Roots and Soil

Chapter 5

Outline

Root Development

Root Structure

Specialized Roots

Mycorrhizae

Root Nodules

Soils

Horizons

Soil Formation

Factors

How Roots Develop

When a seed germinates, the embryo’s radicle grows out and develops into the first root.

May develop into thick taproot with branch roots.

Dicotyledonous Plants

May develop adventitious roots that develop a fibrous root system.

Monocotyledonous Plants

Root Structure

Root Cap - Thimble-shaped mass of parenchyma cells covering each root tip.

Protects tissue from damage.

Function in gravity perception.

Region of Cell Division - Composed of apical meristem in the center of the root tip.

Most cell division occurs at the edge of the inverted cup-shaped zone.

Root Structure

Region of Elongation - Cells become several times their original length.

Vacuoles merge

Region of Maturation - Most cells differentiate into various distinctive cell types.

Root hairs form.

Absorb water and minerals and adhere tightly to soil particles.

Thin cuticle

Region of Maturation

Cortex cells mostly store food.

Contain endodermis

Cell walls impregnated with suberin bands, Casparian Strips.

Forces all water and dissolved substances entering and leaving the central core to pass through plasma membranes of the endodermal cells.

Region of Maturation

Vascular Cylinder lies at the inside of the endodermis.

Pericycle lies directly against the inner boundary of the endodermis.

Lateral Roots

In both roots and stems, growth may be determinate (stops at a certain size) or indeterminate (new tissues added indefinitely).

Specialized Roots

Food Storage Roots

Sweet Potatoes

Water Storage Roots

Pumpkin Family

Propagative Roots

Adventitious Buds develop into suckers.

Fruit Trees

Specialized Roots

Pneumatophores

Spongy roots that extend above the water’s surface and enhance gas exchange between the atmosphere and subsurface roots.

Aerial Roots

Orchids

Specialized Roots

Contractile Roots

Pull plant deeper into the soil.

Lilly Bulbs.

Buttress Roots

Stability - Tropical Trees.

Parasitic Roots

Have no chlorophyll and are dependent on chlorophyll-bearing plants for nutrition.

Dodder

Mycorrhizae

Mycorrhizae form a mutualistic association with plant roots.

Fungus is able to absorb and concentrate phosphorus much better than it can be absorbed by the root hairs.

Particularly susceptible to acid rain.

Mycorhizae

Root Nodules

Few species of bacteria produce enzymes that can convert nitrogen into nitrates and other nitrogenous substances readily absorbed by roots.

Legume Family (Fabaceae)

Root nodules contain large numbers of nitrogen-fixing bacteria.

Soils

Soil is formed through the interaction of climate, parent material, topography, vegetation, and living organisms.

Solid portion of soil consists of minerals and organic matter.

Pore spaces occur between solid particles.

Filled with air or water.

Divided into soil horizons

Soils

A Horizon - Topsoil

Dark, rich soil

B Horizon - Subsoil

More clay, lighter in color

C Horizon - Parent Material

Not broken down into smaller particles.

Soils

Climate

Deserts experience little weathering due to low rainfall.

Grasslands have moderate rainfall and well-developed soils.

Rainforests have excessive rain and nutrients are quickly leached from the soil.

Soils

Living Organisms and Organic Composition

In upper 30 cm of a good agricultural soil, living organisms constitute about one-thousandth of the total soil weight.

Bacteria and fungi in the soil decompose organic material.

Humus, partially decomposed organic matter, gives soil a dark color.

Soils

Topography

Steep areas may erode via wind or water.

Flat areas may be flooded, and thus contain little available oxygen.

Soil Texture and Composition

Best agricultural loams are composed of 40% silt, 40% sand and 20% clay.

Coarse soils drain water too quickly

Dense soils have poor drainage.

Soils

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.

Soil Mineral Components

Stones > 76 mm

Gravel 76 mm - 2.0 mm

Very Coarse Sand 2.0 mm - 1.0 mm

Coarse Sand 1.0 mm - 0.5 mm

Medium Sand 0.5 mm - 0.25 mm

Fine Sand 0.25 mm - 0.10 mm

Very Fine Sand 0.10 mm - 0.05 mm

Silt 0.05 mm - 0.002 mm

Clay < 0.002 mm

Soils

Soil Water

Hygroscopic Water - Physically bound to soil particles and is 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.

