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OCR UNIT 215 PLANT RESPONSES
Specification details:
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Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress

Define the term tropism

Explain how plant responses to environmental changes are co-ordinated by hormones, with reference to
responding to changes in light direction

Evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the
control of stem elongation

Outline the role of hormones in leaf loss in deciduous plants

Describe how plant hormones are used commercially
Why Plants need to respond to their Environment

Plants can avoid abiotic stress factors such as drought and high temperatures. They use plant
hormones to induce stomatal closure in order to reduce transpiration

Plant shoots grow towards light in order to maximise light absorption for increased
photosynthesis. Shoots are positively phototropic and bend towards a light source

Plant roots grow into the soil, towards the pull of gravity, in order to maximise the absorption of
water and mineral ions for increased photosynthesis

In order to avoid competition, plants will grow taller to out-compete surrounding plants for light.
Their growth is often spindly referred to as etiolation. They will grow deeper roots to absorb
water and mineral ions from soil where there is less competition. These growth responses
require auxins and cytokinins

Some plants avoid grazing by herbivores. Some plants produce toxins. Others have spines on
their leaves

Some plants produce chemicals such as phenols that are leached into the soil inhibiting the
germination of seeds of other plants. This interspecific effect is called allelopathy

By using plant hormones, the plant ensures that its seeds only germinate in suitable conditions
Tropisms

A tropism is a growth response to a stimulus in plants

The direction of growth is always related to the direction of the stimulus

Shoots are positively phototropic – they grow towards a light source, to absorb light for
photosynthesis

Roots are positively geotropic – they grow towards the pull of gravity. This anchors them in
the soil and enables the roots to absorb water and mineral ions from the soil

However, shoots are negatively geotropic and grow away from the direction of gravity
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Plant Growth Substances/Plant Hormones

Plant responses to external stimuli are coordinated by plant hormones

Plant hormones may affect the cells that produce them or they may be transported to other
target cells where they have an effect

Plant hormones are not produced by endocrine glands - unlike animal hormones

Plant hormones bind to specific receptor molecules on the plasma membranes of target cells.
These receptor molecules have a complementary shape to that of the plant hormone - this is
the same mechanism as animal hormones

After binding to receptors on target cells, plant hormones cause a response - a sequence of
events to occur within the cells, just like animal hormones

Plant hormones may be transported by mass flow in xylem vessels or phloem sieve tubes

They may move into target cells by diffusion or active transport

Two or more hormones may have the same effects on plant growth and together have an
enhanced effect. This is called synergy

Two hormones may have opposite effects on plant growth. This is called antagonism
Types of Plant Growth Substances
There are five hormone types that control physiological processes in plants:

Auxins - the parent molecule is indole acetic acid (IAA)

Cytokinins

Gibberellins
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Abscisic acid

Ethene – the only gaseous hormone
The Role of Auxin in the Phototropic Response

Shoots are positively phototropic, meaning that they grow (by bending) towards a light
source

If the light source is unidirectional (shining from one side), cell elongation in the shoot, occurs
at a faster rate on the shaded side of the shoot and this produces the bending growth towards
the light source

Auxins cause cell elongation of cells in the zone of elongation, just behind the shoot apex as
seen in the diagram below
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

Coleoptiles like the one shown above, were used by various scientists to study the effect of
light on shoot growth in germinating seeds
Auxins are produced in cells at the shoot apex (this is where mitosis occurs)

When a shoot is subjected to all round lighting, the auxins move down from the apex to the
cells in the zone of cell elongation. The auxins are probably, actively transported from cell
to cell

When the shoot is subjected to one sided lighting, the auxins are transported away from the
light source and accumulate in the cells on the shaded side
Auxin Effect on Cell Elongation of Shoots subjected to All Round Lighting
Auxin Effect on Cell Elongation of Shoots subjected to Unidirectional Light
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
Auxins cause the cell walls of cells to be less rigid – they increase the stretchiness of the cell
walls. This allows the newly produced cells (without vacuoles) to take in more water and
produce a large, permanent central vacuole containing cell sap. When the cell has done this, it
is elongated

Auxins bind to receptors in the plasma membranes of plant cells. Auxins then stimulate the
active transport of H+ from the cytoplasm into the cell wall. This decreases the cell wall pH
which activates expansin enzymes in the cell wall. Expansins hydrolyse glycosidic bonds
in the cellulose molecules and the H+ disrupts some of the hydrogen bonds between the
cellulose molecules. These effects lead to the cell walls becoming less rigid
Senescence (ageing) and Abscission (shedding) of Deciduous Leaves

