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Plant hormones

Some people think that plants
can’t move. Are they correct? Well
a plant cannot move its entire
self to a different location. Some
plants, such as tumbleweeds
often seen blowing across roads
in Wild West American films, can
be moved by other things like the
wind, but they cannot move all
by themselves. Plants are able to
move parts of themselves though.
Roots grow downwards seeking
water, while shoots spiral upwards
looking for and growing directly
towards light. The Venus fly trap
closes its traps to catch insects.
The Mimosa plant folds its leaves
to stop it being eaten. Much
of your growth is controlled by
hormones, and these are essential
for growth in plants too.
Specification coverage
Plant hormones
This chapter covers specification points and and is called Plant
It covers control and coordination and
the use of plant hormones.
Prior knowledge
13 Plant hormones
Previously you could have learnt:
› that plants make carbohydrates in their leaves by
photosynthesis and gain mineral nutrients and water from
the soil via their roots.
Test yourself on prior knowledge
1 Name the reactants in photosynthesis.
2 Describe how plants maximise the volume of carbon dioxide they
3 Explain why plants grow towards the light.
Control and coordination
Positive phototropism The ability of
plant stems to grow towards the light.
Positive gravitropism The ability of
plant roots to grow downwards.
All plants want to grow towards the light. Most do this quite slowly so it
doesn’t seem as if it is much of a race to us, but plants do race upwards
towards the light. More light means more photosynthesis, which means more
glucose produced. This in turn means more growth and reproduction. Plant
growth of stems towards light is called positive phototropism. Shortly
before his death, Charles Darwin (1809–1882), a scientist better known for
his theory of evolution by natural section, and his son were involved in the
initial discovery that grass seedlings grow towards the light.
Plants also need to grow towards water, which is usually downwards, for
photosynthesis. Their roots therefore must grow in the opposite direction
into the ground. This is called positive gravitropism (or geotropism).
They are responding to gravity and growing into the ground.
● Auxins
▲ Figure 13.1 This plant has been tilted
on its side. Its stem has begun to grow
upwards toward the light. This shows
positive phototropism.
Auxin A type of plant hormone
responsible for cell elongation.
Cell elongation The lengthening of
specific cells by plants as a result of
Hydrotropism A plant’s ability to
grow roots towards water.
Auxins are a group of plant hormones that affect plant growth. They
are involved in phototropism and gravitropism. The presence of higher
concentrations of auxins makes individual cells grow fast and become
longer. We call this cell elongation. If the cells on one side of a plant
shoot have more auxins they will elongate more. This will cause the
stem to curve. In order to make a stem curve towards the light the
auxins concentrate on the other side of the stem. These are the cells
that need to grow longer to bend the stem towards the light. We say
that the auxins concentrate on the dark side of the stem.
Gravitropism works in the same way. Here roots need to grow
downwards towards water. If a root is growing horizontally, the cells on
the top side will need to grow longer to curve the tip downwards.
Auxins are mainly produced in the tips of shoots and roots but do
diffuse through the plant to other parts where needed.
Plant roots are also able to sense and grow towards water. This response
is called hydrotropism and usually overpowers gravitropism. If you
were to grow cress seeds in a Petri dish with droplets of water that have
condensed on the surface of the lid, the roots may grow upwards towards
the water and not downwards as a result of gravity.
Required practical 8
Control and coordination
Investigate the effect of light or gravity on the growth of germinating seeds
In this experiment you will investigate the effect of light on the growth of
germinating seeds.
1 Place some cotton wool in the lids of three Petri
dishes and add water to moisten it. Sprinkle five to six
radish seeds (or other small, fast-growing seeds) over
2 Over one place a yoghurt pot painted black (or other
container that can exclude light).
3 Over the second place a yoghurt pot painted black
with a hole cut out so that light can only come in from
the top. Place this under a light bank, bench light or
on a windowsill.
light source
shaded side
shaded side
light source
mica barrier inserted
between tip and stem
gelatine barrier inserted
between tip and stem
▲ Figure 13.2 A number of experiments have proved that auxins
are responsible for phototropism.
You do not need to know specifically
how gibberellins and ethene work, but
you do need to know their effects.
Impermeable Through which a
substance cannot pass.
Gibberellins Plant hormones
responsible for cell elongation, seed
dormancy and germination.
Ethene A plant hormone that ripens
4 Over the third place a yoghurt pot painted black with
a hole cut in the side so light can only reach it from
the side. Place this so a bench light is shining on it
from the side or on a windowsill.
5 After 3 to 4 days remove the yoghurt pots and record
what has happened to the seedlings.
