Roots
Roots - 1 As we have discussed and observed in the laboratory, plants have two general systems that grow along an axis. The above ground portion of most plants comprises the shoot system (with leaves and stems); root systems constitute the below ground portion of most plants. Because they are unseen, many people don't think about the root systems of plants, unless the roots have grown so much that one's sidewalks get cracked and lifted, or one's septic system needs cleaning because roots have invaded the drain tiles. In this section we will discuss the structure and function of roots. You will recognize the tissues we have previously discussed and see how they are organized in roots, and how tissues are specialized for optimal root functions. Root • • • •
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Functions Anchor Absorb (and conduct) H20 and minerals Store food reserve materials Produce certain hormones (the gibberellins and cytokinins) in the root meristem that are translocated through the plant to control growth and development Produce some secondary metabolites that are translocated upward
Most root systems are fairly shallow (2 - 3 feet), deep enough to anchor the plant, but still growing in soil that contains nutrients that cycle through the ecosystem. Most of the smaller roots active in water and nutrient uptake are found very close to the soil surface. This is one reason why some trees are fragile when planted in traffic areas and lawns. Pressure can damage roots. Roots of some plants access deep water sources; roots have been measured at depths of almost 175 feet in some desert areas. Roots can easily spread in width many times the distance of a tree’s crown. Roots of herbaceous plants have proportionately wide and deep root systems, too. For most plants there is a balance between the root area and shoot area. Shoots must have adequate surface area for photosynthesis for the entire plant and roots must have adequate surface area to provide water and nutrients to the entire plant. If roots are damaged, shoot growth can be affected. Poor shoot growth limits the available fuel needed for root growth. Fortunately, plants are able to replace damaged roots with new root growth pretty rapidly in most cases, just as new leaves and shoots are readily produced. A common problem with houseplants is “root rot”, in which roots do not obtain enough oxygen and suffocate (generally caused by water-logged soils). Once the roots die, the shoot system also dies.
Roots - 2 There are two major root systems (plus adventitious roots) that differ in the structure and number of roots. When a seed germinates, the primary root, or radicle as it is called in the embryo, emerges from the seed coat. That primary root may develop into a t aproot or die and be replaced by a system of f ibrous roots. In addition, adventitious roots arise from stem or leaf cuttings. Root Systems Taproot system The taproot system develops from the primary root. It is comprised of one major root that is thicker at its base (junction with the stem) and tapers toward the growing tip. Taproot systems are found in many dicots and gymnosperms. Taproots are: • Deep penetrating in soil • Often used for food storage • Form small lateral roots from the tap root • Taproots are often deep and provide excellent anchorage for the plant
Tap root system
Fibrous root system
Adventitious roots
Fibrous root system Many, if not all fibrous root systems develop from small adventitious roots that develop from stem tissue soon after germination, particularly in monocots. The primary root is short-lived. (Recall that adventitious structures are structures that develop from other than apical meristem) Fibrous roots are: • Shallow, wide spreading • Several, equal diameter rots or root masses • Good for soil erosion control. Fibrous roots cling to soil particles well. • Fibrous roots are found in many different kinds of plants.
Roots - 3 Root Structure: Root Tips and Meristem Each root tip has four easily defined zones: Root cap • Protective layer of unspecialized parenchyma cells that are continuously worn away by moving through the soil as the root grows, and replaced by new root cap cells. The root cap cells are coated with mucigel, a slime sheath that helps to lubricate the root as it penetrates through soil. • Cells of the central column of the root cap are positively geotropic, so that roots grow toward gravity. Amyloplasts, calcium ions and hormones all influence this growth direction. Meristem (Zone of Cell Division) • The root tip meristem is comprised of the apical meristem initials and three derivative meristem zones (protoderm, procambium and ground meristem). There are slight pattern variations in the meristem initials that are genetic. As roots mature the tip region of meristem becomes inactive and is called the quiescent zone. The active meristem is just above the quiescent zone. Zone of Elongation • Cells (in their fixed positions) elongate to reach mature dimensions in the region of elongation. The vacuole plays a major role in this process, using water pressure to push against the walls to stretch the cellulose fibers as the cells elongate. Virtually all increase in root length occurs in the elongation region. Zone of Maturation • Cells mature for their specific functions and tissues in the maturation region.
