Plant%20Hormones

1
Plant Hormones

• How plant hormones affect growth.

2
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

3
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.

4
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.

5
The Effect of Auxin on Shoots and Roots.

• Ref pg 16

6
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.

7
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

8
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

9
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.

10
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.

11
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.