Transcript Hormonal Control in Plants

Hormonal Control in Plants
Requirements for Growth & Reproduction
2010
Why would plants need
hormones?
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Controlling growth
Responses to environmental changes
Controlling reproductive cycles
Responses to competition for
resources
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When a seed
begins to grow
into a plant, why
does the root
grow downwards
and the shoot
grow upwards?
Gravitropism
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Heavier organelles settle on the lower
side of cells
Auxin concentrates in cells on lower side,
causing differential growth
Mechanism of Action
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loosens cell wall cells elongate
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auxin stimulates proton pumps
pump protons (H+) into cell wall
 expansins modify hydrogen bonds between
cellulose molecules
 molecules slide past one another, allowing for
elongation
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Phototropism
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Photo=light
Tropism=growth
Plants grow towards a
light source.
Charles Darwin and his
son carried out a series
of experiments to
determine what
controlled phototropism
(1881)
Experiment 1
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Oat shoots were used
A light source was
placed on one side
The top of one shoot
was covered with
tinfoil to keep out the
light
Write down your predictions:
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Why was the top of the shoot covered with
tinfoil?
Results
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The uncovered shoot
bends towards the
light
The covered shoot
remains straight
Therefore the tip of
the shoot is required
to detect the light
stimulus
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In another experiment, they covered the tip
with a transparent glass tube. Predict what
occurred.
Experiment 2
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a) the tip was cut off
b) the tip was cut off,
then replaced
c) the tip was cut off,
then replaced on a
block of agar
d) the tip was cut off,
then replaced on a
metal disc
Write down your predictions:
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a)
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b)
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c)
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d)
Results
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The tip is required for growth
The hormone was able to
diffuse through the agar and
cause growth
The metal disc prevented
growth from occurring
Therefore the tip produces a
hormone which diffuses
downwards and promotes
growth
Experiment 3
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This experiment was
carried out by Frits
Went in 1926
He placed tips of shoots
on blocks of agar, so
that the hormone
diffused into them.
He than placed the agar
blocks on different areas
of the cut tips.
Write down your predictions:
Results
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The growth hormone
diffused into the agar
Uneven
concentrations on one
side of the shoot
caused bending
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The hormone involved is a member of a
group of hormones called auxins.
The specific hormone is IAA (indole
acetic acid).
If the tip is exposed to light on one side,
the IAA drifts to the darker side. This
prompts that side to grow more, bending
the tip towards the light source.
Questions
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What is the advantage of phototropism?
What would happen to a plant placed
under lights which shone on it from all
directions?
Photoperiodism
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Photo=light
Period=time
Why do flowers
which are open
during the day close
at night?
What controls the
time of year when
plants flower?
Circadian rhythms
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The plants’ biological
clock is set to a 21-27 hour
cycle.
Some plants close their
flowers or leaves at night.
This prevents loss of heat
and frost damage.
Photosynthesis, auxin
production and cell
division also rise & fall
with the circadian rhythm.
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Long day plants
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Long days of
sunlight and short
nights stimulate
flowering
E.g. lettuce,
clover
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Short day plants
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Short days and
long nights
stimulate
flowering
E.g. dahlias,
chrysanthemums
Questions
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At what time of the year would long day
plants flower?
What is the advantage of this?
At what time of the year would short day
plants flower?
What is the advantage of this?
What about tropical plants growing on the
equator? When would they flower?
Phytochrome
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A pigment called phytochrome detects the
amount of daylight in each diurnal (day/night)
cycle.
It has two forms
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Pr (biologically inactive)
Pfr (biologically active)
Pr is converted into Pfr by exposure to light
It is involved in seed germination, leaf growth,
flowering and dormancy.
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Inhibits flowering in short day plants
Induces flowering in long day plants
(note: Northern hemisphere data)