First Root of the Plant

Roots

Chapter 24

Root functions

Anchorage

Absorption

Storage

Conduction

Root structure

Primary root

–First root of the plant

–Also termed the taproot

Taproot grows down and lateral roots or branching roots form

When taproot is prominent, root system is termed a taproot system

–Common in eudicots

Taproot can extend great distances

Root systems

In monocots, primary root is short lived

Main shoot system arises from the stem

Forms a fibrous root system

Generally are shallower than taproot systems

Root to shoot ratio (R:S)

Plants allocate energy to roots or shoots depending on environmental conditions

Root to shoot ratio is a common measure in plant ecology

Root to shoot lower in high moisture environment than in low moisture environment

Root to shoot ratio higher in high light environment compared to low light environment

Root growth

Roots grow continuously from apical meristem

–Promeristem is most distal and least determined part of the apical meristem

Root cap protects apical meristem

–Parenchyma cells

Hydrated polysaccharide and sloughed root cells form slimy sheath over root cap, called mucigel

Gravitropism

Roots move towards gravity

Perception to gravity is in central column of cells in the rootcap, called the columella

Root zones

Region of actively dividing cells is called the region of cell division

Region of elongation is small, but responsible for most of the increase in root length

Region of maturation is where root hairs are produced

Primary structure

Three tissues

–Epidermis

–Cortex

–Vascular

Simple structure

Tissues easily distinguishable

Primary structure

Three tissues

–Epidermis

–Cortex

–Vascular

Simple structure

Tissues easily distinguishable

Pith in center

Root hairs

Root hairs are tubular extensions of epidermis

Greatly increase surface area of roots

Important in uptake of water and minerals

Mucigel provides intimate contact with environment

–Layer soil bound to roots with sloughed cells and microorganisms is called the rhizosphere

Mycorrhizae

Mutualistic relationship between plants and fungi occuring in many vascular plants

Fungi increase plant’s ability to capture water and essential nutrients, especially phosphorus

Fungi provide protection against attack by pathogenic fungi and nematodes

Fungi receive carbohydrates and vitamins

Endomycorrhizae

Penetrate root cells

Most common (80%)

Fungus penetrates cells, forming branched structures called arbuscules

–Most exchange occurs in arbuscules

In some cases, form terminal swellings called vesicles

–Storage?

Also called vesicular-arbuscular, V/A or VAM

Fungal hyphae extend out to soil

Ectomycorrhizae

Surround rather than penetrate root cells

Prevalent in some families: Fagaceae, Salicaceae, Pinaceae

In conifers, Hartig net eventually surrounds many of the cortical and epidermal cells

Causes roots to thicken rather than elongate

Mantle is a sheath of hyphae that covers the root surface

Cortex

Contains numerous intercellular spaces that provide aeration to roots

Substances can move through apoplast (between cells) or symplast (through plasmodesmata)

Innermost layer of cortex is endodermis

–Characterized by presence of Casparian strips

–All materials must pass through protoplasts of endodermis (symplast)

Exodermis

In many angiosperms, there is an exodermis, that also has Casparian strips

Vascular cylinder

Primary vascular tissues and nonvascular pericycle

Secondary roots arise from pericycle

Phloem are found between ridges of xylem

First xylem cells are near the pericycle and the are called protoxylem poles

The metaxylem occurs on the inner portion of the vascular cylinders

Secondary growth

Monocots do not undergo

Some eudicots do not undergo

Secondary growth happens after:

–Secondary vascular tissues are formed from vascular cambium

–Periderm is formed from cork cambium

Steps of secondary growth

1. Vascular cambium initiated by divisions of procambium cells located between primary phloem and xylem

2. Pericycle cells opposite protoxylem poles also divide and inner cells contribute to vascular cambium

Steps of secondary growth

3. After these cell divisions, vascular cambium completely surrounds the xylem core

4. Vascular cambium forms secondary xylem on inside and secondary phloem on outside

Steps of secondary growth

5. Periderm formation initiated after secondary vascular tissue is made by cell division in pericycle

6. Cork cambium forms from outer layer of pericycle

7. Cork is produced on outside and phelloderm on inside (cork + cork cambium + phelloderm = periderm)

Lateral roots

Originate in pericycle

Occurs in zone of maturation

Root cap forms on lateral root tip

Two vascular cylinders are joined later

Aerial roots

Prop roots provide support

Air roots or Pneumatophores

Provide oxygen to roots

Fleshy roots

Carrot

Sweet potato

Beet