Transcript student version module 5 5.1.5 plant responses

Module 5 5.1.5 Plant Responses
By Ms Cullen
Responding to stimuli
Plants increase their chances of
survival if they respond to……
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Responses to herbivory
Chemical defences – producing toxic chemicals
when they are eaten
eg tannins – bitter chemicals, in cattle & sheep
they bind to proteins in the gut making them
hard to digest. Can be found in oak
alkaloids – bitter tastes, poisonous
characteristics or noxious smells that deter or
kill herbivores. Tobacco plants produce nicotine
when their tissue is damaged.
Responses to herbivory
Releasing pheromones– signalling chemicals
that produce a response in other organisms
eg corn plants eaten by caterpillars can produce
pheromones which attract parasitic wasps.
Folding when touched – the plant Mimosa
pudica will fold all it’s leaves if it is touched. This
can protect the plant by scaring off herbivores
or knock small insects off it’s leaves
Responses to abiotic stress
• Abiotic stress is anything non-living which is
harmful to the plant.
eg drought - the quiver tree in Africa self
amputates in extreme drought.
Carrots produce antifreeze proteins in cases of
low temperatures, these bind to ice crystals and
prevent more forming.
Tropisms – directional growth
responses
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Phototropism
Geotropism
Chemotropism – pollen tubes
Thermotropism - temperature
Thigmotropism – climbing plants
Nastic Movements
• Nastic movements are plant movements that
occur in response to environmental stimuli
but unlike tropic movements, the direction of
the response is not dependent on the
direction of the stimulus.
• Some of the most spectacular plant
movements are nastic movements. These
include the closing of the carnivorous Venus
Flytrap leaf when it captures prey or the
folding of the mimosa leaf when it is
disturbed.
How is plant growth different to
animal growth?
• Plant growth is continuous throughout the
plant’s life.
• Cell walls around the plant cells inhibit cell
division, so it only occurs in particular areas of
a plant.
• These areas are called meristems, these are
groups of immature cells that are capable of
dividing and specialising.
Meristems in a plant
Longitudinal section of a plant shoot
What controls plant responses?
Hormones! (sometimes called plant growth regulators)
• Chemical messengers which are transported from
area they are produced to a target cell, tissue or
organ.
• In plants there are some hormones which stay in the
cell they are produced and have an effect there.
• Unlike animal hormones they are not produced by
endocrine glands.
Q: Explain how hormones are specific.
How are hormones transported
around the plant?
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• Some hormones can increase the effects of other
hormones and amplify their effect (synergism)
• Other hormones can oppose the effects of others
(antagonism)
Plant Growth Hormones
Hormone
Where are they produced? Effects
Auxins
( Indoleacetic acid - IAA)
In shoot apex and young
leaves.
Promote cell elongation,
inhibit growth of sideshoots and leaf fall.
Cytokinins
Wherever mitosis is
occuring.
Promote cell division
Gibberellins
In chloroplasts of young
leaves, buds, seeds & root
tips.
Promote seed germination
and growth of seeds.
Abscisic acid
In chloroplasts
Inhibits seed germination
& growth; causes stomatal
closure when the plant is
stressed by low water
availability.
Ethene
Produced by all plant
organs
Promotes food ripening.
Plant hormones and seed germination
• When a seed absorbs water it will begin to
produce giberellins.
• Giberellins switch on the genes that produce
amylase and proteases which begin to break
down the cotyledons or endosperm (food
stores) of the seed.
• The embryo produces ATP so it can begin to
grow and break through the seed coat.
• There is also evidence that another plant
hormone abscisic acid (ABA) acts as an
antagonist to giberellin.
Mechanism of the phototropic response in shoots
Senescing
• Cytokinins usually prevent this by making sure
the leaf is a sink for translocation, ensuring
the leaf has a good supply of nutrients.
• If cytokinin production drops, the supply of
nutrients drops and senescence begins.
• Senescence is followed by abscission when
leaves are shed.
The leaf petiole showing the abscission zone
Commercial uses of plant hormones Auxins
• Sprayed on developing fruits to prevent them
dropping too early.
• Sprayed on flowers to start formation of fruit
even if flower has not been fertilised. This is
called parthenocarpy. Why is this useful?
• Used in rooting powders to stimulate root
growth in cuttings.
• Broad leaved plant weedkillers.
Commercial uses of plant
hormones - Gibberellins
• Sprayed on plants to promote parthenocarpy eg
seedless grapes.
• Can make apples elongate to improve their
appearance.
• Used in brewing industry, stimulates barley to
produce α-amylase which converts starch into
maltose sugar. ‘malting’.
• Can promote seed production in the first year of
biennial plants (usually only flower in second year of
life.)
Commercial uses of plant
hormones - Cytokinins
• Used in tissue-culture to help mass produce
plants as they promote bud and shoot growth.
• To prevent leaves discolouring especially in
lettuce and cabbage crops once they have
been picked.
• Can be sprayed on fresh flowers, or added to
their water, so that they last longer. Prevents
abscission.
Commercial uses of plant
hormones - Ethene
• Speeds up ripening of fruit.
• Can prevent fruits ripening by keeping in cold,
oxygen-deficient storage and then use ethene
to ripen fruit when ready.
• Promotes lateral growth in some plants.
• Promotes female sex expression in cucumbers.
This prevents self-pollination which can cause
bitter tasting cucumbers and will also increase
yield.
Commercial uses of plant
hormones – Abscisic Acid
• Can be sprayed on fruit trees to regulate the
fruit drop.
• Why is this useful?