Roots - 4 Primary Root Structure (Mature region of roots) Epidermis • Single layer of cells with very little cuticle or suberin • Root hairs are found in the region of early maturation in the epidermis. The extension of the cell greatly increases surface area for absorption of water and nutrients. In one study (as reported in the Raven text) a rye plant had 14 billion root hairs with an absorption surface area of 401 square meters. As the epidermal cells mature, the root hairs atrophy, and are replaced by newer cells with root hairs in the early stages of maturation.
Root hairs •
Root cap mucigel
The active absorption region of roots is coated by the mucigel that provides an environment favorable to mineral and water uptake and may protect the root from dehydration. There is some evidence that nitrogen-fixing bacteria are attracted to the mucigel, as well. The mucigel-soil area around the root tip is called the r hizosphere.
Cortex • The root cortex is composed of loosely packed ground tissue cells with large diameters. • Absorbed water moves readily between cells through the porous cell walls of the cortex parenchyma. This pathway of interconnected cell walls for water movement is generally regarded as the a poplast. An alternative water pathway, which passes from cell to cell through plasmodesmata through the interiors of the cells is called the s ymplast. (Water movement is discussed with Plant transport.) • Cortex cells typically contain amyloplasts. • The inner layer of cortex is the e ndodermis • The endodermis is a barrier to further cell movement between cells. The walls of the endodermis cells have a casparian strip, a layer or band of suberin on the walls, which are perpendicular to the epidermis, which prevents movement of materials. From this point, water and nutrients can no longer flow between cell spaces (apoplastic movement) as they did in the root cortex. Water and minerals must now move through the endodermis cells and are subject to membrane regulation.
Roots - 5
Endodermis in young root
Endodermis in older root
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The endodermis walls thicken and no longer permit passage of materials as roots mature. In roots that have secondary growth, the entire root exterior to the stele (the vascular tissue cylinder) gets sloughed off.
Stele • The vascular tissue of the root is located with the stele, which forms the center or core of the root inside of the endodermis • The stele contains: • Xylem, the core of the stele with radiating "arms". The number of “arms” is a genetic trait (The vocabulary is diarch, triarch, tetrarch, pentarch, etc.) Xylem is recognized by its large diameter vessels. • Phloem, found in patches between the xylem arms, comprised of sieve elements and companion cells • Pericycle, parenchyma cells just inside of the endodermis, forms the rest of the stele tissue. Older pericycle cells may form secondary walls. • Origin of lateral or branch roots (by dedifferentiation) • Origin of secondary root cambium and cork cambium (by dedifferentiation) • The stele of M onocot roots usually has a thin cylinder of alternating xylem and phloem patches and an internal pith composed of parenchyma cells. The endodermis layer of some monocot roots is very thick.
Roots - 6
Mature Dicot Stele
Mature Monocot Stele
Roots - 7 Lateral (Branch) Root Formation • Root pericycle cell dedifferentiates to form a root primordium. • Root pushes out through stele, cortex and epidermis by root cap secretions. Eventually the vascular tissue of the lateral root is joined to the vascular tissue of the “parent” root to permit continuous flow of substances.
Lateral Root Origin from Pericycle
Lateral Roots Emerging
Secondary Root Growth • A cambium cylinder develops from parenchyma cells between xylem and phloem in the primary root stele. Once formed, the cambium produces xylem inward and phloem outward. Additional parenchyma cells form rays • Pericycle cells dedifferentiate to form a cork cambium, which produces a cork layer as well. • The annual growth of "wood" and cork-like bark in secondary roots is very similar to stem secondary growth (to be discussed soon).
