14.5 Tropic responses
Core
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Define gravitropism as a response in which parts of a plant grow towards or away from gravity
Define phototropism as a response in which parts of a plant grow towards or away from the direction from
which light is coming
• Investigate gravitropism and phototropism in shoots and roots
Supplement
• Explain phototropism and gravitropism of a shoot as examples of the chemical control of plant growth
• Explain the role of auxin in controlling shoot growth, limited to:
o auxin made in shoot tip (only)
o auxin spreads through the plant from the shoot tip
o auxin is unequally distributed in response to light and gravity
o auxin stimulates cell elongation
• Describe the use in weedkillers of the synthetic
plant hormone 2,4-D
Gravitropism = response in which parts of a plant
grow towards or away from gravity
Phototropism = response in which parts of a plant
grow towards or away from the direction from
which light is coming
Positive tropism = towards stimulus
Negative tropisms = away from stimulus
The growth of plants in shoots and in roots are controlled by the plant hormone called auxin. So plant
growth is controlled by chemical substances, in this case hormones. They control the response towards
light and gravity
In the shoot = auxin make cells elongate in response to LIGHT = POSITIVE PHOTOTROPISM
1. auxin is made in the shoot tip only (the very top layer).
2. spread downwards.
3. move to the side where it is shaded.
4. And cause cell elongation (absorb more water)
5. When one side grow longer than the other, cause shoot to bend towards light.
In the roots = auxin make stops cell elongation in response to LIGHT = NEGATIVE PHOTOTROPISM
1. Auxin is made in the root tip.
2. Auxin will again gather at the shaded side.
3. However, in the root, auxin make roots cells absorb less water
4. And inhibits cell elongation
5. Causing root to bend away from light
To test for auxin response to gravity, plants are placed horizontally in the absence of light
In the shoot = auxin make cells grow more quickly in response to GRAVITY = NEGATIVE GEOTROPISM
Auxin gathers at the bottom due to gravity, cause shoot to bend upwards
In the roots = auxin make cells grow more slowly in response to GRAVITY = POSITIVE GEOTROPISM
Auxin gathers at the bottom due to gravity, cause roots to bend downwards
Experiment
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Shoots that have their tip cut off, cannot respond to light = proving that chemicals produced at the
tip controls phototropism in the shoot.
Shoots covered with opaque cap will grow upwards, as they are covered = showing that light
influence the distribution of auxin in the tip
Selective Weedkillers (herbicide) = synthetic plant hormone 2,4-D
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Synthetic plant hormone 2,4-D acts like an auxin plant hormone
It is used to spray on weeds
And cause rapid, uncontrolled cell growth, elongation and cell respiration
Resulting in the death of the weeds
However, different plant species have different
sensitivity to auxins, so synthetic plant hormone
2,4-D are only able to kill dicot weeds with broad
leaves, such as dandelions and not monocots
(narrow leaves weeds such as grasses and cereal
crops), making them useful in such cases.
Other synthetic plant hormones use:
Rooting powder
Rooting powder makes stem cuttings quickly develop roots. Rooting powder contains growth hormones
that causes stem cuttings to grow roots so that valuable plants can be cloned.
Controlling fruit ripening
Some hormones slow the ripening of fruits and others speed it up. These hormones and their inhibitors are
useful for delaying ripening during transport or when fruit is displayed in shops. It is also useful to allow
fruit development at the same time for efficient picking.
Dormancy
Dormancy stops seeds germinating until conditions are ideal for growth. Hormones can be used to remove
the dormancy of a seed so it can germinate at all times of year. Buds and flowers can also be naturally
dormant. Hormones can also be used to make plants grow bushier, make them flower or control the
growth of hedge plants.
Seedless fruits
A hormone spray can make fruits such as apples and grapes develop without fertilisation. Since no
fertilisation has taken place, no seeds are formed. This also reduces the grower’s dependence on
pollinating insects.