Dr' Bennett copy

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CHAPTER 10 ROOT GROWTH
Out of sight, out of mind Because roots are
below-ground, they are difficult to
study Rhizosphere Root environment (air,
water, and mineral phases). The soil environment
is usually easier to modify than the aerial
environment (i.e., tillage, irrigation,
fertilization, liming etc.) Functions of
roots 1) Absorption 2) Storage 3)
Anchorage 4) Transport 5) Propagation 6)
Source of growth regulators Dicots thicker
roots - more storage Monocots fine with little
storage capacity
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Roots (cont)
Absorption Root tips and hairs (younger
roots) Transport and Storage Older
roots Root Study Techniques 1) Trench
profile 2) Framed monolith (p. 248) 3)
Soil moisture depletion (gravimetric or
neutron probe) 4) Core sampling
5) Mini-rhizotron (TV camera, glass plate) 6)
Radioactive isotope method (32P) 7) Allometry
method (SR ratio)
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MEASURES OF ROOTS
Data are expressed as Root density g of
roots per unit volume of soil
(g cm-3) Root length density cm (length)
of root per unit
volume of soil (cm cm-3) Measures based on
length are more closely associated with absorption
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Distance from tip
15-25 mm
5-15 mm
0-5 mm
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• ROOT EXTENSION
• Generally on the order of 1- 3 cm d-1
• Decreases with age
• As a general rule of thumb, most root
• growth is complete by early seed filling in
• most agronomic seed crops
• In sandy Florida soils, root depth generally
• reaches 1.5 - 2.0 m in depth
• 100 days x 2 cm d-1 200 cm 2.0 m

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ROOTS (cont)

• Major uptake of water and nutrients is
  generally
• in the upper 30 cm of soil
• Root morphology is genetically and
• environmentally controlled
• Senescence of roots occurs near crop maturity
  
• or throughout seed filling
• Sloughing-off of roots may occur at the same
  
• time as new roots are being produced
• Root to shoot communication
• (signals - ABA, cytokinin)

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ROOTS (cont)

• Lateral branching arises from the pericycle,
• deep within older or differentiated tissues
• Unlike stems, roots do not have nodes and
• internodes
• Endodermis - layer of cells that restricts
• movement in apoplast
• Root cap (not the meristematic region, but
• external to it)
• 1) Protection from physical damage
• 2) Lubrication for growing tip
• 3) Substrate for microbes
• 4) Production of ABA

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• ROOT HAIRS AND ROOT SUBERIZATION
• Root Hairs
• - specialized tissues
• - lateral extensions of epidermal cells
• - may be several mm in length
• - may be quite numerous (200 mm-2)
• produce mucigel, which promotes microbial
• activity
• - provide a very large surface area for interface
  with
• soil particles (enhances absorption of both
  water
• and nutrients)

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DICOT ROOT - SOYBEAN
a
b
a - Primary b - Secondary c - Tertiary d -
cotyledon
c

• Soil factors strongly affect the architecture
• of root systems
• Fineness of roots increases with the
• branching order

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a coleoptile b adventious root c mesocotyl d
seminal root e seed
Adventitious Roots-eventually develop
into primary root system
Seminal Root- From seed. 1st roots to appear
after germination
Monocots (Grasses) Fibrous Root System
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TYPES OF ROOTS
Seminal roots (Seed roots) - emerge along with
the radicle from the first node of the seed
embryo axis. Adventitious roots (nodal, crown
roots) - emerge from the basal nodes of the grass
shoot just below the soil surface Prop (Brace)
Roots (In Maize) - aerial adventitious
roots. Generally, seminal roots are thought to
contribute primarily early in development and
adventitious roots become dominant for most of
the remainder of the life cycle. Importance of
seminal roots depends on species and
environment.
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SEMINAL ROOTS (cont)

• In Maize,
• 1) Seminal roots are short-lived and contribute
  
• relatively little
• 2) They disintegrate from the plant after a few
  weeks
• Weight, volume, and length of adventitious roots
  is much greater
• 4) Seminal roots are important early in
  development
• Seminal roots are more important in small grains
• Barley, rye, wheat, oats

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Mycorrhizae
MYCORRHIZAE Fungi that infect root cells -
hyphae penetrate directly into root cells. Have
been shown to enhance uptake of water and certain
nutrients, especially P Generally, few root
hairs are present when roots are infested with
mycorrhizae.
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White clover
wheat
carrot
Unbranched tap root (alfalfa)
Branched tap root (birdsfoot trefoil)
Fig. 10.5 (Page 255)
DICOT
TAPROOT
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• FACTORS AFFECTING ROOT GROWTH
• 1) Genotype - controlled by a number of genes.
• Difficult to breed for various root types. Type
  of root system that is desirable is site specific
  (depending on replenishment of deep soil water
  etc., amount and distribution of rainfall, yield
  levels)
• Defoliation - since regrowth of many forage
• crops depends on use of stored CH2O in
  roots, defoliation timing and intensity can
  affect regrowth
• CH2O levels in roots, rhizomes has been shown
  to be related to regrowth potential
• For perennials, usually in late fall, PS is
  strongly partitioned to roots. (Stimulus
  Photoperiod or Temp?)
• 3) Oxygen

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FACTORS AFFECTING ROOT GROWTH (cont)
4) Soil pH (5-8) - Breeding efforts 5) Optimum
temperatures - may affect angle at which lateral
roots grow (i.e., distribution with depth) 6)
Adequate nutrients 7) Adequate water (drying of
roots in upper layers, then rehydration) In
general, stresses (water, nutrients) seem to
affect root systems less than the shoot Plant
roots do not grow to water or nutrients but
they may proliferate in soil regions where
water and nutrients are optimal See Fig 10.12
(maize roots)
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