Soils

Field Capacity - Water remaining in the soil after drainage by gravity.

Permanent Wilting Point - Rate of water absorption insufficient for plant needs.

Available Water - Soil water between field capacity and the permanent wilting point.

Soils

Soil pH

Alkalinity causes some minerals to become less available.

Add nitrogenous fertilizers.

Acidity may inhibit growth of nitrogen-fixing bacteria.

Add calcium or magnesium compounds.

Review

Root Development

Root Structure

Specialized Roots

Mycorrhizae

Root Nodules

Soils

Horizons

Soil Formation

Factors

. The root is usually the first organ which emerges from the seed. However, because roots spend most of their time below ground, they are understudied and underappreciated. So are the Rhizologists who slave away in anonymity trying to unlock the secrets of the root!!

Root systems can be as extensive or more extensive than shoot systems. Roots are generally

. nonphotosynthetic. However, a few, like a small orchid I spotted in Puerto Rico, are capable of photosynthesis and in this particular case, the root is the major source of photosynthate. This orchid grows in an extremely dry (xeric) environment and does not produce leaves. A stem is only produced when flowering occurs and it is ephemeral (short-lived). The roots are decidedly green and they have velamen.

. The velamen may serve a protective and regulatory function with regard to water loss and photosynthesis. It may act like the large scales of Tillandsia, which are opaque when dry and translucent when wet.

. Some plants produce both kinds of root systems. Large "tap roots" penetrate to great depths in the soil while smaller, shallow roots spread horizontally. Consequently, these plants have the best of both worlds when it comes to accessing soil water.

. Root Anatomy- Apical Meristems
Root anatomy is very simple.

The root is composed of three concentric circles of primary tissues.

The central core is Vascular Tissue. This central core of cells is derived from the Procambium and is called the Stele.

This is surrounded by a ring of Ground Tissue (Cortex).

The Epidermis is the outer most ring of tissue.

. These can be traced back to an Apical Meristem. However, the Root Apical Meristem (RAM) is covered by a Root Cap.Think of the root as being composed of a Cap and a Body.

. The root apical meristem produces the three primary meristems (procambium - ground meristem & protoderm) plus the root cap.

In cases where the root cap has a separate primary meristem, it is called the calyptrogen (calypt = cover & gen = produce). This occurs with closed RAMs

. There are two basic types of root apical meristems. They are called Open or Closed.

Seedless plants like ferns have prominent Apical Cells in their meristems.

All cells in the Root Cap & Root Body can be traced back to the Apical Cell.

There is a clear separation of the Root Cap and Root Body.

. Cells in the Body are not continuous with cells in the Root Cap.

The Root Cap originates from its own Primary Meristem (Calyptrogen).

This is a Closed RAM.

. The Apical Meristems of Seed Plants are Multicellular but there may appear to be cells analogous to Apical Cells in the roots of some species like corn. These cells are called Initials

. Apical Cells are not found in seed plants but all the cells in the body of some roots can be traced back to a few apical cell-like "Initials". Cell files converge upon these and they bear a resemblance to apices which have an apical cell. Consequently, these are Closed as well.

The Root Cap of these roots appears to have a separate origin from the cells of the body, and a distinct

. boundary can be seen between the Root Cap and Body of these closed RAMs. The Root Cap of closed RAMs can be readily plucked from the body. This trait was used to devise some imaginative experiments on the root cap and geotropism.

. Closed roots have a distinct, macroscopic Root Cap like the one below.

. The major difference between Open & Closed RAMs is that there are continuous cell files which span the Cortex and the Root Cap in Open RAMs.

The origin of the Epidermis and the ability to distinguish a Protoderm is less distinct in Open RAMs because of this.

The outer covering layer comes off in sheets which include portions of the Dermis/Cortex and the Root Cap.

. Root Anatomy - The Body

A typical Root BODY can be dividend into the following Zones

Apical Meristem Zone

Elongation Zone

Differentiation (Maturation) Zone (Root Hairs Present)

The Apical Meristem is the zone in which cell divisions occur.

. These cells Elongate, and subsequently Differentiate into cells and tissues which have mature traits. These intergrade and overlap when all of the tissues are considered but it is possible to locate these in general terms.

Root Hairs develop in the Maturation Zone. They would be destroyed if they differentiated in the Elongation Zone!

