Root Structure and Function Penetration of Soil Gravitropism Downward Growth Water and Mineral Intake Conduction (Xylem and Phloem) Storage of Materials.

Root Structure and Function

Penetration of Soil

Gravitropism Downward Growth

Water and Mineral Intake

Conduction (Xylem and Phloem)

Storage of Materials

Branching

Anchorage

Notice how the growing zone has no root hairs or lateral roots!

Growth among soil particles would result in shear forces.

Zone of Maturation - cell differentiationProtodermGround MeristemProvascular

Zone of Cell Elongation - cell expansion

Zone of Cell Division - new cells by mitosis

Root Cap - penetration, padding

Mucilage Slough Cells

Root Tip Senses GravityAuxin Hormone ProducedAuxin Accumulates on LowerGrowth Inhibited on Lower…Relative to UpperRoot Curves Downward

Gravitropism

Root Hairs Increase Surface Area

Root Hairs Secrete Acid (H+)

H+ Cation Exchange w/Minerals

Mineral Uptake into Roots

Water and Mineral Uptake

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Radish seedlings have roots with long root hairs that increase the surface area for water and mineral uptake

Osmosis: passive movement of water from pure to polluted area

cytoplasmic solutesmore concentrated

soil solutes more dilute

cell membrane

Water potential low Water potential high

cell wall

water flow

Root hairs are responsible for cation exchange

soil particles covered with capillary waterand minerals

water

voids with air space

root hair penetrates soil spacesepidermal cell

cortex cell

H+

Ca2+H+Ca2+

Ca2+

intercellular gas space

to vascular cylinder

Dicot Mature Root Structure - Anatomy

Epidermis

Cortex

Vascular Cylinder

Ranunculus acris - buttercup

What does all of this autumn color (leaf senescence) have to do with roots?

Root Vascular Cylinder and Cortex Ranunculus acris - buttercup

Cortex

Endodermis

Meta-xylem

Phloem

PericycleProto-xylem

Endarch: protoxylem is inside the metaxylem

Exarch: protoxylem is outside the metaxylem √

haplostele

siphonostele

eustele

solenostele dictyostele

atactostele

leaf gap

leaf trace

cortexcortex phloem

xylem

cortexcortex phloem

xylem pithpith

dicot stem

monocot stem

Protosteles:actinostele plectostele

Specialized Versions

dicot root

monocot root

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Symplastic

xylem inside cortex outside

endodermis

suberin-waxy barrierto apoplastic

movement

cell membrane proteins (active transporters) determine which minerals may be taken up

The endodermis is thus responsible for selective mineral uptake.

minerals cannot go between cells

minerals must go through cells

Mineral uptake: Active transport against concentration gradient

cytoplasmic solutesmore concentrated

soil solutes more dilute

cell membrane

Water potential low Water potential high

cell wall

water flow

Ca2+

ATP

ADP + Pi

Ca2+Ca2+

Calcium transport protein

Osmosis: passive movement of water from pure to polluted area

too expensive?

Root Anatomy: Dicot Root Cross Section

Epidermis - root hairs, mineral and water intake

Cortex - storage, defense

Endodermis - selective mineral uptake

Pericycle - lateral root formation (periderm)

(Vascular Cambium - makes 2° tissues)

Phloem - CH2O delivery from leaves

Xylem - conduct water and mineral upwards

Casparian strips in radial walls

One Vascular Cylinder (Phloem + (Cambium) + Xylem)Radial Xylem-Phloem ArrangementExarch Xylem MaturationTetrarch (this example)

Monocot RootCross Section

Smilax-catbrier

Pith

Xylem/Phloem Arr?

Xylem Maturation?

______arch?

How is this section different?Smilax - catbrier

A closer look…What do these features tell you?

StarchCutin/Suberin

Mitochondria

Sieve Tube ElementCompanion Cell

Vessel with Lignin

Xylem Parenchyma

Lignified Pith Parenchyma

What is the Pericycle doing?

Root Cap

Zone of Cell Division

Growing out through cortex

In fibrous root systems, there is much lateral root formation.

Here you can see two root apices initiating from the

pericycle.

Notice their connection to the ridges of xylem

In shrubs like this tea plant (Camellia sinensis), the root system will be more tap root than fibrous root.

Notice the diameter of this tap root compared to this man’s waist!

But shrubs also generally have some compromise for uprooting forces…feeder roots extending laterally.

Tropical soils are nutrient poor.

Roots must traverse the surface for minerals, so roots grow on the surface (no tap root).

So, to keep this tall baobab tree standing upright, the roots grow in diameter but only in the vertical dimensions to form ridge roots…called buttress roots.

My wife here is as large as I am so you can see these roots are a meter tall!

These roots inspired gothic cathedral architects to design buttress walls.

http://www.dublincity.ie/dublin/citywalls/buttress.jpg

http://www.oxc.com.hk/raoul_nathalie/gallery/images/04%20Buttress.jpg

Pandanus utilis - screw pineProp roots such as these inspired flying buttresses.

http://www.contrib.andrew.cmu.edu/~ajm/Pages/Graphics/flyingbuttress.JPG

http://williamcalvin.com/BHM/img/FlyingButtressND.jpg

Avicennia germinans (black mangrove)

pneumatophores