Chapter 7
ROOTS
FUNCTIONS OF THE ROOT
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Anchorage.
Absorption and conduction of water and minerals.
Storage of food.
Production of some hormones and secondary metabolites.
The cylindrical shape of the root allows all sides to have the same absorptive capacity.
The cylindrical shape also facilitates the penetration of the root through the soil.
EXTERNAL STRUCTURE OF THE ROOTS.
Taproot system.
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Produced by gymnosperms, eudicots and magnoliids.
The primary root originates in the embryo derived from the embryonic radicle.
It grows downward and is called then a taproot.
It produces lateral roots (branch roots).
This system generally penetrates deep into the soil.
Fibrous root system.
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It is found in monocots.
The primary root originates from the embryonic radicle.
It is short lived.
Adventitious roots develop from the stem.
This system is usually shallow.
Many eudicots are perennial and undergo secondary growth, resulting in an increased quantity of
functional tissue. The taproot system allows an increase in the absorptive capacity of the roots to
satisfy the demands of the above ground tissues.
Monocots do not have secondary growth and after their tracheary elements mature their conducting
capacity cannot increase. Extra leaves could not be supplied with water and minerals and the demands
of the secondary growth of a taproot system could not be satisfied. The fibrous root system is the most
practical for monocots.
Some monocots add new tissue through the growth of stolons. The stolons produce adventitious roots
at regular intervals to supply the new growth with an adequate supply of water and nutrients.
Feeder roots are those laterals that are actively engaged in absorption of water and minerals.
Most of the absorption of water and minerals takes place in the upper 15 cm of soil, the area richest in
organic material.
Desert plants can have very long taproots that penetrate deep into the soil.
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A mesquite bush near Tucson, AZ, had roots 53.3 m or 175 feet deep into the soil.
Acacia and Tamarix trees found in Egypt grow roots 30 m deep into the soil.
Plants maintain a balance between its shoot and root systems.
In young plants, the absorbing surface of roots far exceeds that of photosynthesizing parts.
As the plant ages, the root-to-shoot ratio decreases.
Damage to the roots decreases the shoot production.
Damage to the shoot decreases the availability of food and hormones for the roots.
STRUCTURE OF INDIVIDUAL ROOTS
Root growth is a continuous process that stops only during very adverse conditions of low temperature
and heavy drought.
Roots follow the path of least resistance through the soil.
The tip of the root is covered by a rootcap that produces mucigel.
The rootcap is a thimble-like mass of living parenchyma cells that covers the apical meristem behind it
and helps the root to penetrate the soil.
Behind the rootcap and root apical meristem is a zone of elongation; behind it is the root hair zone or
zone of maturation.
INTERNAL STRUCTURE OF ROOTS
Root cap
As the root grows it pushed the rootcap forward and the peripheral cells are sloughed off.
The sloughed off cells and the cap is covered with a slimy mucigel that lubricates the path of the root
through the soil.
New cells produced by the apical meristem are added to the cap to replace the ones that have been
sloughed off.
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As cells are pushed closer to the edge of the cap, their ER becomes less conspicuous, starch
grains are digested, and the dictyosomes secrete copious amounts of mucigel by exocytosis; the
middle lamella breaks down and releases the cells. The life of these root cap cells is estimated
to be between 4 or 5 days.
The mucigel is a hydrated polysaccharide secreted by the rootcap cells.
The mucigel accumulates in the Golgi vesicles that release the mucigel into the cell wall and
eventually leaks to the outside.
The rootcap also plays a role in gravitropism.
The central column of cells or columella contains starch grains or amyloplasts, which probably act as
gravity sensors.
Root apical meristem
Meristematic initials are the dividing cells of the meristem.
Longitudinal files or lineages of cells emanate from the apical meristem of the root.
The meristematic initials and its derivatives are polyhedral cells with large nuclei and dense cytoplasm.
The activity of the meristematic initials decreases as the plant develops and divisions become
infrequent. This region is called the quiescent region of the meristem.
Cell divisions occur in cells a bordering the quiescent initials.
The quiescent region is the origin of initials. The quiescent region helps to repopulate the bordering
meristematic regions surrounding it in case of injury.
The quiescent center plays an essential role in organization and development of roots.
Zone of elongation
The region of cell elongation is immediately behind the region of cell division.
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It usually s a few millimeters in length.
The elongation of cells results in most of the increase in root length.
There is no elongation beyond this area.
Only one small portion of the root is pushed through the soil at a given time.
Differentiation begins in this region.
