VCE Biology Unit 2
Area of Study 01
Adaptations of Organisms
Chapter 14
Plant tropisms and hormonal control
Chapter 14.1 Environmental cues
Plants must survive in the immediate
environment
• They depend on the immediate environment
for materials and energy
• They are relatively tolerant of environmental
changes which they cannot escape
Chapter 14.1 Environmental cues
Plant growth and reproduction are synchronised
with seasonal changes and climate conditions
• Environment provides cues for many stages of
plant growth.
• Flowering, ripening of fruit, seed germination
– All which must occur in the ‘most ideal’ conditions
• Also respond to light and temperature
Chapter 14.1 Environmental cues
Communication between cells in different parts of
plant are required for direction and timing of
growth
• Other plant responses are also co-ordinated
• Plants do not have a nervous system
• Internal conditions are controlled by hormones
Chapter 14.1 Environmental cues
Hormonal systems
• A hormone is a chemical produced by cells in one
part of an organism, transported throughout,
accepted by specific receptor sites and affect
specific cells
• Hormones act as intercellular messengers and
regulate cell functions
Chapter 14.1 Environmental cues
Hormonal systems (continued)
• Target cells possess specific receptors
• Involved in metabolic functions, rates of chemical
reactions, transport of substances across cell membranes,
secretion and cell growth
• Alter specific biochemical reactions
– Production of enzymes
– Change in membrane transport
– Switching on of off of specific genes
Chapter 14.1 Environmental cues
Hormonal systems (continued)
• Exert their affects directly by passing through cell
membrane into cell or
• Indirectly by interacting with a receptor on the
outside of the cell membrane
• Hormone-receptor associations trigger particular
biochemical events
• Hormones are affective in low concentrations
Chapter 14.1 Environmental cues
Specificity
• A particular stimulus will only affect a specific group of
hormone-secreting cells. E.g. a sudden shock to
humans causes the release of adrenaline
• Hormones are transported throughout the organism,
but only the cells with specific receptors are able to
respond to the hormone. This specificity is not only
restricted to specific organs, but specific
cells/structures within organs.
Chapter 14.1 Environmental cues
Specificity (continued)
• Flower plants have fewer hormones than animals
and they tend to affect most cells.
• Hormone producing cells in plants are not
organised into glands.
• The cells receiving the environmental cue
(stimulus) produce the required hormone.
– E.g. a growing shoot tip of a plant receiving light.
Chapter 14.1 Environmental cues
Specificity (continued)
•Plant response is specific.
–E.g. growing tip of wheat, lettuce or spinach
receives increasing daylight (i.e. after winter solstice)
will be stimulated to produce flowers.
Chapter 14.1 Environmental cues
Speed
• Hormonal response generally slower than
nerve
• Effects last longer
• Affects cells widely distributed
Chapter 14.1 Environmental cues
Speed (continued)
• Plant hormones slower in response than animals
(why?)
• Transported through phloem and xylem
• Transported between cells
• Transported through air to other plants
• Transported through soil (inhibit root growth from
neighbouring plants)
Chapter 14.1 Environmental cues
Speed (continued)
• Movement of a plant hormone requires the
expenditure of energy (ATP)
• Active transport is 10× faster than diffusion
(passive)
• Still may be as slow as 1 cm/hour (1 cmhour-1)
Chapter 14.1 Environmental cues
Hormones and responses of flowering plants
• Orientate the growth of roots, stems and
leaves
• Timing of flowering, fruit ripening and seed
germination
Chapter 14.1 Environmental cues
Hormones and responses of flowering plants
• Response triggered by environmental factor
• Tropism
– When the response is directed towards the
environmental factor, this is called “Tropism”
– Positive towards
– Negative away
Chapter 14.1 Environmental cues
Hormones and responses of flowering plants
• Hormones in plants are responsible for
–
–
–
–
–
Phototropism – growth in response to light
Geotropism – growth in response to gravity
Apical dominance – inhibition of lateral branches
Ripening of fruit – conversion of starches to sugars
Abscission – shedding of leaves and flowers
Chapter 14.1 Environmental cues
Auxins
• Phototropism – reaching for light
• Growing plants bend and grow towards light
• Stimulated by chemical – auxin (“to grow”)
• Chemical diffuses downwards from growing
tip (meristem)
Chapter 14.1 Environmental cues
Auxins (continued)
• Produced continuously in meristem
• Diffuses through layers of cells
• Light interacts with receptors that control
membrane permeability to auxin
• Auxin moves laterally away from light
Chapter 14.1 Environmental cues
Auxins (continued)
• Higher concentration of auxin on dark side of tip
• Those cells grow faster (elongation not cell division)
• Softens cell walls allowing elongation under turgor
pressure
• Cells outside growing area lack auxin receptors
• Auxins used as herbicides – excess auxin causes growth
to the detriment of the plant (cells rupture, etc.)