1 In which direction do the shoots grow in each pot?
Draw a sketch and describe your observations.
2 Explain why the growth was different in each pot.
3 Why is it important that plant shoots can respond to
● Experiments involving
Figure 13.2 shows the results from a number of
famous experiments involving auxins. In the first
diagram you can see the shoot bending towards the
light. The second experiment shows a shoot that has
had its top cut off. This has not bent towards the
light, which shows that the auxins are produced in
the tip. The third experiment shows a shoot with a
cover on the tip. This has not bent towards the light,
which shows that the light-sensitive cells are in the
tip. The fourth experiment shows a shoot that has
had an impermeable mica barrier inserted to stop
hormones diffusing between cells. The shoot with the
insert on the dark side did not bend whilst the one
with the insert on the light side did. This shows that
hormones diffuse down the dark side of the shoot.
The fifth experiment shows a similar experiment to
the fourth except the barrier was permeable gelatine
not mica. The shoot did bend towards the light,
showing that auxins can diffuse through gelatine.
● Other hormones
Plants have other hormones called gibberellins, which are involved in
plant growth. Some help with stem elongation while others are involved
in the dormant period before a seed germinates, and the germination
process itself. Others help form flowers and fruits.
Ethene is a third plant hormone. Its main role is the ripening of fruit,
but it does control cell division and help flowers open and is involved
in the dropping of leaves. It is present in high concentrations in rotting
fruit. If fresh fruit is kept near rotting fruit, the ethene will diffuse and
cause the other fruit to rapidly rot, producing more ethene. Perhaps
this is the cause of the saying “one bad apple spoils the bunch”.
13 Plant hormones
Auxin and plant growth
Barley seedlings were grown, being lit from directly
above for 5 days, so that the seedlings produced
showed straight vertical growth. A toothpick was
placed next to the plant to show the direction of
starting growth.
angle of growth (°)
Auxin (IAA) of different concentrations was mixed
with lanolin paste (a natural grease) and applied to
the left-hand side of a seedling between 5 mm and
10 mm from the tip of the shoot.
The seedlings were then placed in the dark for
a further 5 days. When they were removed the
angle of growth away from the toothpick could be
measured. The results of the experiment are shown
in Table 13.1.
1 Plot the results as a line graph.
▲ Figure 13.3 How to measure the angle of growth.
2 Use your graph to predict the angle of growth if
6 mg/dm3 of IAA was used.
Table 13.1
3 Why was it important that the seedlings were kept
in the dark following the treatment with lanolin?
Concentration of IAA in
Angle of growth in
4 Explain why the growth angle increased as the
concentration of IAA increased.
Test yourself
1 Name a plant hormone other than auxin.
2 Name the process by which plant roots grow downwards.
3 Describe an experiment in which you prove that auxins are made in the
tip of shoots.
4 Describe why ripe bananas are often not kept with other fruit.
Show you can...
▲ Figure 13.4 Ripening bananas
produce lots of ethene. Some people
keep bananas away from other fruit to
prevent the other fruit from ripening too
Explain how plants grow towards the light. Draw a diagram as part of
your answer.
Use of plant hormones
Agriculture is the growth of animals and crops for food, fuels or
medicines. Horticulture is the growth of plants. It may include crops
or vegetables but also includes plants that are grown for the way they
look or smell. Plant hormones are widely used in both agriculture and
Use of plant hormones
Selective weedkillers
You should be able to understand
how the use of hormones as
weedkillers has an effect on
Auxins are used in many selective weedkillers. These will kill
broadleaved plants such as dandelions and daisies but not the narrower
leaved grass plants. This is because broadleaved plants have a greater
surface area of leaf to absorb the weedkiller. This means we can spray it
on lawns and it will only kill undesirable weeds. These hormones cause
some plants cells to grow uncontrollably and others to stop growing.
This kills the plant.
Rooting powder
Taking plant cuttings is a way of propagating (or increasing the number
of) plants that a gardener has. This is shown in Figure 13.6. The cut
end of the stem is then dipped in a rooting powder and placed into the
soil. Rooting powder contains auxins, which help the cutting form roots.
Taking cuttings is an artificial asexual form of plant reproduction. The
offspring have the same genetic information (DNA) as their one parent.
They are clones.
▲ Figure 13.5 A selective weedkiller
has been applied and will kill the
dandelion but not the grass.
1 Part of the stem
is cut from the
2 The leaves are
removed from
the bottom of
the cutting.
Tissue culture
Auxins are also used in tissue culture. This is another example of
artificial asexual reproduction in which small numbers of cells are
removed from the parent plant. These are placed into a growth medium
and grow into cloned copies.
