The Root System

1
The Root System

• Chapter 7

2
Functions of Roots

• Absorption of water and minerals
• Anchor plant in soil or to a surface
• Storage of foods
• Conduction of food and water from soil and from
  storage reserves into the shoot

3
Root System

• Becomes more complex as plant grows
• Balance between shoot and root systems
• Root system supplies shoot with enough water
  and minerals
• Shoot system manufactures enough food for root
  system

4
Root System

• Rhizosphere
• Contact zone between root surface and soil
• Few mm thick
• Bacteria and fungi nearer roots richer and more
  diverse than in soil farther away
• Competition among plants
• Competition for water, minerals, light energy by
  plants in close proximity to each other
• Reduce competition by using different parts of
  soil

5
Types of Root Systems
Type of System Description Examples
Fibrous root system Several main roots that branch to form dense mass of roots Corn (Zea mays) Rye (Elymus cereale)
Tap root system Large storage root Carrots (Daucus carota)
6
Different Types of Roots

• Radicle
• Embryonic root
• Forms primary root
• Tap root system
• Develops from one primary root that forms lateral
  roots

7
Different Types of Roots

• Grasses
• Seminal roots
• Embryonic roots just above radicle
• Emerge after radicle
• Radicle and seminal roots form fibrous root
  system

8
Different Types of Roots

• Adventitious roots
• Originate on leaves and stems
• Examples
• Prop roots
• Develop on stem just above soil
• Absorb water and minerals
• Support plant in soil
• Example corn
• Aerial roots
• Pneumatophores

9
Different Types of Roots

• Aerial roots
• Formed by branches of tree
• Absorb water and minerals
• Most important function is to hold up tree
• Example banyan tree
• Pneumatophores
• Small, adventitious roots
• Stick up from mud
• Absorb oxygen to supply submerged roots
• Example mangrove tree

10
Root Propagation

• Place stem of plants such as blackberry or willow
  in moist soil to induce root formation
• Leaves of plants such as Begonia can be rooted by
  soaking them in water

11
Root Tip

• Organized into three regions
• Root apical meristem (RAM)
• Region of elongation
• Region of maturation

12
Root Tip

• Root cap
• Protects RAM
• Site of gravity perception
• Cells constantly sloughed off and replaced
• Sloughed cells called border cells
• Border cells remain alive in soil for short
  period of time
• Provide nutrients for soil bacteria and fungi in
  rhizosphere

13
Root Tip

• RAM
• Regularly shaped cells
• Organized into two areas
• Quiescent center (QC)
• Small, centrally located portion
• Slow rate of cell division
• Seems to be activated in times of stress
• May be site where hormones controlling root
  development are synthesized
• Cells apical to QC
• Divide to produce cells that form root cap

14
Root Tip

• Region of elongation
• Cells elongate
• Region of maturation
• Site of root hair formation
• Site of maturation of other cell types

15
Primary Meristems of Root

• RAM differentiates into
• Protoderm
• Ground meristem
• procambium

16
Summary of Tissues and Meristems in Roots
17
Epidermis

• Formed by differentiation of protoderm cells
• Root hairs
• Epidermal cell extension into surrounding soil
• Thin cell walls composed of pectin and cellulose
• Sticky
• soil particles cling to root hairs
• Short life
• Function only a few days or weeks

18
Epidermis

• Root hairs
• Not formed by all plants
• Some gymnosperms (Example firs)
• Many aquatic plants
• Usually one cell layer thick
• In aerial roots of plants like orchids can be
  multilayered

19
Cortex

• Derived from ground meristem
• Mainly composed of parenchyma cells
• Endodermis
• Innermost layer of cortex

20
Cortex

• Casparian strip
• Waxy material embedded in upper, lower, and side
  walls of endodermal cells
• Function
• Guarantee minerals that reach vascular cylinder
  do so by first passing through at least one
  plasma membrane
• Provides mechanism to increase concentration of
  needed minerals through pumps in endodermis cell
  membrane

21
Cortex

• Exodermis
• Present in many grass roots and aerial roots of
  orchids
• Occurs at outer layer of cortex
• Functions to regulate ion absorption and
  accumulation