. Furthermore, the internal tissues of the root are in an optimal state for water uptake and translocation in the Maturation Zone. New Root Hairs are continuously produced as roots grow through the soil.

. Roots are composed of three concentric rings of tissues. These are vascular, ground & epidermis. This organization is illustrated by Selaginella which is a seedless plant.

It has a solid core of xylem, surrounded by a ring of Phloem which is surrounded by ground tissue (Cortex). The innermost layer of the Cortex is the Endodermis.

An Epidermis is the outermost ring.

. Cross-Section of a Selaginella root. Locate the concentric circles of tissues.

. Center of a Selaginella root with the vascular tissues labeled.

. The classic root which is studied in all Botany courses is that of Ranunculus, a dicot. Its anatomy is identical to Selaginella, except for the fact that the

central xylem is

star-shaped

. Cross-Section of Ranunculus Root - Locate the concentric circles of tissues. The vascular tissues are at the bottom center of the image. The densely stained purple cells in the Cortex are Parenchyma that contain many Amyloplasts. Starch storage is one important adaptation of roots.

. Vascular cylinder from a Ranunculus root. The xylem has a star-like appearance. The Phloem occupies the indentations between the xylem arms. The Endodermis is

. Diagram showing the organization of Vascular Tissues and the Endodermis.

. Monocot Roots

The organization of Monocot roots like Sugarcane is similar to that found in dicots like Ranunculus. However, tracheary elements may be absent from the center of the root

. Corn Root: Interface of the Stele & the Cortex-The cells with the largest Diameters are Vessel Members. Follow the Cortical Parenchyma from the top of the image to the first cells which have thick, red-stained walls. These are the Endodermis.

. The Endodermis usually develops exceedingly thick secondary walls at levels in the root which are no longer absorbing water. This obscures the Casparian Strips but makes the Endodermis more obvious.

There are usually more xylem arms in monocots.

The number of arms is indicated by the following terms.

.Monocots are usually Polyarch.

Bundles of Phloem alternate with the Xylem arms. The xylem contains many lignified cells in older portions of the root. The phloem stands out because its cells have thin, unlignified cell walls.

. Smilax Stele showing bundles of Phloem alternating with Xylem Vessel Members.

Smilax root showing the Stele, Endodermis, Cortex, Exodermis & Epidermis. The Exodermis has the same anatomy as the Endodermis and probably restricts water loss to the outside in older roots.

.Water Hyacinth Roots

Water Hyacinth(Eichornia crassipes) is an aquatic plant which floats on lakes and slow-moving streams of rivers. It is incredibly beautiful but it can be an environmental nightmare! It grows at an incredible rate and can be a serious environmental pest. It doesen't help that its natural herbivor, the Manatee has been driven to the brink of extinction.

. It quickly becomes a complete mat which crowds out other floating aquatic plants like Lemna or Azolla.

It also intercepts all of the incident illumination that phytoplankton might use otherwise.

The dead biomass produced by Eichornia accumulates on the bottom and is decomposed by by microorganisms

. Because there is a tremendous amount of dead biomass, the environment becomes anaerobic as the available oxygen is consumed by the decomposers.

This is a deadly circle which further reduces the ability of the ecosystem to support other species.

. The roots are totally submerged and have some adaptations associated with growth in a Hydric environment. Such traits are called Hydromorphic.

. Cross-section of Water Hyacinth root stained with Toluidine Blue.

. The tissues starting from the outside to the Inside are

Epidermis

Ground Tissue (Outer Cortex)

Ground Tissue (Cortex) Aerenchyma
Aerenchyma is typically found in submerged organs.
It is a Hydromorphic Trait.

Ground Tissue (Inner Cortex)

. Endodermis

Pericycle

Phloem

Xylem

Ground Tissue

Pericycle

The Pericycle (around the circle) is parenchyma located immediately inside the Endodermis. It may be one to several cell layers thick.

. It tends to be several layers thick in monocot roots. It is one source of Lateral Root initiation.

To locate the Endodermis, find a file of cells from the Inner Cortex and follow it as far as you can towards the center of the root. This unicellular layer located by this method is the Endodermis. It will be hard to see the Casparian Strips.

. The Pericycle is hard to see. It appears to be one cell thick and appears incomplete because some of it has collapsed due to the cell expansion in the stele.