In temperate climates (as in the UK), deciduous plants lose their leaves in the Autumn, to
reduce transpiration in the Winter, when water may freeze in the soil

Leaf senescence is the ageing of leaves – they change colour as the chlorophyll breaks down

Cytokinins are plant hormones that prevent leaf senescence by maintaining a nutrient supply in
the leaves. When the cytokinin levels decrease, the leaves have less nutrients and they start to
die

Leaf senescence is followed by leaf abscission. This is when the leaves are shed from the
plant

At least three different plant hormones control leaf abscission – auxin, ethene and possibly
abscisic acid

Usually auxin (produced at the leaf tip) inhibits leaf abscission

Leaf senescence causes a decrease in the production of auxin at the leaf tip
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
A decrease in auxin concentration causes an increase in ethene production

The change in the proportion of auxin to ethene – less auxin and more ethene, results in the
development of an abscission layer at the base of the leaf stalk

The abscission layer is made of thin walled cells. Ethene stimulates the synthesis of
cellulase enzymes in these cells that hydrolyse the cellulose molecules in the cell walls.
Eventually, this layer of cells is so weak that the petiole breaks and the leaf falls. Ethene
therefore, causes the shedding of leaves

Before the leaf falls, the tree grows a layer of protective tissue with suberin in the cell walls,
below the abscission layer. This protective layer forms a scar to prevent the entry of pathogens

Although abscisic acid was named because of its involvement in leaf abscission, its role is now
less clear. It may control the production or activity of ethene
Apical Dominance – the Experimental Evidence for the Role of Auxins

Definition - Apical dominance is when the apical bud, growing at the tip of a shoot inhibits
growth of lateral buds further down the shoot

The specification does not just require a summary of the plant growth substances involved in
apical dominance. It requires an understanding of the interpretation of the experimental
findings that led to our current understanding of the process

When the tip of a shoot (the apical meristem) is cut, the lateral buds will develop and side
branches will grow. The plant becomes bushy
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
Since auxin is produced at the tip of the shoot in the apical meristem, it was assumed that
removing the auxin supply, allowed the side branches to grow and that apical dominance was
the effect of auxin inhibiting the growth of side branches

To test this assumption, auxin paste was applied to the cut end left after cutting off the shoot tip,
and again, the side branches did not grow

However, the information observed so far, does not exclude the involvement of other factors eg
the cut end is exposed to oxygen. Perhaps this exposure stimulates the production of another
hormone that causes side branches to grow

Thimann and Skoog then applied a ring of auxin transport inhibitor below the intact shoot apex.
The side branches grew suggesting even more strongly, that normal auxin concentrations inhibit
side branch growth

Several years later, Gocal disproved a direct causative link between auxin concentration and
and lateral bud growth inhibition. He found that removal of the apical meristem actually caused
an increase in auxin concentration in the lateral buds

It is now thought that two other plant hormones are also involved

Abscisic acid inhibits the growth of lateral buds. High concentrations of auxin produced in the
intact apical meristem may maintain abscisic acid concentrations at a high level to inhibit lateral
bud growth. When the shoot tip is cut, the concentration of auxin decreases causing the
abscisic acid levels to fall. The lateral buds then grow

Cytokinins promote lateral bud growth. High concentrations of auxin at the shoot tip make the
tip a sink for cytokinins produced in the roots. When the shoot tips are intact, most of the
cytokinin moves to the tips. When the tip is removed, the cytokinin is distributed more evenly
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around the plant and its presence in the lateral buds stimulates their growth
Gibberellins and Stem Elongation

In Japan, rice plants grow very tall when infected by a fungus. The fungal compounds
responsible were isolated and identified as gibberellins. One of these was gibberellic acid GA 3

Gibberellic acid was then tested on dwarf varieties of different plants such as peas and maize.
These plants grew taller when gibberellic acid was spread over the stems

These experiments did not prove that in nature, gibberellic acid causes stem growth. It is
important to test the low concentrations of gibberellins found naturally in plants and to test the
effect on plant cells that gibberellins would normally reach

Researchers analysed the levels of GA1 (another gibberellin) in tall pea plants (homozygous
for the dominant allele Le Le) and dwarf pea plants (homozygous for the recessive allele le le).
The taller plants had higher GA1 concentrations in their stem cells

To show that GA1 causes stem growth directly, researches looked at the metabolic pathway to
produce GA1 and used plants with mutations
The complex metabolic pathway for gibberellin synthesis is shown below:
Ent kaurene
Site of mutation - plants
only grow 1cm tall
GA12 aldehyde
GA12
GA53
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GA44
GA19
GA20
Enzyme produced by
the Le allele acts here
GA1

