Chapter 3
Lecture Outline
Roots and Soils
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Outline

Function of Roots

Root Development

Root Structure

Specialized Roots

Mycorrhizae

Root Nodules

Human Relevance of Roots

Soils
Function of Roots

Anchor plants into soil

Absorption of water and minerals

Store food or water

Other specialized functions
Root Development

Upon germination,
embryo’s radicle
grows out and
develops into first
root.
• Radicle may develop
into thick taproot with
thinner branch roots.
–
Dicotyledonous plants
(dicots)
Taproot system
Root Development
• Or, after radicle formation,
adventitious roots may
arise that develop into a
fibrous root system.
–
Adventitious roots do not
develop from another root,
but instead from a stem or
leaf.
–
Fibrous roots - Large number
of fine roots of similar
diameter
–
Monocotyledonous plants
(monocots) and some dicots
Fibrous root system
Root Structure

4 regions:
• Root cap
• Region of cell division
• Region of cell elongation
• Region of maturation
• Root Cap - Thimble-shaped
mass of parenchyma cells
covering each root tip
–
Protects tissues from damage
as root grows
o Secretes mucilage that
acts as lubricant
–
Functions in gravitropism (gravity perception)
Longitudinal section
through root tip
Root Structure

Region of Cell Division Composed of apical
meristem in the center
of root tip
• Subdivided into 3
meristematic areas:
–
Protoderm - Gives rise to
epidermis
–
Ground meristem - Gives
rise to cortex and pith
–
Procambium - Gives rise to
primary xylem and primary
phloem
Root tip showing
primary meristems
Root Structure

Region of Elongation - Cells become several
times their original length.
• Vacuoles merge

Region of Maturation - Cells differentiate into
various distinctive cell types.
• Root hairs form.
–
Epidermal cell extensions
with thin cuticle
–
Absorb water and minerals
–
Adhere tightly to soil particles
–
Increase total absorptive
surface of root
Root hair zone of
radish seedling
Root Structure
Region of Maturation

Cortex - Parenchyma cells between epidermis
and vascular cylinder
• Mostly stores food
Cross section
of dicot root
Root Structure
Region of Maturation
• Endodermis - Inner boundary of cortex, consisting of
a single-layered cylinder of compact cells
–
Cell walls impregnated with suberin and lignin on radial and
transverse walls
–
bands called Casparian strips
o
Forces water and dissolved substances entering and
leaving the central core to pass through endodermis
Regulates types of minerals absorbed
Eventually inner cell
walls become
thickened with
suberin, except for
passage cells.
o
–
Enlargement of vascular cylinder of dicot root
Root Structure
Region of Maturation

Vascular cylinder - Core of tissues inside
endodermis
• Pericycle - Outer boundary of vascular cylinder
Region of
endodermis
and pericycle
in dicot root
–
Continues to divide, even
after mature
–
Forms lateral (branch)
roots and part of the
vascular cambium
Lateral root formation
Root Structure
Region of Maturation

Most of cells of vascular cylinder are
primary xylem or primary phloem.
• In dicot or conifer roots - Solid core of xylem, with
“arms” in cross section
• In monocots, xylem
surrounds pith.
• Phloem in patches
between xylem arms
• Vascular cambium forms
secondary phloem to the
outside and secondary
xylem to the inside.
Vascular cylinder
of dicot root
Root Structure

Growth
• Determinate growth - Growth that stops after an
organ is fully expanded or after a plant has
reached a certain size
• Indeterminate growth - New tissues are added
indefinitely, season after season
Specialized Roots

Food Storage Roots
• Starch and other
carbohydrates
• Sweet Potatoes

Water Storage Roots
• Pumpkin family,
especially in arid regions

Manroot, water storage root
Propagative Roots
• Adventitious buds on roots - Develop into suckers
(aerial stems)
–
Fruit Trees
Specialized Roots

Pneumatophores
• In plants with roots
growing in water
• Spongy roots that
extend above the
water’s surface and
enhance gas
exchange between
atmosphere and
subsurface roots
Mangrove pneumatophores
Specialized Roots

Aerial Roots
• Orchids - Velamen
roots, with epidermis
several layers thick
to reduce water loss
• Corn - Prop roots
support plants in
high wind
• Ivies (English ivy,
Virginia creeper) Aerial roots aid
plants in climbing
Orchid aerial (velamen) roots
Specialized Roots

Contractile Roots
• Pull plant deeper into
the soil
–

Lilly bulbs, dandelions
Buttress Roots
• Stability in shallow soil
–

Tropical Trees
Buttress roots of
tropical fig tree
Parasitic Roots
• Most have no chlorophyll and dependent on
chlorophyll-bearing plants for nutrition
Mycorrhizae