. The large Vessel Members (VMs) of the Metaxylem are easy to find. Locate the other

. represent Protoxylem. This is EXARCH development. The direction of xylem maturation is from the outside (Ex) towards the Inside.
It is Centripetal.

. Phloem alternates with the xylem. You can see an enlarged Sieve Tube Member in each. The phloem cell walls will be pink because they lack lignin. Locate the Phloem in the picture above.

The center of the stele is composed of Parenchyma. This is typical of monocots but virtually absent in Dicots. These cells may be sclerified in terrestrial monocot roots. Absence of Sclerenchyma is another Hydromorphic Trait.

. Two illustrations of corn roots are found below. Find the tissue layers and compare with Water Hyacinth. Note the presence of a few Root Hairs. Absence of Root Hairs is a Hydromorphic Traits

. Endodermis

The Endodermis is one of the most important adaptations of terrestrial plants. In the absence of an endodermis plants would not be able to regulate water uptake by the roots and maintain the water balance of the plant. Bryophytes lack an endodermis and they are unable to maintain large aerial systems. Plants began to develop large shoots only

. after they developed this adaptation. I will try to illustrate how the endodermis regulates solute movement and consequently regulates water movement at an elementary level.

Keep in mind that molecules move from areas of high concentration to areas of low concentration. This includes water. Solute molecules like Sucrose, Na & Cl reduce the water

. concentration. We usually think of diluting solutions by adding water, however, dissolving 50 grams of sucrose in 100 ml of water would "dilute" the water molecules. If this solution was placed in one side of a U-Tube which was divided into two by a semipermeable membrane (permeable to water but impermeable to sucrose) & Pure water was placed in the other half, what would

. happen??? The sucrose molecules can't go anywhere, but the water molecules can cross the membrane.

. Sucrose solution is in the left half of the tube. Pure water is in the right half. The membrane will allow water molecules to pass through but sugar is blocked. There is a greater concentration of water molecules on the right side of the membrane. Thus, there will be a net movement of water molecules from the right to the left side of the membrane.

. What would happen if there were no membrane?

The Sucrose and water molecules would become evenly distributed in the solution.

The endodermis separates the Cortex from the Stele. If the endodermal cell walls were purely composed of Cellulose & there were a lot of Sucrose molecules in the

. cortex, but none in the stele, what would happen to the sucrose molecules.

Sucrose molecules are represented as red balls below. These are a thousand fold bigger than actual sucrose molecules, and can easily fit between cellulose fibrils in the cell wall.

. I separated these two cell stacks to emphasize that the Casparian Strip forms an unbroken barrier

. Lateral (Secondary) Roots

Lateral (Secondary) Roots originate in the Stele or the Endodermis. They grow through the Cortex and Epidermis.

Early Stages in Lateral Root Initiation

. Tap root system of Zamia

. Lateral roots are produced in a definite pattern. The youngest roots are close (proximal) to the apex. The pattern of lateral root production is readily observed with tap root systems. The tap root can also be called the primary root while its branches are called secondary or lateral roots

. Plant with a Fibrous Root System. These have more than one dominant root.

The origin of lateral roots can be unraveled for species that have fibrous root systems. A clear pattern emerges with careful study

. Prop Roots

Roots may have other specialized functions. Species like Pandanus (hala) have weak, brittle stems which would break under the strain of their large leaves which are clustered about the shoot tips. Hala produces prop roots which compensate somewhat for its weak stems.

. Legume Root Symbioses

Besides the absorption of water and nutrients, virtually all roots have symbiotic associations with soil microorganisms. The term "Rhizosphere" is used to indicate the interface of the root Epidermis and its immediate zone of soil contact. It has been estimated that 90% of land plants have mycorrhizal associations with soil fungi. This is

. probably an underestimate. Those who want to raise endangered species for out-planting to nature, achieve good results when the propagules have been grown in medium containing the soil microflora of the out-planting sites.

. There are a few famous symbiotic relationships. The most well known is that between legumes and soil bacteria in the genus Rhizobium. The Rhizobia can fix nitrogen gas when the symbiosis is achieved. They can't fix nitrogen in the free-living state, however!

Fixed nitrogen is obtained by the legume and the Rhizobium gets a nice place to live with all of the amenities!

