Hormonal Control in Plants
Requirements for Growth & Reproduction
2010
Why would plants need hormones? Controlling growth Responses to environmental changes Controlling reproductive cycles Responses to competition for
resources
When a seed begins to grow into a plant, why does the root grow downwards and the shoot grow upwards?
Gravitropism Heavier organelles settle on the lower
side of cells Auxin concentrates in cells on lower
side, causing differential growth
Mechanism of Action loosens cell wall cells elongate
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
Phototropism 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 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: Why was the top of the shoot covered with
tinfoil?
Results 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
In another experiment, they covered the tip with a transparent glass tube. Predict what occurred.
Experiment 2 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: a)
b)
c)
d)
Results 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 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 The growth hormone
diffused into the agar Uneven
concentrations on one side of the shoot caused bending
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 What is the advantage of phototropism? What would happen to a plant placed
under lights which shone on it from all directions?
Photoperiodism 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 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.
Long day plants Long days of
sunlight and short nights stimulate flowering
E.g. lettuce, clover
Short day plants Short days and
long nights stimulate flowering
E.g. dahlias, chrysanthemums
Questions 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 A pigment called phytochrome detects the
amount of daylight in each diurnal (day/night) cycle.
It has two forms 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. Inhibits flowering in short day plants Induces flowering in long day plants
(note: Northern hemisphere data)