3.5.2 Responses in the Flowering Plant
Objectives – What you will need to
know from this section
 Explain the term: growth regulation.
 Define the following terms: tropisms, phototropism,
geotropism, thigmotropism, hydrotropism,
chemotropism.
 Name examples of phototropism & geotropism.
 Define the term: growth regulator.
 Outline the transport of regulators via the vascular
system.
 Explain the term: Combined Effect
 Explain the term: Growth Promoter.
 Explain the term: Growth Inhibitor
 Name 4 methods of anatomical/chemical adaptations
that protect plants.
 State 2 examples of the use of plant regulators.
 Mandatory Activity :
Effects of I.A.A. Growth Regulators on Plants
3.5.2 Responses in the Flowering Plant
The ability to detect change and to respond is called
sensitivity. Response is a form of defence that allows
organisms to survive.
Plant adapt to new situations by modifying their growth,
by means of chemicals called growth regulators
[hormones].
 Growth is the increase in the number, size and volume
of cells.
 A hormone is a chemical produced in one part of an
organism, transported to other tissues where it has
its effect.
 External factors that regulate the growth of plants are light
intensity, day length, gravity, temperature.
 Development means the
changes that lead to specialised
tissues and organs.
 Plant growth regulators [hormones] are chemicals that
interact with one another to control a particular
development or response.
 Plants only grow at their tips,
in small regions of active cell
division called meristems—
the tips of shoots, roots and
side buds.
 Plant growth regulators are
produced in the meristems
and transported through
the vascular system of the
plant.
Plant Growth Regulators
• They are active in very small amounts
• They are produced in the meristems
• They are transported in the xylem and
phloem
• Their effects are dependent on
concentration
• A small amount of growth regulator can
have great effect on growth
LEARNING CHECK
• What is the difference between growth and
development?
• What is a growth regulator?
• List the properties of plant growth
regulators
• What is a meristem?
 A tropism is a plant’s response to a stimulus coming from
one direction, e.g. sunlight, gravity.
 Phototropism is a
growth response of
a stem towards
light, so that it can
receive the
maximum amount
of light for
photosynthesis.
 The coleoptile is the sheath
around the leaves and shoot
of grass seedlings, such as
corn or oats.
 Geotropism is the
growth of a plant in
response to gravity—it
allows the root to get the
water and nutrients it
needs from
the soil, and raises the
leaves up into the light
for photosynthesis.
 Thigmotropism is the
growth of a plant in
response to contact,
e.g. vines wrap around
objects which help support
the plant.
 Hydrotropism is the
response of roots to water
– they will grow towards
it.
 Chemotropism is the
response to chemicals,
e.g. pollen tubes
growing down the
carpel in response to
chemicals released
by the ovule.
LEARNING CHECK
•
•
•
•
What is meant by the term “tropism”.
What is a coleoptile?
What is a stimulus.
Name 5 different tropisms and the stimulus
involved in each case.
• List the advantage to the plant of each
tropism.
Plant growth regulators
 Plant growth regulators interact with one another to
control a particular development or response.
 Some regulators
promote growth e.g.
auxins, gibberellins,
cytokinins.
 Some regulators inhibit growth
e.g. abscisic acid and ethene.
Growth shut down for
winter
New roots develop
Auxins
 promote cell enlargement and growth,
 are involved in phototropism and apical dominance
PHOTOTROPISM
APICAL
DOMINANCE
In shoots, light causes auxin to move down the shaded part of
the stem, causing the shoot to grow towards the sunlight.
This is called PHOTOTROPISM.
Quicker growth
here due to more
hormones
Artificial auxins can also be used to kill weeds,
stimulate root formation in cuttings, …
Gibberellins
 cause stem lengthening,
 Mobilise the stored food in
germinating seeds
 Break dormancy in buds
and seeds in Spring.
Cytokinin
 Cytokinins stimulate cell division (mitosis)
and trigger leaf growth in spring.
Abscisin
 Abscisic acid triggers bud and seed
dormancy in autumn and inhibits
cell growth.
Ethene
 Ethene promotes ripening of fruit and the
fall of leaves, flowers and fruits.
Uses of plant regulators
Artificial auxins are used in rooting
powders to stimulate root formation in
stem cuttings.
A synthetic auxin [2,4-D] is used as a
selective weedkiller, making the plants
grow too fast.
Fruit is transported green and
unripe, and can then be
quickly ripened by spraying it
with ethene.
LEARNING CHECK
• What is a plant growth regulator?
• Explain the terms “promote” and “inhibit”.
• Name 5 growth regulators and a function of
each.
• Give 3 examples of how we make use of
plants growth regulators [natural or
artificial]
Plant Defences
 The growth and development responses
described so far are a form of defence that
allows a plant to survive difficult conditions
[environmental stress] in its habitat.
 Plants can’t move and so need to defend
themselves against being eaten or damaged.
Some adaptations that plants use to protect themselves
include:
1. Spines, thorns or stinging
hairs to deter animals from
eating them, e.g. cacti,
nettles.
2. Toxins that cause illness or death,
e.g. Cassava is an important food source in tropical
countries, but its leaves and roots produce cyanide
poison to protect it against insects and other herbivores.
The roots must be processed properly if humans are not
to be poisoned.
Cassava plant
Cassava root tuber
3. In autumn, trees recycle all the nutrients they can out of
the leaves for storage in phloem tissue.
The red pigments of autumn serve as a kind of
botanical sunscreen, a defence mechanism against sun
damage that could interfere with the storage process
and cause a leaf to drop before the tree was done with
it.
4. Heat-shock proteins [stress proteins] are created when
cells are exposed to higher temperatures or to other
kinds of environmental stress, such as UV light. Their
activities are part of a cell's repair system and allow
the plant to tolerate extra heat, light, etc. for a limited
period, and resume normal cellular activities when the
stress ends.
Stress proteins act like chaperons and provide
"shelters" in which new protein chains can
be "incubated" until they have folded
properly
Plant protection
Plants need to protect themselves from their
environment
They protect themselves against:
• Loss of water
• Overheating
• Infection from micro organisms
• Being eaten by herbivores
Adaptations for Protection
in Plants
Plants can adapt themselves for protection in
two ways:
1. Structural or anatomical adaptations
2. Chemical adaptations
Structural Adaptations
• Bark/epidermis prevents entry of microbes
and reduces loss of water
• Thick cuticle on stem or leaves in plants that
live in dry places
• Thorns prevent plants from being eaten by
herbivores
• Stinging cell in epidermis prevent plants been
eaten
• Guard cells change shape when they lose water
which causes stomata to close and this reduces
water loss
Chemical Adaptations
• Production of Tannins to make the plant
indigestible
• Production of toxic chemicals to prevent
growth of insect larvae
• Production of strychnine and nicotine in
legumes that damage nerves and muscles
• Production of heat shock proteins to protect
enzymes when temperatures are high
Most important adaptation
Plants can grow and regenerate throughout
their life.
They can produce new roots, stems, leaves.
LEARNING CHECK
• Why is defence important to plants?
• List four methods used by plants to defend
themselves.
• Explain the term “structural adaptation”.
• What is a toxin?
• What advantage is it to a plant that it grows
throughout its life [from meristems]?