. This includes photosynthate, water and minerals. This is an extremely important relationship because Nitrogen is usually the most limiting element in terrestrial ecosystems. Furthermore, many legumes, like soybean, form the basis for agriculture on a world-scale. Some legume seeds, like soybeans, contain high levels of protein. These are the most important agricultural sources of protein in the world.

. Nodule Types

Two basic types of root nodules are produced by legumes. One type is ephemeral and lasts days or a few weeks. This is called a determinate structure. It has a short, predestined life-span. Consequently, new nodules are being formed as the root grows in the soil and others are being lost on older parts of the root system.

. Soybean nodules are like this. The nodule is a spherical elaboration of the ground tissue system in the root cortex and has a specialized anatomy

. The outer part of the nodule becomes sclerotic as parenchyma cells are converted into sclereids. This tissue blocks gas exchange to some extent. More internal Vascular Bundles surround a large central mass of parenchyma which contains cells infected with bacteroids that fix nitrogen. The term Bacteroid refers to the growth form of the Bacteria in N-fixing Cells.

. These are structurally modified compared to free-living bacteria. Uninfected Parenchyma cells are also present.

. Sections through Soybean Determinate Nodules: Note the extent of cells infected with N-fixing Bacteria. Also note the Sclerenchyma that helps reduce the Oxygen levels inside the Nodule. Note the many Vascular Bundles which facilitate transport of Sugar into the nodule and nitrogenous compounds out of the nodule.

. Cells from a Legume/Rhizobium Nodule seen with Bright-Field Optics. The densely stained cells contain Rhizobium. This is where atmospheric Nitrogen is fixed.

. The second nodule type is illustrated by Clover. In this case the nodule has an Apical Meristem which functions for many months. It is called Indeterminate in that meristematic activity is theoretically unlimited.

These are elongate compared to the determinate nodules. They are tumescent, (swollen). The apical meristem continuously produces new

. cells which become infected with bacteria from older cells.

These nodules have a much more extensive vascular system which surrounds the nitrogen-fixing parenchyma that occupy the center of the nodule. This central location is not a coincidence.

The enzyme which fixes nitrogen (Nitrogenase) needs an anaerobic environment. Consequently, the

. location of the bacteroids, inside living, non-photosynthetic cells favors N-fixation. Lignified external layers may also limit gas exchange.

Cork, which can be impervious to air can develop especially on Indeterminate Nodules. This also promotes anoxia.

Furthermore, the bacteroids stimulate the production of

. Leghaemoglobin which acts like animal hemoglobin and binds oxygen, thus reducing oxygen levels.

. Mycorrhizae

Mycorrhizae means Fungus Root. A Mycorrhizae consists of a root and an associated fungus whaich have a symbiotic relationship.

There are two general types of Mycorrhizae, Endomycorrhizae & Ectomycorrhizae.

Endomycorrhizae have fungal hyphae inside cortical cells of roots. They produce characteristic Vesicles (globular structures) and Arbuscules (highly branched structures) inside the walls of cortical cells. Consequently, they are often called Vesicular Arbuscular Mycorrhizae. Internal hyphae are continuous with hyphae on the root surface and in the soil.

. Whole Mount of a Root with Endomycorrhizal Fungus: Note the prominent Blue Vesicles in the Cortex

. Ectomycorrhizae form a thick hyphal mat that surrounds the root. Hyphae penetrate the Root Cortex and have an extensive network therein. The hyphal network in the Cortex is often called the "Hartig Net".

. Images of Ectomycorrhizae in Root cross-sections: Note the presence of the thick Fungal Sheath (Mantle) that surrounds the root. Hyphae penetrate the Cell Walls of the Cortex where they form the "Hartig Net" but do not enter the protoplasts of these cells.

. Diagram of an Ectomycorrhizal Root: Note the developmental progression of Mycorrhizal roots on the Left-hand picture. The roots infected with Ectomycorrhizal fungi are swollen and branch dichotomously. Note the Fungal Mantle on the outside of the Root and the Hartig Net inside the Cortex.

. Mycorrhizae are incredibly important! Over 90% of terrestrial plants have these. They vastly extend the area available for the absorption of water and minerals.

Presence of appropriate fungal partners in nursery soils is critical for the establishment of plants in nature.

. This has been widely applied in Forestry BUT is also proving to be important in out-planting native species to re-establish native ecosystems!