Plant Growth in Angiosperms
Plants have hormones:
Substances produced in one part of body,
transported to another part
where it has a physiological effect
(binds to receptor, triggers response in nearby
cells/tissues)
Auxin
= Indole Acetic Acid (IAA)
1)
Functions in phototropism
= movement
toward light
- Auxin concentrated
on dark side
- Induces cell
expansion /
elongation
Auxin
2) Functions in gravitropism
= movement relative to
gravity
Shoots negatively gravitropic
Roots positively gravitropic
In both, auxin concentration
induces cell expansion /
elongation
Auxin
Acts by loosening cellulose
microfibrils of cell walls
Causes cell expansion
Auxin
3) Functions (along with cytokinens)
in apical dominance
Auxins produced by apical meristem
Inhibits growth of lateral branches
If apical meristem damaged,
auxin production stops
No longer inhibits lateral branch
growth
lateral buds ---> branches
Auxin
4) Promotes secondary growth
(& wood production)
Cytokinins
1) Promote cell division
2) Stimulate bud formation --> lateral branches
3) Inhibit stem elongation
Balance of cytokinins & auxins
---> many plant growth processes!
Gibberellins
1) Function in seed germination
- Embryo releases gibberellins
- Causes aleurone layer (in seed coat) to release
enzymes (alpha-amylase): break down starch in
endosperm to sugars (e.g., maltose)
E.g.,
germination
of barley
(beer
production)
Gibberellins
2) Fruit development
Seedless fruit crops (e.g., grapes) may be
artificially sprayed with gibberellins - make
fruits bigger
Gibberellins
3) Stem growth (elongation)
Induce “bolting”
- elongation of flowering
stem of some plants
gibberellins
added
Ethylene (a gas)
1) Involved in fruit ripening
applied commercially to
ripen some fruits
2) Apical hook
some Eudicot seedlings
- ethylene inhibits growth on
inside of hook
- protects apical meristem
Phytochromes
Photoreceptor proteins
Two forms:
P r = absorbs red light, converted to P fr
P fr = absorbs far red light, converted to P r
red light
Pr
P fr
far red light
Phytochromes
1) Involved in seed
germination
Only P fr form
(after exposure
to red light)
will cause seeds
to germinate:
Fig. 39.18
Transport of water and sugar in
plants
• Chapter 36
How does water get up a tree?
Transpiration Cohesion Tension Mechanism
Transpiration, evaporation of water from leaves,
moves water molecules in xylem up from the roots
Cohesion of water molecules for one another allows
continuous water column to be maintained.
Water follows a water potential () gradient (affected
by pressure, gravity, and solute concentration), from
high to low.
Water potential at the leaf is very low (-10 to -100
MPa), becoming increasingly higher at
the roots (-0.6 MPa).
Tension of water column is maintained by water flow
and cohesion.
Low water potential
(-10 to -100 MPs)
WATER
FLOW
Medium water potential
(-0.8 MPs)
WATER
FLOW
High water potential
(-0.3 MPs)
How is sugar transported?
Movement occurs from a region of high
concentration to low concentration:
sugar source (where sugar produced, e.g.,
from leaf or by breakdown of starch)
to a sugar sink (where it is utilized)
How is sugar transported?
Water entering sieve elements causes positive
osmotic pressure, greatest where sugars are most
concentrated.
Thus, sugars move by pressure flow,
from a region of high concentration (high pressure)
to a region of low concentration (low pressure)
Osmosis-movement of water across a membrane from low to high
concentration of solutes – results in increased osmotic pressure.
Pressure Flow:
Movement by osmotic
pressure within sieve
elements from high sugar
concentration to low sugar
concentration:
sugar source (where
sugar produced, e.g., from
leaf or by breakdown of
starch)
to a sugar sink (where it
is utilized)