Chapter 24 The Root: Structure and Development

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Chapter 24The Root Structure and Development
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Outline

• Intro
• Root systems
• 1º structure
• 2º growth
• Origin of lateral roots
• Aerial roots and air roots
• Adaptations for food storage

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Intro

• Out of sight out of mind
• poorly understood
• must disturb to observe
• highly extensive branched
• 1st to emerge upon germination
• Functions
• anchorage
• absorption
• storage (biennial plants)
• conduction
• hormone (cytokinin gibberellin) and 2
  metabolite synthesis

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Root Systems

• 1º root 1st root
• gives rise to taproot in dicots
• taproot system lateral roots branching off
  taproot
• fibrous root systems in dicots
• short-lived in monocots
• fibrous root system adventitious roots arising
  from stems and lateral roots branching off
  adventitious roots all roots similarly sized
• Suitability for soil stabilization

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Root Systems

• Distribution of root system (depth spread)
• depends on moisture temp. soil characteristics
• feeder/fine roots near soil surface
• better gas exchange OM nutrients
• responsible for water/nutrient uptake
• sp. dependent
• difficult-to-transplant sp.
• extent
• 1-7 x spread of tree crown
• SA and soil volume of rye
• 639 m2 130 x shoot 6 L soil
• mesquite 175 ft

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RootShoot Ratio

• Photosynthetic SA (shoots) balanced
  w/ absorbing SA (roots)
• effects of age
• rootshoot high in seedlings
• rootshoot decreases w/ age
• effects of stress (frost/pests/
  transplanting/compaction)
• damage to root system water nuts. hormones
  shoot death or of shoot growth until balance
  restored
• damage to shoot system photosynthates
  hormones root death or decreased root growth
• Seasonal root turnover

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Origin and Growth of Primary Tissues

• Johannas Talk

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Primary Structure Dermal

• Root hairs (epidermal extensions) responsible for
  absorption
• 14 B hairs on 4-mo old rye plant 401 m2 SA
  10000 km long (6 L soil volume)
• confined to region of maturation
• high turnover rates
• Mucigel
• facilitates absorption via better contact
• favorable for N2 fixing bacteria
• protection from desiccation
• Rhizosphere soil cells microbes
• Mycorrhizae SA aid absorption

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Primary Structure Ground

• Cortex
• substantial portion of 1 plant body
• plastids contain starch lack chlorophyll
• shed in dicots retained in monocots
• contains many intercellular spaces for aeration
• connected via plasmodesmata
• symplastic path move btwn cells via
  plasmodesmata
• apoplastic path move btwn cells via CW

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Primary Structure Ground

• Endodermis
• inner layer of cortex cells tightly packed lack
  air spaces
• Casparian strip
• strip of 1 CW and middle lamella impregnated w/
  suberin and lignin
• impermeable to water/nutrients
• prevents water loss
• forces water to move symplastically

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Primary Structure Ground

• Roots w/ 2 growth
• cortex/endodermis shed when 2 growth occurs
• absorption occurs in young roots
• old roots transport water from young roots

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Primary Structure Vascular

• Vascular cylinder 1 vascular tissue
  pericycle
• pericycle nonvascular cells surrounding
  vascular tissue
• from procambium
• origin of lateral roots
• gives rise to CC and more pericycle
• lacks pith
• some monocots appear to have pith)
• solid core of 1 X in
  center of
  vascular cyclinder
  w/ projections/ridges
• diarch 2 ridges
• tri- tetr- polyarch

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Secondary Growth

• Present in some dicots
  lacking in most monocots
• Forms vascular tissues
  and periderm
• X and P produced by VC
• periderm produced by CC
• VC forms btwn 1 X and P
• X to inside
• P to outside

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Secondary Growth

• Additional X and P added
• Radially extending lines of parenchyma form rays
• 1 P obliterated/shed
• Periderm becomes outermost layer
• produced by CC
• gives rise to lenticels
• CC forms from pericycle
• phelloderm to inside
• cork to outside
• Periderm CC phelloderm cork

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Secondary Growth

• Layers of 1-yr old twig
• remains of epidermis and cortex
• periderm
• cork
• CC
• phelloderm
• pericycle
• 1 P fibers
• 2 P
• VC
• 2 X
• 1 X

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Lateral Root Formation

• Form from pericycle
• behind region of elongation
• Young root pushes through cortex develops
  rootcap and apical meristem

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Special Roots

• Aerial roots adventitious roots from shoot
  structures
• prop roots
• Pneumatophores (air roots) roots adapted to wet
  environments
• cypress knees
• gas exchange
• Roots of epiphytes
• photosynthetic
• thick epidermis
  (velamen)
• Storage
• parenchymal cells
• carrot sweet potato beet

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