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Hormonal Influences on
growth
Chapter 30
Animal Hormones
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Endocrine glands produce hormones
Secreted into bloodstream
Pituitary gland (anterior)
Produces growth hormone (somatotrophin)
- accelerates amino acid transport into soft tissue and
bone cells
- rapid protein synthesis occurs
Also produces thyroid stimulating hormone (TSH)
- controls thyroid gland
- thyroid then produces thyroxine
- regulates metabolic processes
Plant Growth Substances
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Based on work on coleoptiles of oat seedlings:
- shoot tip is essential for growth, & produces a
chemical
- chemical diffuses down shoot & makes cells
elongate
- chemical can diffuse through agar, but not
metal
Chemical in question is one of many auxins
Indole Acetic Acid (IAA)
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Most common auxin in plants
Produced by root and shoot tips and leaf meristems
Can move by diffusion or by translocation
At cell level
- stimulate cell division (primary growth)
- if transported to cambium, stimulates secondary
growth
- promotes cell elongation – increases cell wall plasticity
- aids differentiation of cells for specialisation
- occurs behind root & shoot tips
- and in cells formed from cambium division
Indole Acetic Acid
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At organ level
- v low concentrations stimulate root elongation
- higher concentrations stimulate shoots, but
inhibit roots
- v high concentrations inhibit shoot growth
- higher concentrations can also cause curvature
Refer to pg. 251-253
Phototropism
Directional growth in response to light
from one direction
 Light from one side causes a higher conc
of auixn on non-illuminated (shaded) side
 Cells on shaded side elongate more
 Therefore shoot bends towards the light
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Apical Dominance
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V high auxin concentrations inhibit growth
Apical bud often inhibits development of lateral
(side) buds
Produces v high auxin concentrations
This is apical dominance
Dominance can be very strong e.g. sunflower
- rarely any side buds
Or very weak e.g. tomato plant
- lots of side buds
Apical buds often removed to promote side
branching
Other roles of auxin
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Promotes formation of fruit coat from ovary wall
Also promotes abscission of leaves or fruit
- auxin conc. drops sharply to form an abscission layer
Commercial
- Parthenocarpy
-(fruit development without fertilisation)
- induced by treating unfertilised flowers with
auxin
- produces ‘seedless’ fruits
- Delaying abscission
- spraying crops with synthetic auxin
- Rooting powder
- stimulates production of adventitious roots
- Herbicides
- Auxins used as selective weedkillers
- stimulate growth so much that plant becomes exhausted
- generally only broad leaved plants absorb the auxins
Gibberellins
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Originally found in a fungus, causing abnormal growth in
another plant
Usually occur in low concentrations
Stimulate cell division and elongation in stems
Play no role in phototropism or shoot bending
Most common is Giberellic Acid (GA)
Aid the following:
- increase length of internodes
- induction of a-amylase (for starch digestion in seeds)
- stimulates bud opening (at certain times of the year)