Download Lesson 3 Auxins - MrHay

Animal Behaviour and
Plant Responses.
Year 13 Biology
Plant structure
Plant structure
Plant Hormones
• Plants use hormones to regulate their
growth and development.
• Plant hormones (or phytohormones) are
organic compounds produced in one part
of the plant and transported to another
part, where they produce a growth
response.
Plant Hormones
• There are 5 groups of plant hormones;
– Auxins (indolacetic acid or IAA)
– Cytokinins
– Gibberellins
– Ethene
– Abscisic acid (ABA)
• Together they control growth and
development on the plant at various
stages.
Auxin experiments
Auxin – indolacetic acid IAA
• The effect of auxin on roots and shoots
– The effect of auxin is different on the shoot,
lateral buds (side buds found in the axis of the
leaves where the leaf joins the stem), and the
root.
– It all depends on the concentration of the
auxin
Auxin Concentrations
• Low auxin concentrations do
not stimulate the growth of
shoots (stems in graph), but
high concentrations do.
• Low auxin concentrations
stimulate root growth, and
high concentrations inhibit
root growth.
• Low auxin concentrations
stimulate growth of lateral
buds; high concentrations
inhibit this.
Apical dominance
Continuing Biology page 189
• Apical dominance
– If you take a bean shoot and leave it to grow,
the intact tip keeps the lateral buds from
growing.
– If, however you remove the apical bud (the
bud at the tip of the main growing shoot) the
two lateral buds start to grow within hours.
– If you were to place a plug onto the apical bud
that contained auxin you would find that the
growth of the lateral buds is inhibited.
Apical dominance
Apical dominance
• Remember
Low auxin concentrations
stimulate growth of lateral
buds; high concentrations
inhibit this.
This is why some trees
have a triangular shape
Geotropism
• Continuing Biology page 189
Other tropisms
Continuing biology page 190
Other effects of auxin
Continuing Biology page 191
• Cell elongation
• Encourages root development
• Involved in the abscission (dropping of
leaves and fruit). It appears to delay this.
• Stimulates growth of the cambium when a
tree stem is under stress
• Suppression of root elongation
• Initiations of flowering in some plants