Plant structure and function Roots

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Plant structure and function Roots

• Objectives of todays lecture
• Learn about the functions of the root system and
  its importance to plant growth and development
• Learn the structures that allow the root to
  perform these functions
• Learn some of the modifications that occur in
  roots

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Functions of the Root

• anchorage to the soil (or other substrate)
• absorption of water
• acquisition of mineral nutrients
• a tissue for energy (food) storage
• for propagation and dispersal of some species

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Functions of the Root

• Root morphology is highly variable

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Functions of the Root

• Root morphology is highly variable
• most dicotyledonous plants (those with 2 seed
  leaves) have a prominent primary root
• monocots (with one cotyledon or seed leaf)
  typically have a more diffuse fibrous root system

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How extensive are root systems?

• A 40 ft. tall oak tree
• primary tap root penetrated 14 ft. into the soil
• lateral roots extended to a radius of 60 ft.

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How extensive are root systems?

• A single winter rye plant growing in a pot with 2
  cu. ft. of soil
• clump of about 80 shoots, total leaf area of 51
  sq. ft.
• 143 primary roots
• 35,000 secondary roots
• 2 million tertiary roots
• 11 million quaternary roots
• total root surface area of 6,500 sq. ft.

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Importance of the root system in horticulture

• Essential for growth of virtually all plants
• Are there examples of plants that dont have
  roots?

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Importance of the root system in horticulture

• Many horticultural plants are transplanted
• in ornamental landscaping
• Trees and shrubs
• Perennials and annuals

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Importance of the root system in horticulture

• Vegetable seedlings and fruit trees are
  transplanted
• Successful transplanting depends on a vigorous
  and healthy root system
• Many horticultural products are roots
• Primarily vegetables

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Growth of the root system

• Root growth depends on the activity of the root
  apical meristem
• located just behind the root cap at the tip of
  the root
• provides a source of new cells for extension and
  elongation of the root

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Growth of the root system

• Growth of the root occurs at the tip

Ink dots are placed on the roots of a seedling
(A). As the seedling grows, the dots at the top
of the root (near the stem) stay in position. The
dots near the tip move apart (B), indicating that
this region is growing.
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Tissues of the root
Root tissues can be regarded as a series of
concentric rings of different tissues
Epidermis
Cortex
Endodermis
Pericycle
Phloem
Vascular System
Cambium
Xylem
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Epidermis

• Single layer of cells on the exterior of the root
• NOT covered with a cuticle
• root is designed for water uptake
• cuticle is a barrier to water

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Epidermis

• Root hairs are specialized epidermal cells
• single cells that elongate perpendicular to the
  root

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Epidermis

• Root hairs contribute the largest fraction of
  root surface area
• In the winter rye example given earlier, more
  than 4,000 sq. ft. of the 6,500 sq. ft. of root
  surface area comes from root hairs
• Epidermis functions in the uptake of water and
  minerals
• Large surface area facilitates uptake

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Cortex

• Comprised of large undifferentiated cells
• Functions in the storage of food reserves
  (proteins and starches) in root tissues

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Endodermis

• Single layer of cells outside vascular system
• Spaces between the cells are covered with a waxy
  layer, called suberin
• Similar to grout between ceramic tiles, and
  serves a similar function

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Endodermis

• Blocks passage of water and dissolved nutrients
  into the vascular system
• Water, nutrients and other solutes must enter a
  cell, cannot go round the endodermis

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Endodermis

• Endodermis functions to regulate the uptake of
  water and nutrients into the plant

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Pericycle

• A thin layer of undifferentiated cells
• A meristem tissue
• The source of new lateral, or secondary, roots

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Vascular tissue

• The transport system

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Vascular tissue

• Phloem
• living cells comprised of seive tube elements and
  companion cells
• Cambium
• a meristematic tissue supplying cells that will
  develop into vascular structures

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Vascular tissue

• Xylem
• dead cells with thickened cell walls, typically
  larger than phloem cells
• functions in the transport of water and mineral
  nutrients

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A long look at the root

• A longitudinal section through the root (from
  bottom to top) shows these various tissues and
  how they develop.

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A long look at the root

• Root cap
• Cells are sloughed off
• Protects the apical meristem
• Apical meristem
• Source of cells for the root

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A long look at the root

• Elongation zone
• Where cells elongate and differentiate
• Pushes the root tip through the soil
• Maturation zone
• Cells develop their final form
• Root hairs form, xylem cells thicken and die

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Organ
Tissue
Cell Type
Epidermis
Cortex
Parenchyma
Root
Casparian Strip
Endodermis
Parenchyma
Pericycle
Vascular System
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Transpiration

• The continuum of water from soil, through the
  plant and into the air

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Root modifications

• The organ that is consumed in many vegetable
  crops, such as carrot and sweet potato, is a
  modified root that serves as a storage tissue
• Other modifications for support