Lecture 7_ Root and Shoot Systems

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Lecture 7_ Root and Shoot Systems Powered By Docstoc
					Root System and Shoot System

  Reading: Solomon & Berg 8th ed. Chap. 34-35
                     Root and Shoot Systems
                                                                   The Shoot System
                                                                   Elevates the plant above the soil
                                                                   Many functions including:
                                                                        reproduction & dispersal
                                                                        food and water conduction
                                                                   Includes the leaves and the reproductive

                                                                   The Root System
                                                                   Underground (usually)
                                                                   Anchor the plant in the soil
                                                                   Absorb water and nutrients
                                                                   Conduct water and nutrients
                                                                   Food Storage
Monocots vs. Dicots

                           Root System
•   Made up of Roots
    – Roots are plant organ that originates and develops at root apical
    – Roots are the        first structure to emerge from the seed

• Function of Roots
    –   specialized descending structures
    – spread down and outward anchoring plant
    – absorbs water and dissolved nutrients (minerals)
    – store food (starch)
             Types of Root Systems
•   Tap Roots
    – one large root with many smaller lateral
      (branch) roots coming out of it (ex. carrot,
      beets, sweet potato, turnips)
    – Forms from the seedlings radicle        
      (embryonic root)                                                                            /

    – Found in dicot plants
•   Fibrous Roots
    – Several roots of the same size developing
      from the end of the stem; with lateral roots
      branching off these roots
    – Also called adventitious roots
    – Found in monocots
• Specialized roots: prop roots, buttress
    roots, aerial roots pneumatophores     
                 Basic Root Structure
•       Root Cap – covers/protects the root apical meristem
•       Root hairs- tubular extensions of epidermal cells that
        increase absorption
•       Epidermis- single layer of protective covering
•       Cortex – loosely arranged parenchyma (for storage) with
        large intercellular spaces (for water transport, aeration)
•       Endodermis
    –      Single layer of cells in roots that contains a suberized Casparian
    –      controls amount and kinds of minerals entering xylem
•       pericycle – single layer; where lateral roots arise
•       Vascular tissues (xylem and phloem)
•       In dicots roots the central area called vascular column,
        central cylinder or stele
•       In monocots central region is called pith
                                Root Structure
207%20-%202008%20-%20Roots%20and%20Stems.html                 7/plants/structure.htm
                                         Root Structure        
arplants/dicot_root.gif                                               20Pictures/Monocot%20Roots/Monocot%20root%20stele.jpg
          Shoot System
Main Organs
• Stems
• Leaves
• Reproductive

•   Link roots and leaves
•   plant organ that originates and develops at meristems
•   derived from embroyonic shoots that form within seeds
•   range from vines to massive trunks
•   Herbaceous or woody
•   usually circular

• Main functions
    – Support leaves and reproductive structures
    – Provide internal transport
    – Produce new living tissue (i.e. buds)

            External Stem Structure
    1. Buds: embryonic shoots
        - Terminal buds- at tip of stem
        - Axillary buds- lateral buds
        - Bud scales- outer protective layer

    2. Nodes: area where leaves attach
    3. Internode: region between two nodes
    4. Lenticels- cells that allow oxygen to
       diffuse into woody stem
               Dicot vs Monocot Stems
online/ibc99/koning/stems.html          ces/plantsstructure/StemShoot/StemShoot.htm
     Monocot vs Dicot Stems
         Transport through Plants
• Water and dissolved
  – Transported from soil through
    roots to xylem and then up
    xylem of stem to rest of plant
  – Unidirectional transport
• Sap (dissolved nutrients
  – Transported via the phloem to
    all parts of plant
  – Bidirectional transport
   Water and Mineral transport
• NOTE: water moves based on changes in
  water potential; minerals are transported
  dissolved in water.

  – Water potential = tendency of water to
    move (free energy of water,); it measures the
    ability to absorb water by osmosis

  – Initially water with dissolved minerals moves
    horizontally through epidermis and cortex
    towards xylem.
Water and Mineral transport cont’d
 • Soil solution is normally very dilute thus
   has higher water potential than solution in
   root hairs
 • This creates root pressure a force created
   from the water potential gradient which
   drives water into root hairs
 • Water with dissolved minerals moves
   horizontally through epidermis and cortex
   towards xylem.
          Routes of Transport
• *Apoplast pathway- movement from cell to cell via
  interconnected cell walls
• *Symplast pathway- movement from one cell’s
  cytoplasm to another via plasmodesmata
Endodermis and Casparian Strip
•       Endodermis- controls water and amount and kinds of minerals that
        enter roots
    –      within endodermal cells is a suberin (waxy, waterproof) strip called the
           Casparian strip which prevents movement

    –      water and minerals travel to the endodermis using both pathways, until the
           Casparian strip is reached which prevents apoplast travel.