Fruit ripening
We have seen that ethene is produced by plants to ripen their fruits.
Because it takes so long to transport fruits by ship from the tropics
to markets in Europe, they are often picked before they are fully ripe.
This stops them rotting on their long boat journey. (It might prove too
expensive to transport them by air.) They are sprayed with ethene a
short time before arriving so that the fruit is beginning to ripen as it
placed on the shelves.
3 The cut end is
dipped in a
rooting powder.
You learnt above that gibberellins are used by plants for stem
elongation, seed dormancy and germination. They are often used
in agriculture and horticulture for beginning germination as well as
helping plants flower and to increase the size of their fruit.
Test yourself
4 The cutting is
planted into
▲ Figure 13.6 How to take a shoot
5 Name three plant hormones.
6 Define the term ‘propagation’.
7 Describe how rooting powder is used.
Show you can...
Explain how plant hormones are used.
13 Plant hormones
Chapter review questions
1 Define the term ‘positive phototropism’.
2 Define the term ‘positive gravitropism’.
3 Describe the effect that auxins have upon cells.
4 Name the process by which auxins move in a plant.
5 Name the process by which roots grow towards water.
6 Give the three main uses of plant hormones.
7 Describe an experiment in which you investigate the effect of light on newly
germinated shoots.
8 Explain in detail why plant roots usually grow downwards.
9 Describe the role of auxins in phototropism.
10 Define the term ‘cell elongation’.
11 Describe what would happen to a shoot if you cut its tip off. Explain why.
12 Describe what would happen to a shoot if you cut its tip off, placed a layer
of impermeable mica on it and then put the tip back on. Explain why.
13 Describe what would happen to a shoot if you cut its tip off, placed a layer
of permeable gelatine on it and then put the tip back on. Explain why.
14 Describe the effects of the plant hormone ethene.
15 Describe the effects of the gibberellin plant hormones.
16 Explain how hormones are used in selective weedkillers.
17 Explain how hormones are used in rooting powder.
18 Explain how hormones are used in fruit ripening.
19 Describe an experiment in which you investigate the effects of auxins on the
growth of shoots.
20 Which famous scientist and his son first discovered phototropism?
21 Explain why auxins diffuse from the tip of a plant’s shoot.
22 Describe what would happen to a shoot if you cut halfway through it on its
dark side and placed a layer of impermeable mica in the gap. Explain why.
23 Describe what would happen to a shoot if you cut halfway through it on its
light side and placed a layer of impermeable mica in the gap. Explain why.
24 Suggest at what point fruit would produce the most ethene.
25 Explain the difference between agriculture and horticulture.
Practice questions
Practice questions
1 Joe wanted to propagate a geranium plant.
He did this by taking cuttings and using rooting powder to
make new roots grow. He decided to compare two different
brands of rooting powder, Rapid-Root and Ready Root,
along with water, to see which was more effective.
The results are shown in Table 13.2 below.
Table 13.2
Number of roots
Ready Root
a) i) Calculate the mean number of roots produced
when Joe used Rapid-Root.
[1 mark]
ii) What conclusion could Joe draw from the
[2 marks]
b) Rapid-Root claims to produce 75% more roots
than other rooting powders. Calculate whether
this claim is true.
[3 marks]
c) i) Name the plant hormone found in rooting
[1 mark]
ii) Suggest another plant hormone that can be
used in agriculture and explain its use. [2 marks]
d) Make a prediction about the appearance of the
plants that will grow from the cuttings.
[1 mark]
▲ Figure 13.7
a) Sketch a diagram to show what each plant would
look like after a week.
[3 marks]
b) The differences in growth of the seedlings was caused
by a hormone, this hormone diffuses into the plant
tissue. In which of the following ways would it
[1 mark]
A Shaded side of the
B Lit side of the
C Top of the seedling
D Bottom of the
c) i) Which of the following correctly defines a tropism?
[1 mark]
A A plant hormone
B A ripening agent
C A plant defence
D A growth movement
ii) Plants respond to light by phototropism. Explain as
fully as you can how this occurs and why it benefits
the plant.
[5 marks]
iii)Name another type of plant tropism.
[1 mark]
2 Charles Darwin and his son Francis famously discovered
that growing seedlings respond to light. Two students
repeated their experiment by observing the growth of five
grass seedlings by doing the following:
• Seedling A: normal
• Seedling B: tip covered in tin foil
• Seedling C: stem covered in tinfoil
• Seedling D: tip covered in clingfilm
• Seedling E: tip cut off
The seedlings were lit from one side by a lamp, and were
left to grow for a week.