The tissues in this zone are quite permeable. Minerals penetrate deep into the root through the apoplast
simply by diffusing along the thin, fully hydrated well walls and intercellular spaces.
Zone of Maturation
In the region of maturation or differentiation is where most of the primary tissues mature.
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Root hairs are produced here.
This region is also called the "root hair zone".
There is gradual transition from one zone to the next.
Some cells may begin to elongate and differentiate in the region of cell division whereas others do it
the region of elongation.
The protoderm, ground meristem and procambium can be distinguished in very close proximity to
the apical meristem. These will differentiate into epidermis, cortex and primary vascular tissues
respectively.
PRIMARY STRUCTURE
There are three tissue systems in the root: epidermis (dermal tissue system), cortex (ground tissue
system) and vascular bundle (vascular tissue system).
Epidermis.
Absorption of water and minerals is facilitated by root hairs.
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A 4-month-old rye plant was estimated to have 14 billion root hairs, with an absorbing surface of
401 m2, and an end-to-end length of over 10,000 km.
Root hairs are short lived.
They are found in the region of maturation.
Production of root hairs occurs in the region of elongation at a rate that matches the rate at which the
older root hairs are dying off at the upper end of the of the root hair zone.
The walls of the epidermal cells offer little resistance to the passage of water and minerals even in
those species with a thin layer of epidermal suberin.
The mucigel produced by the rootcap is favorable to nitrogen-fixing bacteria.
The rhizosphere is the layer of soil bound to the root by mucigel and root hairs. It contains a variety of
microorganisms and sloughed rootcap cells.
Typically the root hairs do not develop into ectomycorrhizae.
Cortex
The cortex represents the ground tissue system in most roots.
The cortex occupies the greatest area of the primary body of most roots.
The plastids of cortical cells store starch; these cells are devoid of chloroplasts
Roots that undergo significant secondary growth shed their cortex early, e.g. eudicots, magnoliids and
gymnosperms.
In monocots, the cortex is retained for life. The parenchymatous cells may develop secondary wall and
become lignified.
There are numerous intercellular air spaces in the cortex.
Substances moving across the cortex may follow the symplast pathway moving from cell to cell
through the plasmodesmata or an apoplastic pathway through the spaces in between cells and spaces
in the cell walls.
The innermost layer of the cortex is the endodermis and is characterized by the Casparian strip in the
radial and transverse walls (anticlinal walls), which are those perpendicular to the surface of the root.
See Fig. 7-13.
The Casparian strip is made of a suberized the primary wall and middle lamella.
The Casparian strip is impermeable to water and ions and prevents their passage via the apoplast into
the vascular tissue.
All substances entering and leaving the vascular cylinder must pass through the protoplasts of the
endodermal cells. This happens by passing through the plasma membrane or by moving through the
symplast formed by the numerous plasmodesmata connecting the endodermal cells with protoplasts of
neighboring cells of the cortex and vascular cylinder.
In roots that undergo secondary growth, the cortex and endodermis are shed early.
Old roots mostly transport water and minerals absorbed by the younger roots they are connected to.
In older roots that conserve the endodermis (cortex), a suberin lamella consisting of alternating layers
of suberin and wax is deposited internally in the endodermal cells.
Opposite the protoxylem of the root some endodermal cells may remain thin walled and allow the
passage of materials. These cells are called passage cells.
The suberin lamella does not necessarily prevent the passage of water and minerals. As long as the
endodermis remains alive, their plasmodesmata remain providing symplastic passage for water and
minerals.
To roots of many angiosperms develop a suberized outermost layer of cortex under the epidermis
called the exodermis.
The exodermis probably reduces the loss of water to the soil and minimizes the attachment of the root
by microorganisms.
Abstract Roots of virtually all vascular plants have an endodermis with a Casparian band, and the majority of angiosperm
roots tested also have an exodermis with a Casparian band. Both the endodermis and exodermis may develop suberin
lamellae and thick, tertiary walls. Each of these wall modifications has its own function(s). The endodermal Casparian
band prevents the unimpeded movement of apoplastic substances into the stele and also prevents the backflow of ions that
have moved into the stele symplastically and then were released into its apoplast. In roots with a mature exodermis, the
barrier to apoplastic inflow of ions occurs near the root surface, but prevention of backflow of ions from the stele remains a
function of the endodermis. The suberin lamellae protect against pathogen invasion and possibly root drying during times
of stress. Tertiary walls of the endodermis and exodermis are believed to function in mechanical support of the root, but
this idea remains to be tested. During stress, root growth rates decline, and the endodermis and exodermis develop closer
to the root tip. In two cases, stress is known to induce the formation of an exodermis, and in several other cases to
accelerate the development of both the exodermis and endodermis. The responses of the endodermis and exodermis to
drought, exposure to moist air, flooding, salinity, ion deficiency, acidity, and mechanical impedance are discussed.
