Download Plant Hormones - NCEA Level 3 Biology

Plant Hormones
How plant hormones affect
growth.
Auxins
The name comes from the Greek word
auxein which means “to grow”
These are produced in the tips of shoots
and roots.
Auxin is water soluble
Auxin causes elongation of cells promoting
growth
Auxin Cont
It is auxin which allows plants to move or
bend in response to light.
The main auxin is called Indole Acetic Acid
or IAA.
Artificial auxins can be used as weed
killers.
The Effect of Auxin on Shoots and
Roots.
This depends on the concentration of
auxin.
– Low concentrations stimulate roots, while high
concentrations inhibit growth.
– Low concentrations stimulate growth of lateral
buds, while high concentrations inhibit growth.
– Low concentrations do not stimulate the
stems, while high concentrations do.
The Effect of Auxin on Shoots and
Roots.
Ref pg 16
Apical Dominance
The auxin from the top of the plant, the
apical meristem is produced in relatively
high conc, so the shoot is stimulated.
However the lateral buds remain dormant
as high concs of auxin inhibit their growth.
As the auxin diffuses down the plant it
becomes less concentrated, and is
inactivated by enzymes and hormones in
the lower part of the plant.
Apical Dominance Cont
Near the bottom of the plant the
concentration of auxin is low enough to
stimulate the growth of lateral buds.
This causes the triangular shape of trees
such as conifers.
The process is known as Apical
Dominance
Ref pg17
Geotropism in Seedlings
When the shoot and root emerge from a
seed, gravity receptors come in to play.
These cause an uneven movement of
auxin from the top of the shoot or root to
the bottom, so the conc of auxin on the
underside is greatest.
Ref pg 18
Geotropism in Seedlings
Higher conc of auxin stimulates the shoot,
so the underside elongates the most and
causes the shoot to curve upwards.
Higher conc of auxin inhibits the root, so
the top side grows faster and so the root
curves downwards.
Other Effects of Auxin
Auxin encourages root development even
on stems and leaves. This is important in
the propagation of new plants.
IAA, along with other hormones is involved
in abscission. IAA appears to delay this
abscission – used by fruit growers to kepp
fruit on trees until it is ripe.
Other Effects of Auxin
Auxin stimulates the growth of cambium
when a tree stem is under stress – if a
branch is in danger of breaking due to
heavy fruit or leaves, the cambium lays
down more wood on one side to
strengthen the branch.
Suppression of root elongation.
Initiation of flowering in some plants.