A shoot of a mutant plant that could not produce any gibberellins (see site of mutation above)
was grafted onto a dwarf pea plant, homozygous recessive for (le le)

The dwarf pea plant could not produce GA1 because of its inability to produce the enzyme to
convert GA20 to GA1

However, the dwarf pea plant did produce GA20 and the mutant grafted shoot was able to
convert GA20 to GA1 (it produced the enzyme required since it had at least one dominant Le
allele). The shoot grew tall confirming that GA1 caused stem elongation

Further work has shown that gibberellins cause stem growth by causing loosening of the cell
walls and cell elongation. They also promote cell division.Commercial Use of Plant
Hormones
Auxins
Commercial Use
Taking plant cuttings
to produce new plants
Production of seedless
fruit eg grapes
Selective weedkillers
(herbicides)
Explanation
Artificial vegetative propagation. Auxins are an ingredient in hormone
rooting powder, bought from garden centres. The ends of stem cuttings
are dipped into the rooting powder and then planted in compost. The
auxins stimulate root growth at the end of the cutting
Unpollinated (and therefore unfertilised) flowers are sprayed with auxins.
The auxins stimulate fruit development. Since the ovary is not fertilised,
no seeds will be present in the fruit
Artificial auxins are sprayed onto broadleaved weeds eg dandelion. The
auxins are transported in phloem and are not broken down by the
enzymes that usually break down natural auxins (the shape of the
artificial auxin does not fit the enzyme active sites). These auxins
promote excessive rapid shoot growth. The stem cannot support itself
and the plant collapses and dies.
Narrow leaved plants such as grasses and cereals are not killed by
these weedkillers. These weedkillers are used to kill weeds in lawns
and in fields of cereal crops
Gibberellins
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Commercial Use
Delays senescence in citrus
fruit. Fruits can be left unpicked
to extend the availability time in
shops
Causes elongation of grape
stalks, giving more room for the
grapes to grow
Brewing Industry – gibberellins
stimulate the germination of
barley seeds to produce malted
barley
Sugar production from sugar
cane – gibberellins increase the
length of the sugar cane
internodes
Promoting seed production in
some plants – promote sexual
maturity in conifers
Production of short stocky
plants
Explanation
Citrus fruits are sprayed with gibberellins
Developing fruits are sprayed with gibberellins
When barley seeds germinate they take up water. Gibberellins
stimulate the synthesis of amylase in the aleurone layer of the
seed. The amylase hydrolyses the starch in the seed
endosperm to produce respiratory substrates for ATP synthesis
in the seed during its growth. The germination process of the
seeds is allowed to continue for a few days to produce maltose
and glucose respiratory substrates. The product is then dried
and ground up to form malted barley
Beer is made when yeast cells use the sugars in the malted
barley as substrates for their respiration
Spraying the internodes of sugar cane stems with gibberellins
increases the number of cells resulting in stem elongation.
Since the sucrose sugar is contained in these stem cells, the
more cells, the higher the sugar yield
Spraying young conifer plants with gibberellins can promote
seed formation (conifer plants take a long time to become
reproductively active).
Biennials plants (that produce seeds in the second year of life)
can be induced to produce seeds in the first year by spraying
with gibberellins
Spraying plants with gibberellin synthesis inhibitors can inhibit
the growth promoted by gibberellins. This process is used to
produce short poinsettias for the Christmas market
Cytokinins
Commercial uses
Used in tissue culture to mass
produce genetically identical plants
Sprayed onto lettuce leaves after
picking to prevent yellowing
Explanation
Cytokinins promote bud and shoot growth from small
pieces of tissue taken from the parent plant. This
produces a short shoot with lots of side branches. One
plant can be divided into several small plants and each is
grown separately
Cytokinins delay leaf senescence which involves the
yellowing of the leaves
Ethene
Ethene is a gas and cannot be sprayed. Scientists have developed 2-chloroethylphosphonic acid
which can be sprayed in solution onto plants. This compound is easily absorbed and releases ethene
inside the plant.
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Commercial uses
Stimulates ripening of apples, tomatoes
and citrus fruits
Promotes fruit drop in cherry and
walnut trees
Promotes female sex expression in
cucumber flowers
Explanation
Ethene naturally causes ripening of fruits. Commercially the
fruits indicated will be sprayed with the alkene
This process may be carried out either to reduce the
number of fruits so that those left will grow bigger or to
harvest the fruits all at the same time
If cucumber flowers are pollinated, the fruits tend to be
bitter. Spraying the flowers prevents bitter fruit
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