Roots - 8 Specialized and Modified Roots "Food" Storage Roots • The most common specialized root is the storage root. Many of our important food crops are roots (some of which include the basal stem portions derived from the embryonic hypocotyl). The storage tissue is parenchyma that is produced from cambium regions in vascular tissue. In some plants, the cambium proliferates around secondary xylem in many locations. • Some examples of storage roots that are economically important are: Cassava Taro Sweet potato Beets and Sugar beets Carrots and Parsnips Radishes, Turnips and Rutabaga
Aerial Roots Roots that do not grow underground are referred to as aerial roots. There are a number of different types of aerial roots. • Prop roots, which generally arise at stem nodes, provide additional support for the plant. Prop roots are found in corn and some tree species.
Roots - 9 •
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Buttress roots are similar to prop roots, except they originate at the base of the stem, rather than further up the stem. They are pretty impressive structures on some tropical tree specimens.
Climbing roots (such as ivy roots) are used to attach the plant to its substrate. They are common on e piphytes. Other epiphytes have air roots that absorb moisture from humid air.
Clinging and climbing roots •
Aerial roots
The v elamen layer roots of orchids are aerial roots, which may be used for gas exchange or for protection. The velamen is a multi-layered modified epidermis, which contains chloroplasts for photosynthesis. Velamen also provides mechanical protection and prevents water loss.
Roots - 10 •
Pneumatophores are negatively geotropic roots found on some plants in swampy areas that grow above the water line or tide-line. These roots help absorb oxygen. They are common on mangroves. The knees of bald cypress are similar to pneumatophores.
Water Storage Roots • Water storage roots are found in members of the Cucurbitaceae, including one that grows in Washington. They can be huge, weighing more than 50 pounds.
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One unique variation for specialized roots is found in the “flower pot plant”. It has modified leaves that form a “pot” in which water and debris collects. Adventitious roots form in the pot for absorption of collected water and nutrients.
Roots - 11 Propagation Roots It is not uncommon for some plants to be able to produce adventitious shoots (or suckers) from roots. • The common thistle can do this, as can false dandelions, found in our yards. • Many trees can produce "suckers" from roots. Suckers are adventitious woody shoots • Aspen clones, in which many trees are formed from one original, can cover several acres, and grow from root suckers. • Trees in the rose family also produce suckers. Contractile Roots Contractile roots are roots that help to pull stem tissue into the soil to provide additional support and anchoring of the plant, and provide more stable temperature conditions in the soil for the dormant period. These are common on bulb and corm plants. Parasitic Roots Some plants choose to parasitize others rather than to photosynthesize on their own, or prefer to obtain water and nutrients from some other plant than from the soil. Root-like structures called h austoria penetrate other plant tissues to obtain needed materials. Root Nodules Some legumes, and a few other plants, produce root nodules, which contain bacteria or cyanobacteria that can "fix" nitrogen (make nitrogen into a form that is useful as a plant nutrient). When the bacteria enter the root, they induce the root to form "tumors" which are the nodules. The bacteria then grow and produce the nitrogen products in the nodules. You can pull up clover and other herbaceous legumes and examine the conspicuous nodules on their roots.
Roots - 12 Mycorrhizae We have learned that roots function to absorb water and minerals (nutrients). However, the majority of vascular plants form root associations with fungi to increase their absorption of mineral nutrients. Fungi, which live by absorbing nutrients from their surroundings, are ideal organisms to make these associations. The fungus obtains carbohydrates (the product of photosynthesis) from the host plant, and absorbs water and nutrients from the environment for the plant. The fungus can absorb nutrients actively so it can obtain nutrients even when they are in low supply. Roots absorb passively, so a gradient must exist from the soil to the root tissue. There are both e ndomycorrhizae and e ctomycorrhizae. • Endomycorrhizae penetrate cells of the cortex with their hyphae (the threadlike cells of a fungus) forming much branched arbuscules, sometimes with swollen vesicles at their tips; some endomycorrhizae are found in the cortex spaces. • Ectomycorrhizae surround the exterior of roots, forming a hyphal sheath, or mantle, but do not penetrate the root cells. Ectomycorrhizae are important root associates in trees in most forest biomes. Ectomycorrhizae of conifers penetrate the epidermal layers to surround the cortex, forming a network of fungal cells called the Hartig net. Essentially, mycorrhizae can function as sophisticated root hairs, and plants that associate with ectomycorrhizae often do not produce root hairs. Some plants absolutely require mycorrhizae, such as orchids and the gametophytes of many lower vascular plants.
