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)