Smaller cells, more dense.

• Wood is secondary xylem

• Lignin (polymer) in secondary walls of tracheids and vessel elements. Provides strength and rigidity to the cell walls.

• Heartwood vs. sapwood

  • Heartwood: Older, non-conducting xylem filled with resins and tannins, providing structural support and decay resistance.

  • Sapwood: Younger, conducting xylem that transports water and nutrients.

Lignin

• Deposited in cell walls, fills spaces, and binds cellulose, hemicellulose & pectin

• Gives strength to wood & bark

• Can occur in cell walls of non-woody plants (palm trees, bamboo, wheat straw, …)

3-Year-Old Woody Stem

• Cork: Protective outer layer.

• Phloem: Transports sugars.

• Vascular cambium: Produces secondary xylem and phloem.

• 3 years of secondary xylem: Shows annual growth rings.

• Cork cambium: Produces cork.

• Annual growth rings: Indicate the age of the stem and past environmental conditions.

Cork Cambium (Phellogen)

• New lateral meristem

• Arises from cylinder of cortex cells outside the vascular cambium & secondary phloem

• Produces the “periderm”: 3 layers:

  • Phelloderm to inside (some woody species)

  • Thin layer of living parenchymal cells

• Cork cambium itself

• Cork to outside

  • Suberized, dead cells

  • Protects woody plant (there is no more epidermis)

Anatomy of a Tree Trunk

• Heartwood: Non-conducting xylem.

• Sapwood: Conducting xylem.

• Vascular cambium: Meristematic layer producing secondary xylem and phloem.

• Living phloem: Transports sugars.

• Bark: all tissues outside the vascular cambium

• Periderm: Protective outer layer, including cork, cork cambium, and phelloderm.

• Cork cambium: Produces cork.

• Cork: Outermost protective layer.

Cork

• Quercus suber: The cork oak, a major source of commercial cork.

Primary and Secondary Growth of a 3-Year-Old Woody Stem

• Tilia stem: Example of a woody stem.

• Bark is all tissues outside vascular cambium:

  • Secondary phloem

  • Periderm

Primary & Secondary Growth

• Primary growth from the activity of the apical meristem is nearing completion. The vascular cambium has just formed.

• Although primary growth continues in the apical bud, only secondary growth occurs in this region. The stem thickens as the vascular cambium forms secondary xylem to the inside and secondary phloem to the outside.

• Some initials of the vascular cambium give rise to vascular rays.

• As the vascular cambium's diameter increases, the secondary phloem and other tissues external to the cambium can't keep pace because their cells no longer divide. As a result, these tissues, including the epidermis, will eventually rupture. A second lateral meristem, the cork cambium, develops from parenchyma cells in the cortex. The cork cambium produces cork cells, which replace the epidermis.

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

• As the stem's diameter increases, the outermost tissues exterior to the cork cambium rupture and are sloughed off.

• In many cases, the cork cambium re-forms deeper in the cortex. When none of the cortex is left, the cambium develops from phloem parenchyma cells.

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

• Bark consists of all tissues exterior to the vascular cambium.

Tissues of Woody Stem

• Primary Meristems

  • Protoderm -> Epidermis

  • Procambium -> Primary phloem, vascular cambium, primary xylem

  • Ground meristem -> Ground tissue, pith, cortex

• Lateral Meristem

  • Cork cambium -> Cork, periderm, secondary pholem, rays, secondary xylem

Transport

• 1st Law of Thermodynamics: Cannot create or destroy energy. Can only change from one form to another (e.g., electricity to light)

• 2nd Law of Thermodynamics: Energy spontaneously tends to flow only from being concentrated in one place to becoming spread out, or for a combined system and surroundings, entropy never decreases

The 2nd Law in Life

• Overall Messages

  • The movement of fluid in plants follows the 2nd Law of Thermodynamics

  • The most equitable distribution of energy corresponds to maximum entropy

• Some examples

  • Osmosis: The movement of water across a semipermeable membrane from an area of high water concentration to an area of low water concentration.

  • Diffusion: The movement of molecules from an area of high concentration to an area of low concentration.

  • Fluid movement because of differences in hydrostatic pressure

Transport: Overview

• Water and minerals enter the plant through the roots and are transported via xylem.

• Carbon dioxide enters the plant and oxygen exits through the leaves.

• Sugars are produced in the leaves through photosynthesis and transported via phloem.

Routes of Water Movement Within a Plant

• 3 Cell compartments

• 3 Transport routes

• Cytoplasm: all material inside cell membrane

• Cytosol: the part of cytoplasm excluding organelles

Cellulose

• Main component of cell walls

• Highly absorbent (hydrophilic)

• Polysaccharide (polymer)

• Most abundant organic compound on earth

• Examples: Rayon, Cotton, Paper, Cellulose sponges

Water Potential, Psi ($\Psi$)

• Water potential energy

• Unit: megapascal (MPa)

  • $1 MPa = ~10$ atmospheres (bars)

• Combines effects of solute concentration & pressure

• $\Psi = 0$ MPa for pure water at sea level and at room temperature

• Determines DIRECTION of movement of water: Water flows from regions of higher to lower water potential

Water Potential, Psi ($\Psi$)

• Is water potential energy

• Two components

Water Potential: 2 Components

• Solute potential & Pressure potential

• Solutes have a negative effect on $\Psi$ by binding water molecules.