Roots - 13
Ectomycorrhizae
Vesicles and Arbuscules of Endomycorrhizae
•
Functions Anchor Absorb (and conduct) H20 and minerals Store food reserve materials Produce certain hormones (the gibberellins and cytokinins) in the root meristem that are translocated through the plant to control growth and development Produce some secondary metabolites that are translocated upward
Most root systems are fairly shallow (2 - 3 feet), deep enough to anchor the plant, but still growing in soil that contains nutrients that cycle through the ecosystem. Most of the smaller roots active in water and nutrient uptake are found very close to the soil surface. This is one reason why some trees are fragile when planted in traffic areas and lawns. Pressure can damage roots. Roots of some plants access deep water sources; roots have been measured at depths of almost 175 feet in some desert areas. Roots can easily spread in width many times the distance of a tree’s crown. Roots of herbaceous plants have proportionately wide and deep root systems, too. For most plants there is a balance between the root area and shoot area. Shoots must have adequate surface area for photosynthesis for the entire plant and roots must have adequate surface area to provide water and nutrients to the entire plant. If roots are damaged, shoot growth can be affected. Poor shoot growth limits the available fuel needed for root growth. Fortunately, plants are able to replace damaged roots with new root growth pretty rapidly in most cases, just as new leaves and shoots are readily produced. A common problem with houseplants is “root rot”, in which roots do not obtain enough oxygen and suffocate (generally caused by water-logged soils). Once the roots die, the shoot system also dies.
Roots - 2 There are two major root systems (plus adventitious roots) that differ in the structure and number of roots. When a seed germinates, the primary root, or radicle as it is called in the embryo, emerges from the seed coat. That primary root may develop into a t aproot or die and be replaced by a system of f ibrous roots. In addition, adventitious roots arise from stem or leaf cuttings. Root Systems Taproot system The taproot system develops from the primary root. It is comprised of one major root that is thicker at its base (junction with the stem) and tapers toward the growing tip. Taproot systems are found in many dicots and gymnosperms. Taproots are: • Deep penetrating in soil • Often used for food storage • Form small lateral roots from the tap root • Taproots are often deep and provide excellent anchorage for the plant
Tap root system
Fibrous root system
Adventitious roots
Fibrous root system Many, if not all fibrous root systems develop from small adventitious roots that develop from stem tissue soon after germination, particularly in monocots. The primary root is short-lived. (Recall that adventitious structures are structures that develop from other than apical meristem) Fibrous roots are: • Shallow, wide spreading • Several, equal diameter rots or root masses • Good for soil erosion control. Fibrous roots cling to soil particles well. • Fibrous roots are found in many different kinds of plants.
Roots - 3 Root Structure: Root Tips and Meristem Each root tip has four easily defined zones: Root cap • Protective layer of unspecialized parenchyma cells that are continuously worn away by moving through the soil as the root grows, and replaced by new root cap cells. The root cap cells are coated with mucigel, a slime sheath that helps to lubricate the root as it penetrates through soil. • Cells of the central column of the root cap are positively geotropic, so that roots grow toward gravity. Amyloplasts, calcium ions and hormones all influence this growth direction. Meristem (Zone of Cell Division) • The root tip meristem is comprised of the apical meristem initials and three derivative meristem zones (protoderm, procambium and ground meristem). There are slight pattern variations in the meristem initials that are genetic. As roots mature the tip region of meristem becomes inactive and is called the quiescent zone. The active meristem is just above the quiescent zone. Zone of Elongation • Cells (in their fixed positions) elongate to reach mature dimensions in the region of elongation. The vacuole plays a major role in this process, using water pressure to push against the walls to stretch the cellulose fibers as the cells elongate. Virtually all increase in root length occurs in the elongation region. Zone of Maturation • Cells mature for their specific functions and tissues in the maturation region.
