Download Uses of plant hormones

AQA Certificate
6.3 Plant Responses
January 2012
6.3 Plant Responses
Learning outcomes:
a) Plants are sensitive to light (phototropism), moisture (hydrotropism) and gravity (gravitropism):
•their shoots grow towards light and against the force of gravity
•their roots grow towards moisture and in the direction of the force of gravity.
b) Plants produce hormones to coordinate and control growth. The hormone auxin controls
phototropism and gravitropism (geotropism).
c) The responses of plant roots and shoots to light, gravity and moisture are the result of unequal
distribution of auxin, causing unequal growth rates.
d) Plant growth hormones are used in agriculture and horticulture as weed killers and as rooting
hormones.
Names of specific weed killers and rooting hormones are not required.
Experiments and investigations that could be used to help develop skills and understanding:
• observing the effects of light and / or water on the growth of seedlings
• observing the effect of gravity on growth in germinating seedlings
• using a motion sensor to measure the growth of plants and seedlings
• investigating the effect of rooting compounds and weed killers on the growth of plants.
Videos:- 243 science bank 2- (put onto DVD) last bit of responding to stimuli
Video 102- old, but good, (spin through a bit). Covers roots well and touch and nastic
movements.
1
AQA Certificate
6.3 Plant Responses
January 2012
Remember, plants are living organisms, just as much as animals. Therefore they must carry out the 7
key processes of living organisms, (MRS GREN)
S = Sensitivity.
There are 3 key factors, (stimuli) that plants must be able to sense:
1: light
2: gravity
3: moisture
For each of the above factors, think about how each part of the plant must respond to these stimuli.
Use the table below to help you.
Stimulus
Part of Plant
Description of Response
light
Root
Grow away from light
Shoot/ Stem/
Leaves
Grow towards light
gravity
Root
Grow towards gravity
Shoot/ Stem/
Leaves
Grow away from gravity
moisture
Root
Grow towards Moisture
Shoot/ Stem/
Leaves
No response
Plant Responses:
Responses of plants to stimuli are known as TROPISMS.
Gravity- GEOTROPISM (also known as GRAVITROPISM)
Light- PHOTOTROPISM
Water- HYDROTROPISM
2
AQA Certificate
6.3 Plant Responses
January 2012
Depending on the direction of the plant organ’s response to the stimulus depends on whether the
response is described as being NEGATIVE or POSITIVE.
e.g. A shoot growing TOWARDS light would be POSITIVELY PHOTOTROPIC.
Whereas, as root growing AWAY from light would be NEGATIVELY PHOTOTROPIC.
Complete the following sentences:
Shoots grow away from gravity. They are described as being negatively geotropic .
A root growing downwards into the soil is described as being positively geotropic(in
response to gravity).
The responses to these stimuli in the plants are controlled by chemical hormones called AUXINS.
Auxins control the growth of cells, (cell elongation) in the plants by increasing the growth in shoots
of plants, and decreasing growth in the roots of plants.
Auxin is made at the tips of stems and roots. It is moved in solution to older parts of the stem and
root where it changes the elasticity of the cells. More elastic cells absorb more water and grow
longer, causing bending in the stem or root. It's thought that light ,gravity and moisture can interfere
with the transport of auxin causing it to be unevenly distributed.
This control is fine enough that different sides of the same root or stem can experience different
results in response to the stimulus.
This is how a shoot bends to grow towards the light.
3
AQA Certificate
6.3 Plant Responses
January 2012
Root response to gravity:
Sketch a diagram similar to that above for light, to show the effect of gravity on auxin action and
root growth.
Draw root on its side. Auxin collects on the underside of the root and inhibits growth.
Uses of plant hormones
Weedkillers
Selective weedkillers kill some plants but not others. This can be useful for getting rid of dandelions
in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat plants.
The selective weedkiller contains growth hormone that causes the weeds to grow too quickly. The
weedkiller is absorbed in larger quantities by the weeds than the beneficial plants.
Rooting powder
Rooting powder makes stem cuttings, (clones) quickly develop roots. Rooting powder contains
growth hormones.
4