Chapter 5

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 growso Secretes mucilage that

acts as lubricant

Longitudinal section through root tip

– Functions in gravitropism (gravity perception)

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 rootRoot hair zone of radish seedling

Root StructureRegion of Maturation

Cortex - Parenchyma cells between epidermis and vascular cylinder

• Mostly stores food

Cross section of dicot root

Root StructureRegion of Maturation

• Endodermis - Inner boundary of cortex, consisting of a single-layered cylinder of compact cells– Cell walls with suberin bands called casparian strips on

radial and tangential wallso Forces water and dissolved substances entering and

leaving the central core to pass through endodermiso Regulates types of minerals absorbed

– Eventually inner cell walls become thickened with suberin, except for passage cells.

Enlargement of vascular cylinder of dicot root

Root StructureRegion of Maturation

Vascular cylinder - Core of tissues inside endodermis

• Pericycle - Outer boundary of vascular cylinder

– Continues to divide, even after mature

– Forms lateral (branch) roots and part of the vascular cambium

Region of endodermis and pericycle in dicot root

Lateral root formation

Root StructureRegion 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

Propagative Roots• Adventitious buds on roots - Develop into suckers

(aerial stems)– Fruit Trees

Manroot, water storage root

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

Parasitic Roots

• No chlorophyll and dependent on chlorophyll-bearing plants for nutrition

Buttress roots of tropical fig tree

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 of water and nutrients,

especially phosphorus for roots.

– Plant furnishes sugars and amino acids to fungus.

• Particularly susceptible to acid rain

Mycorrhizae

Root Nodules

A few species of bacteria produce enzymes that can convert nitrogen from the atmosphere into 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

– A horizon - Dark loam, with more organic material than lower layers

– E horizon - Light loam

• B Horizon - Subsoil– More clay, lighter in color

• C Horizon - Parent material

Soil profile

SoilsParent 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

SoilsClimate

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.

SoilsLiving 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

SoilsTopography

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.

SoilsSoil 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.

SoilsWater 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.

SoilsWater 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

SoilsSoil 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