Roots - 4 Primary Root Structure (Mature region of roots) Epidermis • Single layer of cells with very little cuticle or suberin • Root hairs are found in the region of early maturation in the epidermis. The extension of the cell greatly increases surface area for absorption of water and nutrients. In one study (as reported in the Raven text) a rye plant had 14 billion root hairs with an absorption surface area of 401 square meters. As the epidermal cells mature, the root hairs atrophy, and are replaced by newer cells with root hairs in the early stages of maturation.
Root hairs •
Root cap mucigel
The active absorption region of roots is coated by the mucigel that provides an environment favorable to mineral and water uptake and may protect the root from dehydration. There is some evidence that nitrogen-fixing bacteria are attracted to the mucigel, as well. The mucigel-soil area around the root tip is called the r hizosphere.
Cortex • The root cortex is composed of loosely packed ground tissue cells with large diameters. • Absorbed water moves readily between cells through the porous cell walls of the cortex parenchyma. This pathway of interconnected cell walls for water movement is generally regarded as the a poplast. An alternative water pathway, which passes from cell to cell through plasmodesmata through the interiors of the cells is called the s ymplast. (Water movement is discussed with Plant transport.) • Cortex cells typically contain amyloplasts. • The inner layer of cortex is the e ndodermis • The endodermis is a barrier to further cell movement between cells. The walls of the endodermis cells have a casparian strip, a layer or band of suberin on the walls, which are perpendicular to the epidermis, which prevents movement of materials. From this point, water and nutrients can no longer flow between cell spaces (apoplastic movement) as they did in the root cortex. Water and minerals must now move through the endodermis cells and are subject to membrane regulation.
Roots - 5
Endodermis in young root
Endodermis in older root
•
The endodermis walls thicken and no longer permit passage of materials as roots mature. In roots that have secondary growth, the entire root exterior to the stele (the vascular tissue cylinder) gets sloughed off.
Stele • The vascular tissue of the root is located with the stele, which forms the center or core of the root inside of the endodermis • The stele contains: • Xylem, the core of the stele with radiating "arms". The number of “arms” is a genetic trait (The vocabulary is diarch, triarch, tetrarch, pentarch, etc.) Xylem is recognized by its large diameter vessels. • Phloem, found in patches between the xylem arms, comprised of sieve elements and companion cells • Pericycle, parenchyma cells just inside of the endodermis, forms the rest of the stele tissue. Older pericycle cells may form secondary walls. • Origin of lateral or branch roots (by dedifferentiation) • Origin of secondary root cambium and cork cambium (by dedifferentiation) • The stele of M onocot roots usually has a thin cylinder of alternating xylem and phloem patches and an internal pith composed of parenchyma cells. The endodermis layer of some monocot roots is very thick.
Roots - 6
Mature Dicot Stele
Mature Monocot Stele
Roots - 7 Lateral (Branch) Root Formation • Root pericycle cell dedifferentiates to form a root primordium. • Root pushes out through stele, cortex and epidermis by root cap secretions. Eventually the vascular tissue of the lateral root is joined to the vascular tissue of the “parent” root to permit continuous flow of substances.
Lateral Root Origin from Pericycle
Lateral Roots Emerging
Secondary Root Growth • A cambium cylinder develops from parenchyma cells between xylem and phloem in the primary root stele. Once formed, the cambium produces xylem inward and phloem outward. Additional parenchyma cells form rays • Pericycle cells dedifferentiate to form a cork cambium, which produces a cork layer as well. • The annual growth of "wood" and cork-like bark in secondary roots is very similar to stem secondary growth (to be discussed soon).
Roots - 8 Specialized and Modified Roots "Food" Storage Roots • The most common specialized root is the storage root. Many of our important food crops are roots (some of which include the basal stem portions derived from the embryonic hypocotyl). The storage tissue is parenchyma that is produced from cambium regions in vascular tissue. In some plants, the cambium proliferates around secondary xylem in many locations. • Some examples of storage roots that are economically important are: Cassava Taro Sweet potato Beets and Sugar beets Carrots and Parsnips Radishes, Turnips and Rutabaga
Aerial Roots Roots that do not grow underground are referred to as aerial roots. There are a number of different types of aerial roots. • Prop roots, which generally arise at stem nodes, provide additional support for the plant. Prop roots are found in corn and some tree species.
