Download Chapter 24 Plant Hormones and Tropisms

Chapter 24
Plant Hormones and
Tropisms
Hormone
Substance produced in one part of an
organsim that affects another part of
that same organism.
Target Cell
Portion of an organism affected by a
hormone
Plant Hormone
• Chemical substance
• Controls plant’s:
• patterns of growth & development
• responses to environmental
conditions
• Produced in apical meristem
Auxins
• Transported downward to rest
of plant
• Stimulate cell elongation
Auxins
The above diagram shows
how auxin production in a
plant causes it to grow
toward sunlight.
• Light hits one side of stem
Auxin and
Phototropism
• Higher concentration of
auxins develops in shaded
part of the stem
• Change in concentration
stimulates cells in the dark
side to elongate
• Result: stem bends away
from the shaded side and
toward the light
How does auxin affect plant
growth in the presence of gravity?
• Built up on the lower sides of roots and
stems but slows growth
• Result: roots grow downward
How does auxin affect plant
height or width?
• As a stem grows in length, it produces lateral buds
• Lateral bud gives rise to side branches on the side
of stem
• Growth at lateral buds is inhibited by auxin,
which is on the stem’s tip
• The closer a lateral bud is to stem’s tip, the more
it is inhibited (apical dominance)
cytokinins
• Produced in growing roots
and developing fruits & seeds
• Stimulate cell division and
growth of lateral buds
• Cause dormant seeds to
sprout
• Delays aging of leaves
As a result of cytokinin the
top carrot is 2x bigger than
the other.
• Inhibit elongation and cause
cells to grow thicker
How do gibberellins affect plant
growth?
• Produce increase in size in stem and fruit
• Responsible for rapid early growth in many plants
Notice the difference in sizes of the fruit bunches and plants
How does ethylene gas affect
plants and how is it used on a
fruit?
• Trees suffer leaf loss
• Ethylene stimulates fruits to ripen
• Use to treat fruit picked before ripening to ripen them
when being handled for shipment
Homeostasis
• Plant hormones have opposing effect!
– Auxins (made in meristem – stimulate root
growth) VS. Cytokinins (made in roots/fruits –
stimulate leaf/tip growth)
– Gibberelins (growth of stems and fruit) VS.
Absicisic Acid (halt growth until conditions
improve)
Tropism
response of a plant to an
environmental stimulus
Phototropism
tendency for plants to grow
towards light
Thigmotropism
response of plants to touch
Gravitropism
tendency of a plant to grow in a
direction in response to force of
gravity
Tradescantia- A plant that
shows growth response to
gravity upward when
young and downward
when older.
Notice how the trunk
of the tree to the right
grows up away from
gravity.
What Is Photoperiodism?
Photoperiodism is the response of plants to
periods of light and darkness.
Plants such as chrysanthemums and poinsettias
flower when days are short.
Long Day Plants
Plants such as spinach and irises flower
when days are long.
• It is a plant pigment that is held
responsible for photoperiodism.
• Absorbs red light
• Activates a number of signaling pathways
within plant cells