Daryl E. Enstone, Carol A. Peterson and Fengshan Ma (2004) Journal of Plant Growth Regulation 21: 335-351
http://www.springerlink.com/content/hu9r3lwguxft/?p=2b246743aebf4df19b59214cda207b91&pi=0
Vascular cylinder.
The vascular cylinder consists of vascular tissue and the nonvascular pericycle.
The pericycle originates from the procambium and completely surrounds the vascular tissue.
The pericycle is made of parenchyma, which may develop secondary cell wall as they age.
Important functions of the pericycle are:
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Lateral roots originate in the pericycle.
Gives rise to the cork cambium in roots with secondary growth.
The pericycle often produces more pericycle
The center of the vascular cylinder is occupied by the xylem.
The phloem is found in between the "arms" of xylem.
The first xylem to develop is called the protoxylem and is located next to the pericycle.
The metaxylem is found to the inside and center of the cylinder and it matures after the protoxylem.
Some monocot roots have pith considered by some botanist to be part of the vascular cylinder because
of its possible procambial origin.
SECONDARY GROWTH – MATURE PORTIONS OF THE ROOT
The roots of monocots lack secondary growth and consist only of primary tissue.
The roots of many herbaceous eudicots undergo little or no secondary growth and remain mostly
primary in composition.
Secondary growth in roots and stems consists on the formation of
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Secondary vascular xylem and phloem from the vascular cambium.
Periderm composed mostly of cork tissue, from the cork cambium.
Secondary growth begins in portions of the root that are not elongating, in the area between the xylem
and phloem.
Pericycle cells also begin to divide at same time in the area opposite to the protoxylem
Eventually the cambium becomes circular due to the accumulation of secondary xylem toward the
center of the vascular cylinder.
Repeated divisions towards the inside and outside add secondary xylem and secondary phloem added
to the root.
With increases divisions toward the inside secondary xylem and secondary phloem are added to the
root.
Files of parenchyma cells that extend radially form rays in the secondary xylem.
With increases in width of the secondary xylem and phloem, most of the primary phloem is crushed
and obliterated.
The periderm replaces the epidermis in most woody roots.
Division of the pericycle layer causes an increase of pericycle layers in a radial extent.
The formation of the periderm follows the initiation of secondary xylem and phloem production.
As the pericycle cells are pushed out by the addition of secondary xylem and phloem, they multiply
and begin to produce more pericycle cells.
The cork cambium arises in the outer part of the pericycle.
Activity of the cork cambium gives rise to cork cells to the outside and to phelloderm toward the
inner surface.
Collectively, these three tissues, cork, cork cambium and phelloderm, form the periderm.
The remaining pericycle develops into a cortex tissue.
With the formation of the periderm, the epidermis, the cortex and the endodermis eventually die and
are sloughed off.
At the end of the first year’s growth, the following tissues are present in a woody root from outside to
inside: possible remnants of the epidermis and cortex, periderm, pericycle, primary phloem (fibers if
present and crushed soft-walled cells), secondary phloem vascular cambium, secondary xylem, and
primary xylem.
LATERAL ROOTS
Lateral roots commonly arise from the pericycle.
They are called endogenous roots because they arise deep from the parent root.
Lateral roots originate beyond the region of elongation in partially or fully differentiated tissues.
Derivative cells from the pericycle and endodermis contribute to the young root or root primordium.
The young lateral root develops a root cap and an apical meristem.
The young root digests its way through the cortex.
Initially, the vascular cylinders of the young root and the parent root not connected.
The two cylinders become connected at a later stage when the parenchyma cells in between develop
into xylem and phloem.
ROOT ADAPTATIONS
Storage roots like carrots produce an unusual large amount of parenchyma cells in the secondary
xylem to store carbohydrates.
Prop roots provide support. The contract slightly and cause tension on the stem and act as stabilizers.
Aerial roots are adventitious roots produced from structures above the ground.
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The roots of epiphytic orchids produce a multiple epidermis called velamen that provides
protection against abrasion and reduces water loss; it may absorb some water.
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Clinging roots of vines allow the plant to grow up on vertical surfaces.
Contractile roots contract after extending through the soil and becoming firmly anchored; the stem is
pulled downward so that the base of the shoot is either kept at soil level or, in the case of bulbs, is
buried deeper.
