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Chapter 26: Response in the
flowering plant
Leaving Certificate Biology
Higher Level
Growth Regulation
• Growth of plants is controlled by growth
regulators
• Growth regulator levels in the plant are
controlled by tropisms
• A tropism is the response of a plant to a
change in its external environment or to a
specific stimulus
Phototropism
• Phototropism is the growth of a plant in
response to light
• Example is when plants bend towards a window
Geotropism
• Geotropism is the growth of a plant in response
to gravity
• Example: as soon as young root emerges from
seed it grows towards gravity regardless of what
orientation the seed was in
Hydrotropism
• Hydrotropism is the growth of a plant in
response to water
• Example: roots grow towards water
Chemotropism
• Chemotropism is the growth of a plant in
response to chemicals
• Example: roots grow towards minerals and away
from heavy metals
Thigmotropism
• Thigmotropism is the growth of a plant in
response to touch
• Example: ivy growing up a wall or around a tree
Growth Regulators
• A growth regulator is any chemical that
controls the growth of a plant
– Growth regulators are produced in extremely
small amounts usually in the meristematic areas
of a plant
– Growth regulators are thought to be transported
by vascular tissues to other areas of the plant but
can also diffuse through ground tissue
Growth Regulators
• The effects of growth regulators depend on
various factors:
– The area where it is located
– The concentration (high concentrations can have
opposite effects to very low concentrations)
– Growth regulators can cause growth or inhibit
growth depending on interactions with other
growth regulators
Examples of Growth Regulators
– Growth promoters:
• Auxins (indole acetic acid [IAA] – cause stem
and root growth
– Growth inhibitors:
• Ethene (ethylene) – causes fruit to ripen
• Abscisic acid – helps plants deal with harmful
conditions:
– causes stomata to close in very dry conditions
despite presence of light)
– Inhibits germination until ideal conditions are
present
Phototropism
Anatomical & Chemical Protection
• Anatomical:
– Bark/dermal tissue/cuticle: functions in protecting
plant
– Stomata and guard cells: control water loss
• Chemical:
– Heat shock proteins: produced during times of
stress to protect the enzymes within plant cells
– Phytoalexins: produced when plant is infected
with microorganisms and help kill microorganisms,
prevent further spread of the invader, and warn
nearby plant cells
Mandatory Experiment: to
Investigate Effect of IAA on Plant
Tissue
Leaving Certificate Biology
Higher Level
Apparatus:
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Graduated cylinder
Weighing scales
Beakers
Stirrers
Spatula
Petri dishes
Pipettes
Deionised water
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Cotton wool
Filter paper
Masking tape
Ruler
IAA( Indole Acetic Acid)
Ethanol
Cress seeds/Mustard
seeds/Radish seeds
Method:
• Dissolve 100 mg IAA in 2 ml ethanol
• Transfer to 1 L graduated cylinder and top up to 1 L
with deionised water to give 100 mg/L IAA stock
solution
• Transfer 10 ml IAA stock to first petri dish labelled A
• Transfer 1 ml of this solution in A to B and top up with
9 ml deionised water
• Mix B and transfer 1 ml of B to C
• Repeat procedure until dish G
• Dish H is control with 9 ml deionised water
Method (continued)
• Place 5 cress seeds in line in the lid of each
petri dish
• Cover seeds carefully with filter paper
• Transfer the serially-diluted solutions to each
lid
• Place layer of cotton wool into each lid and
place base on
• Seal each dish with masking tape and stand on
sides in incubator set at 25 ˚C
Results
• After 2 – 3 days remove petri dishes and
seedlings
• Using ruler measure length of root and shoot
for each concentration of IAA and record in
table
• Calculate % increase or % decrease for each
concentration
Conclusion
• IAA stimulates growth of roots and shoots up
to a certain concentration
• High concentrations inhibit growth
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