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Plant Hormones and Responses
KEY CONCEPT
Plant hormones guide plant growth and development.
Plant Hormones and Responses
Plant hormones regulate plant functions.
• Hormones are chemical messengers.
– produced in one part of an organism
– stimulates or suppresses activity in another part
– Often have adaptive advantages
Plant Hormones and Responses
Five major groups of plant hormones
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Gibberellins
Ethylene
Cytokinins
Auxins
Abscisic acid
Plant Hormones and Responses
• Gibberellins are plant hormones that produce dramatic
increases in size.
– ending seed dormancy
– rapid growth of young
seedlings
– rapid growth of some
flower stalks
Plant Hormones and Responses
• Ethylene causes the ripening of fruits.
– some fruits picked before
they are ripe
– sprayed with ethylene to
ripen when reach
destination
– Promotes abscission
- The detachment of leaves,
flowers, or fruits
Plant Hormones and Responses
• Cytokinins stimulate cytokinesis.
– final stage in cell division
– produced in growing roots, seeds, and fruits
– involved in growth of side branches
Plant Hormones and Responses
• Auxins lengthen plant cells in the growing tip.
– stimulates growth of
primary stem
– controls some forms of
tropism
• A tropism is the movement
of plant in response to an
environmental stimulus.
Plant Hormones and Responses
• Abscisic acid
– Promotes dormancy in plant buds, maintains
dormancy in seeds, and causes stomata to close
Plant Hormones and Responses
Tropisms
• A tropism is a response in which a plant grows either
toward or away from an environmental stimulus
Plant Hormones and Responses
Plants can respond to light, touch, gravity, and seasonal
changes.
• Phototropism is the
tendency of a plant to
grow toward light.
– auxins build up on
shaded side of stem
– cells on shaded
side lengthen
– causes stem to
bend toward light
Plant Hormones and Responses
• Solar tracking, also called heliotropism, is the motion
of leaves or flowers as they follow the sun’s movement
across the sky.
Plant Hormones and Responses
• Thigmotropism is a plant’s response to touch-like stimuli.
– climbing plants and vines
– plants that grow in direction of constant wind
Plant Hormones and Responses
• Gravitropism is a plant’s
response to Earth’s
gravitational pull.
– positive gravitropism is
downward growth (roots)
– negative gravitropism is
upward growth (shoots)
Plant Hormones and Responses
• Chemotropism
– Plant growth that occurs in response t oa chemical is
called chemotropism
– An example of chemotropism is the growth of a pollen
tube after a flower is pollinated.
Plant Hormones and Responses
Nastic Movements
• Thigmonastic movements
– Occur in response to touch,
such as the closing of the leaf
trap of a Venus’ flytrap around
an insect.
• Nyctinastic movements
– Occur in response to the daily
cycle of light and dark,such as
the cyclical vertical and
horizontal positioning of leaves
in prayer plants.
Plant Hormones and Responses
• Photoperiodism is a response to the changing lengths of
day and night.
– triggers some plants to flower
– triggers fall colors/winter dormancy of deciduous trees
• Vernalization is the promotion of flowering by cold
temperatures
– Farmers often plant wheat seeds in the fall so that the
seedlings can be exposed to winter temperatures and
will flower before summer droughts begin