• Positive pressure has a positive effect on $\Psi$ by pushing water.

• Negative pressure (tension) has a negative effect on $\Psi$ by pulling water.

Water Potential in Plant Cells

• Start with flaccid cell: membrane contacting the cell wall but flaccid

• Now put cell in sugar water

  • “Plasmolysis” is loss of water from cell by osmosis

  • Cell membrane now separated from cell wall

Water Potential in Plant Cells (cont.)

• Start with flaccid cell: membrane contacting the cell wall but flaccid

• Now put cell in sugar water

  • “Plasmolysis” is loss of water from cell by osmosis

  • Cell membrane now separated from cell wall

  • Water moves spontaneously from higher to lower water potential

• Now put cell in PURE water

• Cell membrane now pushing against cell wall: turgid

Xylem

• Water & minerals travel upward in xylem

• Pine tracheids: bordered pits

  • Tracheids (all vascular plants)

  • Vessel elements (flowering plants)

The Ascent of Sap

• Occurs in Xylem

• Herbaceous (nonwoody) eudicot

• Monocot

• Woody plant:

  • Woody eudicot

  • Conifer

• Roots

• Shoots (woody roots similar to woody stems)

Herbaceous Eudicot Root

• Herbaceous (= nonwoody) mature eudicot root

• Ranunculus (buttercup)

  • Epidermis

  • Vascular Cylinder:

  • xylem

  • phloem

• Cortex

• How do water & minerals get from soil to xylem?

Water Enters Root

• Lateral transport of H2O & minerals

• Apoplast = nonliving continuum outside cytosol, including

  • Cell walls

  • Xylem cells (t. & v.e.)

  • Extracellular spaces

• Symplast = continuum of cytosol connected by plasmodesmata

The Endodermis

• Cylinder 1-cell thick

• Stele: all material inside endodermis

  • Xylem & phloem

  • Pith

  • Pericycle (origin of lateral roots)

• Casparian strip

  • Where primary wall & middle lamella were

  • Waterproof & impermeable to ions: suberin

  • All water & ions entering xylem must pass through endodermal cells; must cross cell membrane!

Solute Transport Across Plant Cell Membranes

• Proton pump: uses ATP to pump H+ out of the cell, creating a membrane potential and pH gradient.

• H+/sucrose cotransporter: loads sugars into plant cells.

• H+/NO3- cotransporter: regulates ion fluxes into and out of cells.

• Ion channels: allow specific ions to diffuse across membranes in response to voltage, stretching, or chemical factors.

Mycorrhizae: Mutualism

• Mycorrhizae (mycorrhizal fungi): A mutualism between plants and fungi

  • Increase surface area

  • Aid absorption of minerals

Water & Mineral Pathway in Herbaceous Plant

• After water enters root: what next?

• The pathway of water & minerals in an herbaceous plant

  1. Soil

  2. Root hair or mycorrhizae

  3. Cortex

  4. Endodermis

  5. Xylem

  6. Atmosphere

Ascent of Water & Minerals in Plants

• So, how do water & minerals move up a plant?

• What are the (theoretical) possibilities?

  1. Capillary action

  2. Pumps

  • From above: atmospheric pressure

  • From below: root pressure

3. Transpiration-Cohesion-Tension mechanism

Transpiration-Cohesion-Tension Mechanism

• Water flow in xylem: Ascent of sap

• Steps

  • Water evaporates from moist cells in leaf stomates (transpiration)

  • Water potential is lowered at air-water interface, causing negative pressure (tension) in xylem

  • Hydrogen bonds hold water molecules together (cohesion)

  • Xylem under tension gradient: pressure potential ($\Psi$p) lowest (most negative) at top

  • Thus, water is pulled up by pressure gradient (differences in $\Psi$p, not in $\Psi$s)

  • Water & minerals enter root by osmosis

Transpirational Pull

• Generation of transpirational pull

• Negative pressure (tension) at the air-water interface in the leaf is the basis of transpirational pull, which draws water out of xylem

Ascent of Sap (cont.)

• Water flow in xylem: Ascent of sap

• Facts

  • Total path in xylem from highest (least neg.) to lowest (most neg.) water potential

  • Passive process

  • Tracheids & vessel elements are dead cells

  • Upward only

Cohesion of Water Molecules

• Cohesion of water molecules (movie)

Ascent of Sap Summary

• Water flow in xylem: Ascent of sap

• Transpiration- cohesion-tension mechanism

  • Passive process

  • Xylem under tension

  • Total path in xylem from highest (least neg.) to lowest (most neg.) water potential

Stomate Control of Transpiration

• Control of Transpiration by Stomates

• Cues to open at dawn :

  • Light

  • CO2 depletion

  • Circadian rhythm

• Dry conditions: closure

• Abscisic acid: hormone

  • causes K+ to leave guard cells

  • stimulates stomate closure

Open and Closed Stoma

• An open stoma (left) and closed stoma

Stomates & Epicuticular Waxes

• Stomates