Roots require oxygen for respiration, which is why most plants cannot live in soil that is not
inadequately drained in swampy habitats.
Pneumatophores are negatively gravitropic that grow upward out of water or mud to provide adequate
aeration to the root system.
Mycorrhizae
Mycorrhizae are mutualistic associations between fungi and roots.
It has been found in most cultivated and wild plants.
The mustard family (Brassicaceae) and the sedge family (Cyperaceae) do not form mycorrhizae.
Mutualism:
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The host plant benefits by their increase ability to absorb water and minerals, especially
phosphorous, zinc, copper and manganese.
Mycorrhizal fungi protect the root of plant against attack by parasitic fungi and nematodes.
The fungus receives carbohydrates and other organic molecules synthesized by the plant.
Endomycorrhizae penetrate root cells.
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It occurs in about 80% of the plants.
The fungal component is usually a zygomycete. About 200 species are involved worldwide.
The hypha forms an invagination of the cortical cells of the root and forms arbuscules with
vesicles.
The other portion of the hypha extends into the surrounding soil and absorbs water and
nutrients.
Ectomycorrhizae surrounds but do not penetrate the root cells.
Ectomycorrhizae is commonly found in trees growing in temperate regions. Some have been found in
the tropics also.
Willow family (Salicaceae), birch family (Betulaceae), beech family (Fagaceae) and pine family
(Pinaceae) have ectomycorrhizal associations.
It possibly makes the trees more resistant to cold, dry conditions.
The hyphae grow in between the cortical and epidermal cells of the root forming a network called
Hartig net.
A mantle of hyphae covers the root surface, and mycelium extends from the mantle into the soil.
Most ectomycorrhizal fungi are basidiomycetes, but ascomycetes are also involved.
In the Ericaceae, heather family, the ectomycorrhizal hyphae form a web surrounding the roots.
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The hyphae do not extend into the soil.
The hyphae release enzymes that break down organic matter in the soil, which becomes
available to the plant.
It enhances nitrogen rather than phosphorus uptake.
It allows members of this family to colonize infertile, acidic soils.
Basidiomycetes and ascomycetes are involved in this association.
Orchid seed germinate only in the presence of suitable fungus.
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The fungus supplies the seedling with carbon compounds.
Basidiomycetes and ascomycetes form these associations.
There is evidence that mycorrhizal associations existed very early and it might have been crucial in the
colonization of land by plants.
Root nodules
See chapter 25 notes for details about the fixation of nitrogen.
It is important to note that root nodules are not root knots, which are root swellings in response to
worm invasions.
Root nodules are beneficial bacterial colonies that are visible as small swellings in the root system.
The bacteria aid the plant in fixing, or converting, atmospheric nitrogen in to a form that the plant can
use.
Root nodules are found extensively throughout the legume family.
A nodule develops when a substance leaked into the soil by plant roots stimulates Rhizobium bacteria
to produce another substance that caused root hairs to bend sharply.
The bacterium may attach in the crook of the bend and then ‘invade’ the cell with a tubular infection
thread.
This thread does not penetrate the cell wall and plasma membrane.
The thread, does however, grow through to the cortex, which is stimulated to produce new cells that
will become part of the housing for the bacterium.
As the bacteria multiply and the colony grows, the nodule will swell. It is in the crook of root hairs that
the nitrogen fixing takes place.
HAUSTORIAL ROOTS OF PARASITIC FLOWERING PLANTS
The substrate of parasitic angiosperms is the body of another plant.
The roots of parasitic plants penetrate the host and absorb nutrients and other necessary materials from
the host. As a result, these roots have become highly modified and are known as haustoria.
Parasitism has evolved several times.
The haustoria must adhere securely to the host by secreting and adhesive or by growing around a small
branch or root.
Penetration of the parasitic roots occurs by forcing a shaft of cells through the host’s dermal system or
by expanding the haustorium radially, cracking the host’s epidermis.
In many cases, the cells of the host and the parasite divide and form a mass of parenchyma that
differentiates into xylem vessels forming a continuous vessel from host to parasite
Some parasites attack only the xylem while other contact both xylem and phloem.
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Holoparasites: absorb water, minerals and photosynthates from the host; do not carry on
photosynthesis.
Hemiparasites: photosynthetic parasites that absorb only water and minerals from the host.
Some parasitic plants are very reduced.
Tristerix is a diffuse root system (haustorium) made of parenchyma cells with no stem, leaves,
epidermis or cortex except when it produces flowers.