roots and soils

Roots and Soils

Chapter 5

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• May develop into thick

taproot with thinner branch roots

− Dicotyledonous plants (dicots)

Taproot system

Root Development

Or, after radicle formation, adventitious roots develop into fibrous root system• Adventitious Roots -

develop from 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

Longitudinal section through root tip

Root Structure• Root Cap - thimble-

shaped mass of parenchyma cells covering each root tip

− Protects tissues from damage as root grows

o Secretes mucilage that acts as lubricant

o Functions in gravitropism (gravity perception)

Longitudinal section through root tip

Root Structure

• Region of Cell Division - composed of apical meristem in center of root tip– Subdivided into 3

meristematic areas:o Protoderm - gives rise to

epidermiso Ground meristem - gives

rise to cortex and pitho Procambium - gives rise

to 1° xylem and 1° phloem

Root tip showing primary meristems

Root Structure

• Region of Elongation - cells become several times original length– Vacuoles merge

• Region of Maturation - cells differentiate into various distinctive cell types– Root hairs

Root hair zone of radish seedling

Root Structure

• Root Hairs– Epidermal cell

extensions with thin cuticle

– Absorb water and minerals

– Adhere tightly to soil particles

– Increase total absorptive surface of root

Root 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


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

on radial and tangential walls– Forces water and dissolved substances entering and

leaving central core to pass through endodermis– Regulates types of minerals absorbed– Eventually inner cell walls become thickened with

suberin, except for passage cells


Enlargement of vascular cylinder of dicot root


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 vascular cambium

Region of endodermis and pericycle in dicot root

Lateral root formation


Most cells of vascular cylinder 1° xylem or 1° 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 2°

phloem to outside and 2° xylem to inside

Vascular cylinder of dicot root

Root Structure Growth• Determinate Growth - growth that stops after

organ fully expanded or after plant has reached certain size

• Indeterminate Growth - new tissues 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• Plants with roots

growing in water

• Spongy roots that extend above 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

soil− Lily 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 mutualistic association with plant roots• Mutualistic association: Both fungus and root benefit

and 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 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

Soils Soils divided into horizons: • Topsoil

− A Horizon - dark loam, 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 not 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 globe, as does its role in weathering of rocks• Deserts - little weathering by rain, and soils poorly

developed

• 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

Many kinds of organisms, roots and other plant parts in soil• 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 increasing rate at which minerals dissolve

• Small animals alter soil by activities and wastes• Humus - partially decomposed organic matter, gives

soil 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 - micelleso 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 - granular with pore

spaces occupying between 40-60% of total soil volume

− Particle size 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 rain

Capillary Water - water held against force of gravity in soil pores• Determined by structure and organic matter, by

density and type of vegetation, and by 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 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

roots and soils

Documents