Download Zone of Cell Elongation

THE ROOT
STRUCTURE & FUNCTIONS
FUNCTIONS
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Water & nutrient absorption
Anchor the plant to the soil
Storage
Perception of gravity (root cap)
Root Systems
• Taproot – long root
extending deeply into
the soil to store food
and nutrients and
absorb water from deep
in the ground.
• Fibrous (adventitious)
– an extensive mass of
similarly sized roots.
Root Systems
• Prop roots – fibrous roots below ground
and above ground that help support a tall
upright stem. (Page 156, fig. 7.3 )
• Rhizomes – adventitious roots grown from
underground stems, also serve as
vegetative, asexual, mean of reproduction
for the plant. Where new plants develop
from the rhizome, with out the fusion of
gametes.
Root Cap and Tissues
• Tips of the root are covered and protected by
a thimble-shape Root Cap. The cells that
comprised the root cap produce a slimy
substance known as Mucigel, which is a
mixture of sugars, enzymes and amino acids.
• Mucigel functions for:
– Protection – prevents the roots from drying out,
and inhibit growths of other plants around it.
– Lubrication
– Water Absorption – maintain continuity of
between roots and soil, because soil clings the
mucigel
– Nutrient Absorption – helps nutrients absorb in
the same way it helps water.
Subapical Region
• Region behind the root cap is called the
subapical region.
– Zone of Cell Division – consist of apical meristem
cells, region where cells are dividing (~24-36 hours),
producing upwards of 20,000 new cells a day.
– Zone of Cell Elongation – newly formed cells
elongate by up to 150X. Primarily, by filling vacuoles
with water, this action can push the roots at a
downward rate of up to 4 cm a day.
– Zone of Cell Maturation – where immature cells take
on a specific function, developing root hairs on the
outside of the root.
• Root hairs form only in the mature, non-elongating regions of
the root.
Development
• Primary growth
– From the root apical meristem
tissues & increased root length
primary
• Secondary growth (mostly in woody plants; it is limited in
herbaceous plants
– From the vascular cambium
tissues & increased diameter
secondary
Root Tissues
• Hypodermis is the outermost layer of the
cortex, which is typically coating in a
protective layer called suberin, which is a
waxy substance that prevents water loss
in the roots closest to the surface by as
much as 200X.
• Endodermis is the inner most layer of the
cortex.
Root Tissues
• Stele of the root includes all tissues found
in the middle of the roots and consist of
the pericycle, and vascular tissues.
• Pericycle is a thin walled meristem tissue
that produces branch roots.
Rhizosphere
• This is the narrow zone of soil that
surrounds the roots, extending up to 5mm
from the root surface.
• Very complex and constantly changing
environment.
Rhizosphere
•
Growth of the plant changes the Rhizosphere
by
1. Roots force their way down through the soil creating
crevices, that help to aerate the soil when the plant
dies.
2. Alter chemical composition of the soil thru
respiration, which lowers the oxygen in the soil and
increases the CO2 concentration. This change the
pH of the soil, increasing the acidity which increases
the availability of ions such as iron and decreasing
the availability of potassium.
Rhizosphere
3.
Roots enrich the soil with organic matter. Some plants
can transport as much as 60% of the sugars not used,
into the soil – primarily through the mucigel.
Rhizosphere typically contains large amounts of energy
rich organic molecules that microorganisms use to for
their cellular processes. Upwards of 1010
microorganisms per cubic centimeter
~(10,000,000,000)
Root Growth
• Growth and Distribution of Roots is
primarily controlled by;
– Gravity
– Genetic differences
– Stage of Plant Development
– Soil properties
Environmental Adaptations
• Storage
– Store sugars (ex. Sweet Potato)
– Water storage (ex. Hottentot – uses huge 350
kg / 770 lbs root to store water and starch)
• Vegetative reproduction
– Suckers – shoots that when separated from
the parent plant become new plants.
Environmental Adaptations
• Aeration
– Plants that grow in low oxygen environments
such as tidal marshes, avoid suffocation by
growing roots that are adapted to import
oxygen from the atmosphere. (ex. Black
Mangroves)
– These roots can contain up to 80%
aerenchyma tissues, contain large air spaces,
and act as snorkels.
Environmental Adaptations
• Movement
– Contractile roots, contract the xylem and
cortex which pull the plant deeper into the
soil. (Ex. Dandelion)
• Nutrition
– Parasitic plants attach to the host plant and
siphon nutrients from them – thus harming the
host plant. (Ex. Mistletoe)
Epiphytes
• Epiphytes are plants that grow
independently on other plants, these
include orchids, stagehorn ferns and some
cacti.
• Typically grow slowly and absorb nutrients
through aerial roots.
Mycorrhizae
• Associations between plant roots and fungi in the
soil. Mutualistic relationship.
• Fungi absorb nutrients in the soil that the plant uses
and in turn the plant provides the fungi with sugars,
amino acids and other organic material.
• Mycorrhizae dramatically increase plant growth and
absorption.
– Wheat 200%
– Corn 100%
– Onions 3,000%
% increase in
plant growth
• Helps to improve a plants tolerance to drought,
disease, temp. extremes, and lack of nutrients.
Nitrogen Fixation
• Plants have established a mutualistic
relationship between N2 fixing bacteria,
bacteria species that contain the enzyme
nitrogenase that can convert atmospheric
nitrogen in to ammonia (NH3) which the
plants can utilize for growth and
production of amino acids.
Soil Layers
Plant Nutrition
•
Elements are essential for normal plants
if they:
1. Required for normal growth and
reproduction
2. No other element can replace it
3. Has a direct / indirect action on plant
metabolism
Plant Nutrition
• Macronutrients are elements that are required in
large amounts, usually more that 0.5% of the dry
weight of the plant.
• Nine required macronutrients
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Carbon
Hydrogen
Oxygen
Phosphorus
Nitrogen
Sulfur
Calcium
Magnesium
Plant Nutrition
• Plant also require micronutrients, or trace
elements, which are required for metabolism in
very small quantities (~ppm, or part per millions
or less).
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Iron
Copper
Zinc
Chlorine
Manganese
Molybdenum
Boron
Plant Nutrition
• Functions of essential nutrients
– Part of cellular structures and biological
molecules.
– Involved in energy related chemical reactions
– Activate or inhibit enzymes.
– Alter osmotic pressure of a cell.
Importance and Uses of Essential
Elements
• Some plant can concentrate certain
elements in their bodies at much higher
concentrations than needed; this is called
hyperaccumulators.
• Helps plants to evade “weak stomached”
animals and insects, acts as a defensive
measure.
– Locoweed accumulates selenium, which
affects animals nervous systems causing
confusion, reduced mobility and death.
Environmental Importance of Root
Systems and Hyperaccumultaion
• Phytoremediation – is the use of
hyperaccumulators to remove toxic
substances from contaminated soils.
• Hyperaccumulators also help geologist
explore for ore deposits and contaminated
areas and buried dump sites.