Mycorrhizae - Fungi that form a mutualistic
association with plant roots
• Mutualistic association: Both fungus and root
benefit and are dependent upon association for
normal development
•
–
Fungi facilitate absorption and concentration of
nutrients, especially phosphorus for roots.
–
Plant furnishes sugars and amino acids to fungus.
Plants with mycorrhizae develop few root hairs
compared with those growing without an
associated fungus.
• Particularly susceptible to acid rain
Mycorrhizae
Root Nodules

Plants cannot convert free
nitrogen to usable form

A few species of bacteria
produce enzymes that
converts nitrogen nitrates and
other nitrogenous substances
readily absorbed by roots.
• Root nodules contain large
numbers of nitrogen-fixing
bacteria.
• Legume Family (Fabaceae)
Root nodules on roots
Human Relevance of Roots

Sources of food
• Carrots, sugar beets, turnips, horseradishes,
cassava (tapioca), yams, sweet potatoes

Spices
• Sassafras, sarsaparilla, licorice

Dyes

Drugs
• Aconite, ipecac, gentian, reserpine

Insecticide
• Rotenone
Soils

Soil is formed through the interaction of
climate, parent material, topography,
vegetation, living organisms and time.
• Solid portion of soil consists of minerals and
organic matter.
• Pore spaces between solid particles filled with air
or water.
Soils

Soils divided into horizons:
• Topsoil (10-20cm)
–
A horizon - Dark, with more
organic material than lower layers
–
E horizon - Light
• B Horizon (0.3-0.9m) - Subsoil
–
More clay, lighter in color than
topsoil
• C Horizon (varies) - Parent
material which extends to
bedrock
Soils
Parent Material

Parent material - Rock that has not been
broken down into smaller particles
• Rock types:
–
Igneous – Volcanic
–
Sedimentary - Deposited by glaciers, water or wind
–
Metamorphic - Changes in igneous or sedimentary
rocks from pressure or heat
Soils
Climate

Climate varies throughout the globe, as
does its role in weathering of rocks
• Deserts - Little weathering by rain, and soils
poorly developed
• In areas of moderate rainfall - Well-developed
soils
• Areas of high rainfall - Excessive water flow
through soil leaches out important minerals.
Soils
Living Organisms and Organic Composition

In soil there are many kinds of organisms,
roots and other plant parts.
• Bacteria and fungi decompose organic material
from dead leaves, plants and animals.
• Roots and other living organisms produce carbon
dioxide, which combines with water and forms acid
that increases the rate at which minerals dissolve.
• Small animals alter soil by their activities and by
their wastes.
• Humus - Partially decomposed organic matter,
gives soil a dark color
Soils
Topography

Topography - Surface features
• Steep areas:
–
Soil may erode via wind, water or ice.
• Flat, poorly drained areas:
–
Pools and ponds may appear.
–
Development of soil arrested.
• Ideal topography permits drainage without
erosion.
Soils
Soil Texture and Mineral Composition

Soil Texture - Relative proportion of sand,
silt and clay in soil
• Sand - Many small particles bound together
chemically
• Silt - Particles too small to be seen without
microscope
• Clay - Only seen with electron microscope
–
Individual clay particles called micelles
o
Negatively charged and attract, exchange or retain
positively charged ions, such as Mg++ and K+
Soils

Best agricultural soils - loams composed of
40% silt, 40% sand and 20% clay
• Coarse soils drain water too quickly.
• Clay soils allow little water to pass.

Soil Structure - Arrangement of soil particles
into aggregates
• Productive agricultural soils are granular with
pore spaces occupying between 40-60% of the
total soil volume.
–
Particle size is more important than total volume.
Soils
Water in the Soil

Hygroscopic Water - Physically bound to
soil particles and unavailable to plants

Gravitational Water - Drains out of pore
spaces after a rain

Capillary Water - Water held against the
force of gravity in soil pores
• Determined by structure and organic matter, by
density and type of vegetation, and by the
location of underground water tables
• Plants mostly dependent upon this type.
Soils
Water in the Soil

Field capacity - Water remaining in soil after
water drains away by gravity
• Determined by texture, structure and organic
content of soil

Permanent Wilting Point - Rate of water
absorption insufficient for plant needs
• Plant permanently wilts.

Available Water - Soil water between field
capacity and the permanent wilting point
Soils
Soil pH

Affects nutrient availability

Alkalinity causes some minerals, such as
copper, iron and manganese to become less
available.
• Counteract by adding sulfur, which is converted to
sulfuric acid by bacteria, or by adding nitrogenous
fertilizers

Acidity inhibits growth of nitrogen-fixing
bacteria.
• Counteract by adding calcium or magnesium
compounds = liming
Review

Function of Roots

Root Development

Root Structure

Specialized Roots

Mycorrhizae

Root Nodules

Human Relevance of Roots

Soils