Download Plant hormones and responses

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Responses to environmental changes
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What are plant hormones?
Plants need a method of responding to their environment to
cope with predation and abiotic stresses, such as drought.
Plants do not have a nervous
system, so a major way in
which they respond to the
environment is by using plant
hormones, often called plant
growth factors.
Unlike animal hormones, plant growth factors are not made in
specialized organs but in many tissues all over the plant. They
may be used where they are made, or transported within the
plant. They usually exert their influence by affecting growth.
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Effects of plant growth factors
Plant growth can be divided into three main types:

cell division

cell elongation / enlargement

cell differentiation (specialization).
These types of growth can all be controlled by plant
growth factors.
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Tropisms
A tropism is the movement of part of a plant in response to,
and directed by, an external stimulus. The movement is by
growth and is therefore slow. It is described as positive or
negative depending on whether the growth is towards or away
from the stimulus respectively.
Stimulus
Tropism
Examples
light
phototropism
shoots positive, roots negative
gravity
geotropism
roots positive, shoots negative
chemical
chemotropism
pollen tubes positive
water
hydrotropism
roots positive
air (oxygen)
aerotropism
pollen tubes negative
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Difficulties in studying growth factors
The effects of a plant growth factor can depend on:

the concentration of the growth factor

the tissue being acted on, e.g. stems or roots

the developmental stage of the plant

the species of plant

what other growth factors are present. Another growth
factor can increase the effect of the growth factor being
studied (synergism) or reduce it (antagonism).
The fact that growth factors are only present in minute
amounts adds to the difficulties in studying plant responses.
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Types of plant growth factor
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Which growth factor?
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Fill in the missing words
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Photoreceptors
Photoreceptors are structures or
pigments that are sensitive to light,
often light of specific wavelengths.
light (stimulus)
phototropin (receptor)
cascade of
reactions
Phototropins are a group of
photoreceptors primarily responsible redistribution of auxin
for triggering phototropisms.
phototropism
When light of the right wavelength is present, changes in the
phototropin molecules trigger a cascade of reactions in the
cell. This ultimately results in the redistribution of auxin so that
there is more on the shaded side of the plant.
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IAA – an auxin
Auxins are made continually in
the shoot apex and young
leaves (apical meristems). They
can move by diffusion from cell
to cell, or can be transported
long distances via the phloem,
e.g. from the shoots to the roots.
The primary natural auxin is
indoleacetic acid (IAA). It
is involved in many
important processes,
including cell elongation and
phototropism.
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How does IAA work?
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The discovery of auxin
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Phototropism in the classroom
There are many ways phototropic responses can be shown in
a school laboratory. How would you use this apparatus to
investigate the effects of light on a growing plant?

What variable could
you change?

What control experiment
would you use?

How would you present your results if they were qualitative,
rather than quantitative?

How might you extend this investigation to gain further
information about how plants respond to light?
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True or false?
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Apical dominance
Apical dominance is the growth of the main central stem of a
plant, with reduced production of lateral (side) shoots/branches.
Apical dominance is controlled by auxin. One piece of
evidence for this is a lack of apical dominance when the top
of a plant is pruned.
lateral
apex
High concentrations of auxin
branches
are produced at the stem tip
(apex), preventing the
development of lateral
branches near the apex.
Pruning of the apical bud
causes lateral branches
higher up the plant.
after
apical
dominance
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pruning
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Leaf abscission
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Stem elongation
Gibberellins are formed in young leaves and around growing
tips. They stimulate the growth of shoots (stem elongation)
and leaves. They are also involved in seed germination.
Dwarf plants provide some evidence for the role of gibberellins
in stem elongation because when gibberellins are applied
artificially to dwarf plants, they can stimulate stem elongation,
even in genetically determined dwarfism.
This is because the
dwarfism occurs due to
the absence or mutation
of the gene for
gibberellin production.
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Plant responses quiz
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Commercial applications: introduction
Plant growth factors can be applied to a plant artificially for
commercial reasons.
The growth factors used are usually
synthetically produced since this is
cheaper and more efficient than
extracting them from plants.
The process is relatively inexpensive
because only very low concentrations
are needed to produce an effect.
Synthetic growth factors can be used
by farmers or gardeners to increase
crop yield or quality.
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Commercial applications
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Which growth factor would you use?
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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