Roots - 9 •
•
Buttress roots are similar to prop roots, except they originate at the base of the stem, rather than further up the stem. They are pretty impressive structures on some tropical tree specimens.
Climbing roots (such as ivy roots) are used to attach the plant to its substrate. They are common on e piphytes. Other epiphytes have air roots that absorb moisture from humid air.
Clinging and climbing roots •
Aerial roots
The v elamen layer roots of orchids are aerial roots, which may be used for gas exchange or for protection. The velamen is a multi-layered modified epidermis, which contains chloroplasts for photosynthesis. Velamen also provides mechanical protection and prevents water loss.
Roots - 10 •
Pneumatophores are negatively geotropic roots found on some plants in swampy areas that grow above the water line or tide-line. These roots help absorb oxygen. They are common on mangroves. The knees of bald cypress are similar to pneumatophores.
Water Storage Roots • Water storage roots are found in members of the Cucurbitaceae, including one that grows in Washington. They can be huge, weighing more than 50 pounds.
•
One unique variation for specialized roots is found in the “flower pot plant”. It has modified leaves that form a “pot” in which water and debris collects. Adventitious roots form in the pot for absorption of collected water and nutrients.
Roots - 11 Propagation Roots It is not uncommon for some plants to be able to produce adventitious shoots (or suckers) from roots. • The common thistle can do this, as can false dandelions, found in our yards. • Many trees can produce "suckers" from roots. Suckers are adventitious woody shoots • Aspen clones, in which many trees are formed from one original, can cover several acres, and grow from root suckers. • Trees in the rose family also produce suckers. Contractile Roots Contractile roots are roots that help to pull stem tissue into the soil to provide additional support and anchoring of the plant, and provide more stable temperature conditions in the soil for the dormant period. These are common on bulb and corm plants. Parasitic Roots Some plants choose to parasitize others rather than to photosynthesize on their own, or prefer to obtain water and nutrients from some other plant than from the soil. Root-like structures called h austoria penetrate other plant tissues to obtain needed materials. Root Nodules Some legumes, and a few other plants, produce root nodules, which contain bacteria or cyanobacteria that can "fix" nitrogen (make nitrogen into a form that is useful as a plant nutrient). When the bacteria enter the root, they induce the root to form "tumors" which are the nodules. The bacteria then grow and produce the nitrogen products in the nodules. You can pull up clover and other herbaceous legumes and examine the conspicuous nodules on their roots.
Roots - 12 Mycorrhizae We have learned that roots function to absorb water and minerals (nutrients). However, the majority of vascular plants form root associations with fungi to increase their absorption of mineral nutrients. Fungi, which live by absorbing nutrients from their surroundings, are ideal organisms to make these associations. The fungus obtains carbohydrates (the product of photosynthesis) from the host plant, and absorbs water and nutrients from the environment for the plant. The fungus can absorb nutrients actively so it can obtain nutrients even when they are in low supply. Roots absorb passively, so a gradient must exist from the soil to the root tissue. There are both e ndomycorrhizae and e ctomycorrhizae. • Endomycorrhizae penetrate cells of the cortex with their hyphae (the threadlike cells of a fungus) forming much branched arbuscules, sometimes with swollen vesicles at their tips; some endomycorrhizae are found in the cortex spaces. • Ectomycorrhizae surround the exterior of roots, forming a hyphal sheath, or mantle, but do not penetrate the root cells. Ectomycorrhizae are important root associates in trees in most forest biomes. Ectomycorrhizae of conifers penetrate the epidermal layers to surround the cortex, forming a network of fungal cells called the Hartig net. Essentially, mycorrhizae can function as sophisticated root hairs, and plants that associate with ectomycorrhizae often do not produce root hairs. Some plants absolutely require mycorrhizae, such as orchids and the gametophytes of many lower vascular plants.
Roots - 13
Ectomycorrhizae
Vesicles and Arbuscules of Endomycorrhizae