Phototropism Experiment
Auxin causes elongation of cells on shaded side.
Chapter 14.1 Environmental cues
Auxins
Geototropism – responding to gravity
• Auxins involved in negative geotropism (growing
away from gravity)
• Plant laid on side in dark will bend and grow
away from gravity
• Auxin concentration on lower side of lateral
stems
Negative Geotropism
Chapter 14.1 Environmental cues
Auxins
Apical dominance – one main stem
• Auxin produced in the apical tip of a plant
• Diffuses down stem
• Inhibits lateral bud development
• Results in taller plants with less side branches
• Gardeners cut out apical tips to promote bushiness of
cultivated plant
• Bushfires burn off the apical tips of eucalypts in the crown
Apical dominance
Chapter 14.1 Environmental cues
Gibberellins
• Promote cell elongation
• Promote growth for whole plant
• Promote cell division (mitosis and cytokinesis)
– Flowering
– Fruit enlargement
– Seed germination
Chapter 14.1 Environmental cues
Gibberellins
• Synthesised in flowers, developing fruits, seeds,
growing buds and elongating stems
• Important for triggering cereal grain germination
– Produced in embryo
– Moves to target cells in the endosperm
– Induces the formation of enzymes that digest the
endosperm to produce malt
 Used in the brewing of malt whisky
Chapter 14.1 Environmental cues
Cytokinins
• In the presence of auxins, cytokinins stimulates
cell division and cell differentiation
• Ratio of auxins to cytokinins determines path of
differentiation of new cells
– Stems and leaves develop when more cytokinins than
auxins
– Roots develop when less cytokinins than auxins
Chapter 14.1 Environmental cues
Abscisic acid
• Opposite action to auxins
• Involved in overall regulation of plant
functions
• Little is known about them
Chapter 14.1 Environmental cues
Abscisic acid
• Assist plants to tolerate or avoid adverse
conditions
– E.g. drought, salinity, low temperature
– Promotes leaf drop, bud and seed dormancy and
increased frost resistance
Chapter 14.1 Environmental cues
Abscisic acid (ABA)
• Synthesised mainly in chloroplasts
• Dropping of ripe fruit, unfertilised flowers and
falling of leaves in deciduous trees known as
abscission
• Disintegration of special layer of cells at base of
organ being dropped
Chapter 14.1 Environmental cues
Abscisic acid (ABA)
• Development of seed dormancy and
vernalisation requires ABA
• Appears to act on gene expression in nucleus
Chapter 14.1 Environmental cues
Abscisic acid (ABA)
• Short term effects – stomatal movement
– Plant loses too much water, ABA stimulates guard
cells to close
• [ABA] increases under stressful conditions
Chapter 14.1 Environmental cues
Ethylene (Ethene) (C2H4)
• Released by ripening fruit (apples, pears,
citrus, banana and avocados)
• Stimulates fruit to ripen
• Ripening of fleshy fruit shows colour changes
and softening of flesh
Chapter 14.1 Environmental cues
Ethylene (Ethene) (C2H4)
• Wild plants use fruit to attract animals to eat
them and disperse the seeds
• Ethylene increases rate of respiration and other
processes of fruit ripening (e.g. breaking down
starch and oils into sugars).
• Triggered by auxin and abscisic acid
Summary of plant hormones
Hormone
Where produced
Effective site
Action
Visible effect
auxins
Shoot tip (meristem)
Growing region of shoot
Cells elongate under
turgor pressure
Tip bends towards light, apical
dominance, used as herbicides,
stimulate cuttings to root
gibberellin
Fruits, seeds, growing
buds, elongating stems
Roots, shoots, seeds, also
found in fungi
Growth of cells
Shoot elongation, germination of
seeds, flowering fruit
enlargement
cytokinins
Roots and developing
fruits
Branch and leaf buds, moves
through phloem and xylem
Stimulates cell division,
cell elongation and
tissue differentiation
Growth of lateral branches, used
to promote life of vegetables in
storage
Abscisic acid
chloroplasts
Gene expression in nuclei
Growth inhibition
Seed dormancy, vernalisation,
drought-tolerance
ethylene
Ripening fruits, flowers,
seeds, leaves and roots
Cellular metabolism
Fruit ripening and leaf
drop
Fruit ripening, leaf and fruit drop,
used commercially to ripen
bananas and pineapples