Plant structure and function Roots

Objectives of today’s lecture:

• Learn about the functions of the root system and its importance to plant growth and development

• Learn the structures that allow the root to perform these functions

• Learn some of the modifications that occur in roots

Functions of the Root

• anchorage to the soil (or other substrate)

• absorption of water

• acquisition of mineral nutrients

• a tissue for energy (food) storage

• for propagation and dispersal of some species

Functions of the Root

• Root morphology is highly variable

Functions of the Root

Root morphology is highly variable

• most dicotyledonous plants (those with 2 seed leaves) have a prominent primary root

• monocots (with one cotyledon or seed leaf) typically have a more diffuse fibrous root system

How extensive are root systems?

A 40 ft. tall oak tree

• primary tap root penetrated 14 ft. into the soil

• lateral roots extended to a radius of 60 ft.

How extensive are root systems?

A single winter rye plant growing in a pot with 2 cu. ft. of soil

• clump of about 80 shoots, total leaf area of 51 sq. ft.

• 143 primary roots

• 35,000 secondary roots

• 2 million tertiary roots

• 11 million quaternary roots

• total root surface area of 6,500 sq. ft.

Importance of the root system in horticulture

• Essential for growth of virtually all plants

• Are there examples of plants that don’t have roots?

Importance of the root system in horticulture

• Many horticultural plants are transplanted

• in ornamental landscaping

•Trees and shrubs

•Perennials and annuals

Importance of the root system in horticulture

• Vegetable seedlings and fruit trees are transplanted

• Successful transplanting depends on a vigorous and healthy root system

• Many horticultural products are roots

• Primarily vegetables

Growth of the root system

• Root growth depends on the activity of the root apical meristem

• located just behind the root cap at the tip of the root

• provides a source of new cells for extension and elongation of the root

Growth of the root system

Ink dots are placed on the roots of a seedling (A). As the seedling grows, the dots at the top of the root (near the stem) stay in position. The dots near the tip move apart (B), indicating that this region is growing.

• Growth of the root occurs at the tip

Tissues of the root

Epidermis

Cortex

Endodermis

Pericycle

VascularSystem

Phloem

Cambium

Xylem

Root tissues can be regarded as a series of concentric rings of different tissues

Epidermis

• Single layer of cells on the exterior of the root

• NOT covered with a cuticle

• root is designed for water uptake

• cuticle is a barrier to water

Epidermis

Cortex

Endodermis

Pericycle

VascularSystem

Phloem

Cambium

Xylem

Epidermis

• Root hairs are specialized epidermal cells

• single cells that elongate perpendicular to the root

Epidermis

• Root hairs contribute the largest fraction of root surface area

• In the winter rye example given earlier, more than 4,000 sq. ft. of the 6,500 sq. ft. of root surface area comes from root hairs

• Epidermis functions in the uptake of water and minerals

• Large surface area facilitates uptake

• Comprised of large undifferentiated cells

• Functions in the storage of food reserves (proteins and starches) in root tissues

Cortex

Epidermis

Cortex

Endodermis

Pericycle

VascularSystem

Phloem

Cambium

Xylem

Endodermis

• Single layer of cells outside vascular system

• Spaces between the cells are covered with a waxy layer, called suberin• Similar to grout between

ceramic tiles, and serves a similar function

Epidermis

Cortex

Endodermis

Pericycle

VascularSystem

Phloem

Cambium

Xylem

Endodermis

• Blocks passage of water and dissolved nutrients into the vascular system

• Water, nutrients and other solutes must enter a cell, cannot go round the endodermis

Endodermis

• Endodermis functions to regulate the uptake of water and nutrients into the plant

Pericycle

• A thin layer of undifferentiated cells

• A meristem tissue

• The source of new lateral, or secondary, roots

Vascular tissue

• The transport system

Vascular tissue

• Phloem

• living cells comprised of seive tube elements and companion cells

• Cambium

• a meristematic tissue supplying cells that will develop into vascular structures

Vascular tissue

• Xylem

• dead cells with thickened cell walls, typically larger than phloem cells

• functions in the transport of water and mineral nutrients

A long look at the root

• A longitudinal section through the root (from bottom to top) shows these various tissues and how they develop.

A long look at the root

• Root cap

• Cells are sloughed off

• Protects the apical meristem

• Apical meristem

• Source of cells for the root

A long look at the root

• Elongation zone• Where cells elongate and

differentiate

• Pushes the root tip through the soil

• Maturation zone• Cells develop their final

form

• Root hairs form, xylem cells thicken and die

Organ Tissue Cell Type

Root

Epidermis

VascularSystem

Epidermal

Root hairs

Phloem

Cambium

Xylem

Cortex Parenchyma

Endodermis

Pericycle

CasparianStrip

Parenchyma

Transpiration

• The continuum of water from soil, through the plant and into the air

Root modifications

• The organ that is consumed in many vegetable crops, such as carrot and sweet potato, is a modified root that serves as a storage tissue

• Other modifications for support