22
Vascular Cylinder

• Composed of tissue differentiated from procambium
  cells
• Dicot roots
• Central core of primary xylem elements organized
  into two or more radiating points
• Monocot roots
• Center of root composed of parenchyma (sometimes
  called pith)
• Primary xylem and phloem form in a ring

23
Vascular Cylinder

• Protoxylem
• First xylem element to mature
• Develops at outer points of xylem
• Transports water while root is elongating
• Secondary cell wall in shape of annular rings or
  spirals
• Provides both strength and elasticity

24
Vascular Cylinder

• Metaxylem
• Last primary xylem to mature
• Differentiates in center of vascular cylinder
• Mature in regions of root where elongation is
  completed
• Forms thick secondary cell walls with pits for
  lateral exchange of water and nutrients

25
Vascular Cylinder

• Xylem cell types in roots
• Vessel elements
• Tracheids
• Parenchyma
• Fibers

26
Vascular Cylinder

• Phloem
• Forms in areas between protoxylem arms
• Protophloem
• First part of vascular cylinder to become
  functional
• Cells form at periphery of phloem
• Function mainly during root elongation
• Metaphloem
• Develops toward inside
• Functions during plants adult life

27
Vascular Cylinder

• Phloem cell types
• Parenchyma
• Fibers
• Sieve-tube members
• Companion cells

28
Vascular Cylinder

• Pericycle
• Outermost boundary of vascular cylinder
• Remains capable of dividing for long period of
  time
• Functions in dicot roots
• Site where lateral root development is initiated
• Contributes to formation of vascular cylinder
• Contributes to formation of cork cambium

29
Vascular Cylinder

• Pericycle
• Function in monocot roots
• Lateral root initiation

30
Lateral Roots

• Chemical regulators cause pericycle cells to
  begin dividing at specific sites
• Lateral root primordia result
• Lateral root pushes through and destroys cortical
  cells and outer epidermis
• Digestive enzymes released from lateral root
  primordia probably aid in process
• Emerging lateral root organized into root cap and
  RAM

31
Lateral Roots

• Site where lateral root emerges is healed by
  secretion of mucilage and waxy substances from
  cortical cells
• Vascular system of lateral root connected to
  vascular system of main root axis

32
Secondary Vascular Tissues

• Formed by activation of vascular cambium
• Initiated by activation of pericycle cells and
  residual procambium cells located between arcs of
  xylem and phloem
• Forms secondary xylem toward inside
• Forms secondary phloem toward outside

33
Secondary Vascular Tissues

• Secondary xylem
• continued growth expands root
• Causes splitting, sloughing off, destruction of
  cortex and epidermis
• Pressure stimulates pericycle to divide again
• Converts secondary xylem into cork cambium which
  forms periderm

34
Secondary Vascular Tissues

• Secondary xylem
• Bark on woody roots
• Includes all cells from vascular cambium outward
• Appears similar to bark in stems
• Has thinner, smoother outer surface
• Only monocot plant with secondary growth in roots
  is dragons blood tree (Dracaena draco)

35
Special Functions of Roots

• Haustorial roots
• Parasitic roots
• Grow into vascular tissue of host stem
• Obtain water and nutrients from host

36
Special Functions of Roots

• Root nodules
• Formed by leguminous plants
• Bacteria associated with nodules fix atmospheric
  nitrogen to form usable by plants

37
Special Functions of Roots

• Mycorrhizae
• Association between roots and a fungus found in
  the soil
• Short, forked roots
• Common to about 90 of seed plants
• Not essential for growth of host plant
• Functions
• Make roots more efficient in mineral absorption
• May secrete hormones or antibiotics that reduce
  potential for plant diseases

38
Special Functions of Roots

• Mycorrhizae
• Two types
• Ectotrophic mycorrhizae
• Causes changes in root shape
• Fungus does not penetrate root cells
• Penetrates between cell walls of cortex forming
  sheath of fungal hyphae around root
• These mycorrhizal roots lack root cap and have
  simple vascular cylinder
• Endotrophic mycorrhizae

39
Special Functions of Roots

• Endotrophic mycorrhizae
• Fungus enters cortex cells

40
Special Functions of Roots

• Dandelion roots
• Capable of contracting
• Accomplished by radial expansion of cells in root
  cortex
• Keeps aboveground parts near soil surface