File - Leaving Cert Biology

Chapter 26: Response in the
flowering plant
Leaving Certificate Biology
Higher Level
Growth Regulation
• Growth of plants is controlled by growth
• 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 is the growth of a plant in
response to light
• Example is when plants bend towards a window
• 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 is the growth of a plant in
response to water
• Example: roots grow towards water
• Chemotropism is the growth of a plant in
response to chemicals
• Example: roots grow towards minerals and away
from heavy metals
• 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
– causes stomata to close in very dry conditions
despite presence of light)
– Inhibits germination until ideal conditions are
Anatomical & Chemical Protection
• Anatomical:
– Bark/dermal tissue/cuticle: functions in protecting
– 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
Leaving Certificate Biology
Higher Level
Graduated cylinder
Weighing scales
Petri dishes
Deionised water
Cotton wool
Filter paper
Masking tape
IAA( Indole Acetic Acid)
Cress seeds/Mustard
seeds/Radish seeds
• 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
• 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
• 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
• After 2 – 3 days remove petri dishes and
• Using ruler measure length of root and shoot
for each concentration of IAA and record in
• Calculate % increase or % decrease for each
• IAA stimulates growth of roots and shoots up
to a certain concentration
• High concentrations inhibit growth
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