    –      Minerals then actively transported into xylem through carrier proteins in
           endodermal cell membranes. Water enter via osmosis through aquaporins
    –      Water continues into xylem due to the established water potential
         Tension-Cohesion or
     Transpiration Cohesion Theory
•   water enters xylem vessels at roots

•   water pulled up plant because of tension produced at top

•   Tension due to – transpiration, the evaporation of water from plants
    (mainly through leaf stomata)

•   Upward pull is only possible if an unbroken column of water is formed- this
    is accomplished by the cohesive nature of water molecules

•   Adhesion of water molecules to the lignified walls of xylem also assists in
    the column formation

•   as water moves dissolved minerals are also transported

•   NOTE: transport in xylem is unidirectional

•   Wilting- due to greater loss of water via transpiration than uptake of water
    at roots
     Leaf Structure and Transpiration
                                                                        Leaves main function is to gather
                                                                        the sunlight needed for

                                                                        Most transpiration occurs when      leaf stomata are open as gases are
             Leaf Structure
                            Key Features

• Leaf Internal structure   Epidermis (upper and lower)
                            Mesophyll –photosynthetic tissue
                            (packed with chloroplasts)
                                Palisade Mesophyll
                                Spongy Mesophyll

                            Stomata and Guard cells

                            Veins and vascular bundle
                            (xylem and phloem)
Leaf Structure is related to function
• Cuticle and Epidermis are transparent
• Most have thin flat leaves with large
  surface area
• Mesophyll located close to surface
• Stomata allow the exchange of gases
• Air spaces accumulate gases (eg. CO2)
• Needed water and minerals are provided
  by veins in leaves as well as veins carry
  away the photosynthetic products of the
            Factors affecting the
            Rate of Transpiration
•   Humidity
•   Temperature
•   Wind speed
•   Light
•   Water availability
•   Thickness of cuticle and leaf area
•   Density and distribution of stomata-
    number & location
           Some Reasons Why
           Transpiration is important
•   Water loss via transpiration allows for continuous
    supply of fresh water as water is replaced by uptake at
•   water needed for plant turgor, especially in leaves so
    they can be held out firmly increasing exposure to
    sunlight for photosynthesis
•   herbaceous plants need water for overall support
•   brings continuous supply of minerals
•   cools plant
•   contributes to the water cycle
•   NOTE: however is an inevitable consequence of gas
    exchange in plants
       Conservation of water
• Cuticle- waterproofing (small loss by

• Guard Cells- when closed prevents
  – open stomata represent a trade off between
    gas exchange and water loss
    Stomatal opening and closure
•    open/closing based on changes in pressure
     potential, affected by changes in water

•    Stoma tend to open in light (esp. Blue light)
     and close in dark
•    Theory: in light ATPase pumps H+ ions out of
     guard cells while K+ ions (and Cl- ions) enter
     cell lowering water potential. This causes
     water to enter and stoma opens. In the dark,
     movement of ions and water is reversed,
     stoma closes
     Sap Transport in Phloem
• Sap predominantly contains sucrose
• It also contains small amounts of other
  substances including amino acids, organic
  acids, minerals, hormones, etc.
• Movement in phloem is via a process called
• Sap is moved both upward and downward
• moves from source (loading area) to sink
  (unloading area to be metabolized/stored)
Movement in leaf to phloem
           Pressure Flow Theory
•   dissolved sugars/sap move based on a pressure
•   gradient exist between the source and the sink
•   Sap is actively loaded into companion cells at source
    (usually leaves) then transferred to sieve tube
    elements via plasmodesmata.
    –   This is an active process hence requires ATP
•   Loading of sap causes more negative water potential in
    sieve tube thus water moves by osmosis into sieve
    tubes increasing the hydrostatic pressure and
    causing movement
•   Sap is pushed to sink where hydrostatic pressure is
•   At sink sap actively unloaded and materials stored or
    used. (eg. Sucrose converted and stored as starch)

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