Root Structure and Function

Root Structure and Function
Penetration of Soil
Gravitropism Downward Growth
Water and Mineral Intake
Conduction (Xylem and Phloem)
Storage of Materials
Branching
Anchorage
Zone of Maturation - cell differentiation
Protoderm
Ground Meristem
Provascular
Zone of Cell Elongation - cell expansion
Notice how the growing zone has
no root hairs or lateral roots!
Growth among soil particles
would result in shear forces.
Zone of Cell Division - new cells by mitosis
Root Cap - penetration, padding
Mucilage
Slough Cells
Gravitropism
Root Tip Senses Gravity
Auxin Hormone Produced
Auxin Accumulates on Lower
Growth Inhibited on Lower…
Relative to Upper
Root Curves Downward
Water and Mineral Uptake
Root Hairs Increase Surface Area
Root Hairs Secrete Acid (H+)
H+ Cation Exchange w/Minerals
Mineral Uptake into Roots
©1996 Norton Presentation Maker, W. W. Norton & Company
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
cell membrane cell wall
water flow
cytoplasmic solutes
more concentrated
Water potential low
soil solutes
more dilute
Water potential high
Root hairs are responsible for cation exchange
cortex cell
epidermal cell
root hair penetrates soil spaces
intercellular
gas space
Ca2+Ca
H 2+
soil particles
covered with
capillary water
and minerals
+
to
vascular
cylinder
Ca2+
H+
voids with air
space
water
Dicot Mature Root Structure - Anatomy
Ranunculus acris - buttercup
Epidermis
Cortex
Vascular Cylinder
What does all of this autumn color (leaf senescence) have to do with roots?
Root Vascular Cylinder and Cortex
Ranunculus acris - buttercup
Endodermis
Cortex
Phloem
Metaxylem
Endarch: protoxylem is
inside the metaxylem
Exarch: protoxylem is
outside the metaxylem
Protoxylem
√
Pericycle
Specialized Versions
Protosteles:
haplostele
actinostele
plectostele
siphonostele
solenostele
dictyostele
cortex
phloem
xylem
dicot
root
cortex
phloem
xylem
pith
leaf gap
leaf trace
monocot
root
eustele
dicot
stem
atactostele
monocot
stem
Apoplastic
©1996 Norton Presentation Maker, W. W. Norton & Company
Symplastic
endodermis
xylem inside
The endodermis is
thus responsible for
selective mineral
uptake.
suberinwaxy barrier
to apoplastic
movement
cortex outside
minerals cannot
go between cells
minerals must
go through cells
cell membrane
proteins (active
transporters)
determine which
minerals may be
taken up
Mineral uptake: Active transport against concentration gradient
cell membrane cell wall
Calcium
transport
protein
ADP + Pi
Ca2+
too expensive?
Ca2+
Ca2+
ATP
water flow
cytoplasmic solutes
more concentrated
Water potential low
soil solutes
more dilute
Water potential high
Osmosis: passive movement of water from pure to polluted area
Root Anatomy: Dicot Root Cross Section
Epidermis - root hairs, mineral and water intake
Cortex - storage, defense
Endodermis - selective mineral uptake
Casparian strips in radial walls
Pericycle - lateral root formation (periderm)
(Vascular Cambium - makes 2° tissues)
Phloem - CH2O delivery from leaves
Xylem - conduct water and mineral upwards
One Vascular Cylinder (Phloem + (Cambium) + Xylem)
Radial Xylem-Phloem Arrangement
Exarch Xylem Maturation
Tetrarch (this example)
Monocot Root
Cross Section
Pith
Xylem/Phloem Arr?
Xylem Maturation?
Smilax-catbrier
______arch?
How is this section different?
Smilax - catbrier
A closer look…
What do these features tell you?
Starch
Cutin/Suberin
Mitochondria
Sieve Tube Element
Companion Cell
Vessel with Lignin
Xylem Parenchyma
Lignified Pith Parenchyma
What is the Pericycle doing?
Root Cap
Growing out through cortex
Zone of Cell Division
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
But shrubs also generally have
some compromise for
uprooting forces…feeder roots
extending laterally.
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!
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.oxc.com.hk/raoul_nathalie/gallery/images/04%20Buttress.jpg
http://www.dublincity.ie/dublin/citywalls/buttress.jpg
Prop roots such as these inspired flying buttresses.
Pandanus utilis - screw pine
http://williamcalvin.com/BHM/img/FlyingButtressND.jpg
http://www.contrib.andrew.cmu.edu/~ajm/Pages/Graphics/flyingbuttress.JPG
Avicennia germinans (black mangrove)
pneumatophores