H 3.5.5 AUXINS
Objectives – What you will need to
know from this section
 Study auxin as an example of a plant growth
regulator under the headings of :
 production
 initiator
 production site(s),
 function,
 different effects.
AUXINS -- HIGHER LEVEL
 Auxins affect virtually every aspect of plant development, and
how they respond to environmental stimuli.
 AUXINS influence includes
 phototropism,
 geotropism,
 cell enlargement and growth,
 apical dominance,
 root growth,
 fruit development,
 vascular development,
 and senescence [aging].
AUXINS
 Promote cell enlargement and growth
 Are involved in phototropism—stem bends towards
light.
 Are responsible for apical dominance, where the main
bud inhibits the growth of buds lower down stem.
 Regulate the differentiation of the vascular tissue
(xylem and phloem).
 Indole-3-acetic acid (IAA) is the most abundant
naturally-occurring auxin,
 IAA (Indoleacetic acid) is an auxin that is made in the
meristems of shoots, buds and root, and in the tips of
coleoptiles [of grasses].
 Auxin travels backwards
from the tip in the vascular
tissue and causes cells to
elongate (get longer).
 This makes the zone of
elongation grow and so the
tip is pushed further
upwards (in shoots) and
downwards (in roots).
 Movement is by active
transport from cell to cell –
this requires energy.
Auxins
 promote cell enlargement and growth,
 are involved in phototropism and apical dominance
PHOTOTROPISM
APICAL
DOMINANCE
LEARNING CHECK
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What is an auxin?
List 4 effects of auxins in a plant.
List the properties of auxins.
What does elongation mean?
What is IAA?
What is a meristem?
What is a coleoptile?
 A tropism is a plant’s response to a stimulus coming from
one direction, e.g. sunlight, gravity.
 Phototropism is a
growth response of
a stem towards
light, so that it can
receive the
maximum amount
of light for
photosynthesis.
APICAL
DOMINANCE
 Auxins are
responsible for
apical dominance,
where the main bud
inhibits the growth
of buds lower down
stem.
 This photograph
shows side buds
sprouting when the
main stem is cut off
[pruned].
In shoots, light causes auxin to move down the shaded part of
the stem, causing the shoot to grow towards the sunlight.
This is called PHOTOTROPISM.
Quicker growth
here due to more
hormones
Artificial auxins can also be used to kill weeds,
stimulate root formation in cuttings, …
Uses of plant regulators
Artificial auxins are used in rooting
powders to stimulate root formation in
stem cuttings.
A synthetic auxin [2,4-D] is used as a
selective weedkiller, making the plants
grow too fast.
Fruit is transported green and
unripe, and can then be
quickly ripened by spraying it
with ethene.
LEARNING CHECK
• What is phototropism?
• What is apical dominance?
• If you remove the apical bud from a stem,
what happens next?
• Give 3 uses of plant growth regulators.