Plant Development
Chapter 31 Part 2
31.4 Adjusting the
Direction and Rates of Growth
 Tropisms
• Plants adjust the direction and rate of growth in
response to environmental stimuli such as gravity,
light, contact, and mechanical stress
• Hormones are typically part of this effect
Gravitropism
 Gravitropism
• A growth response to gravity which causes roots
to grow downward and shoots to grow upward
 Statoliths
• Amyloplasts containing heavy starch grains that
sink to the bottom of the cell
• A change in position results in movement of cell’s
auxin efflux carriers
Gravitropism
A Gravitropism of
a corn seedling.
No matter what the
orientation of a
seed in the soil, a
seedling’s primary
root grows down,
and its primary
shoot grows up.
Fig. 31-10a, p. 530
B These seedlings
were rotated 90°
counterclockwise
after they
germinated. The
plant adjusts to
the change by
redistributing auxin,
and the direction
of growth shifts as
a result.
C In the presence of
auxin transport
inhibitors, seedlings
do not adjust their
direction of growth
after a 90°
counterclockwise
rotation. Mutations in
genes that encode
auxin transport
proteins have the
same effect.
Fig. 31-10 (b-c), p. 530
Statoliths and Auxin
Statoliths and Auxin
statoliths
A Heavy, starch-packed statoliths are settled on the
bottom of gravity-sensing cells in a corn root cap.
Fig. 31-11a, p. 530
B Ten minutes after the root was rotated, the statoliths settled
to the new “bottom” of the cells. The redistribution causes
auxin redistribution, and the root tip curves down.
Fig. 31-11b, p. 530
Animation: Gravity and statolith
distribution
Phototropism
 Phototropism
• Orientation of certain plant parts toward light
• Nonphotosynthetic pigments (phototropins)
respond to blue light, initiating signal cascades
• Auxin is redistributed to shady side of plant
Phototropism
Fig. 31-12 (a-b), p. 531
light
A Sunlight
strikes only
one side of
a coleoptile.
B Auxin is
transported to
the shaded side,
where it causes
cells to lengthen.
Fig. 31-12 (a-b), p. 531
Animation: Phototropism
Fig. 31-12c, p. 531
Thigmotropism
 Thigmotropism
• Contact with a solid object changes the direction
of plant growth
• Involves TOUCH genes and calcium ions
• Results in unequal growth rates on opposite sides
of the shoot
 Mechanical stress (such as wind) inhibits stem
lengthening in a similar touch response
Thigmotropism
Mechanical Stress
31.5 Sensing Recurring
Environmental Changes
 Seasonal shifts in night length, temperature, and
light trigger seasonal shifts in plant development
 Flowering plants respond to recurring cues from
the environment with recurring cycles of
development
Biological Clocks
 Biological clock
• Internal mechanism that governs the timing of
rhythmic cycles of activity
 Circadian rhythm
• A cycle of activity that recurs every 24 hours
 Solar tracking
• A circadian rhythm in which a leaf or flower
changes position to continually face the sun
Setting the Clock
 Different wavelengths of sunlight set biological
clocks by activating and inactivating
photoreceptor pigments (phytochromes)
 Active phytochrome cause gene transcription for
components of rubisco, photosystem II,
phototropin, and molecules involved in flowering,
gravitropism, and germination
Conversion of Phytochromes
red
660 nm
far-red
730 nm
red light
Pr
inactive
Pfr
far-red light
activated
response
Pfr influences
gene
expression
Pfr reverts to Pr
in darkness
Fig. 31-15, p. 532
Animation: Phytochrome conversions
When to Flower?
 Photoperiodism
• Long-day plants flower when nights are short;
short-day plants flower when nights are long
• Leaf cells transcribe more or less of a flowering
gene in response to changes in the length of
night relative to the length of day
Seasonal Changes in
the Northern Temperate Zone
JANUARY
dormancy
FEBRUARY
MARCH
APRIL
seed germination or renewed
growth; short-day plant flowering
MAY
JUNE
long-day plant flowering
JULY
short-day plant flowering
AUGUST
SEPTEMBER
onset of dormancy
OCTOBER
dormancy
NOVEMBER
DECEMBER
14
12
10
8
Length of night (hours of darkness)
Fig. 31-16, p. 532
Flowering and Night Length
critical night length
will flower
night
will not flower
day
night
day
0 4 8 12 16 20
Time being measured (hours)
A Long-day plants flower only when
hours of darkness are less than the
critical value for the species. Irises will
flower only when night length is less
than 12 hours.
will not flower
will flower
24
B Short-day plants flower only when
hours of darkness are greater than
the critical value for the species.
Chrysanthemums will flower only
when night length exceeds 12 hours.
Fig. 31-17, p. 533
Phytochromes and Photoperiodism
Long-Day Plant:
Short-Day Plant:
critical night length
did not flower
a
flowered
b
did not flower
0
4
8
12
16
20
Time being measured (hours)
24
flowered
Fig. 31-18, p. 533
Flowering and Vernalization
 Vernalization
• Some biennials and perennials flower in the
spring only after exposure to cold winter
temperatures
Vernalization
Animation: Flowering response
experiments
31.6 Senescence and Dormancy
 Senescence
• The phase of a plant life cycle between full
maturity and death of the plant or plant parts
 Abscission
• The process by which plant parts are shed
• Triggered by many factors, including seasonal
changes in environmental conditions
Abscission in Deciduous Plants
 Midsummer
• Auxin is produced; plants divert nutrients into
flowers, fruits and seeds
 Autumn
• Auxin production declines in leaves and fruits
• Ethylene signals enzymes to digest cell walls in
abscission zones; leaves and fruits drop
Abscission in Deciduous Plants
Delayed Senescence
control (pods
not removed)
experimental plant
(pods removed)
Fig. 31-21, p. 534
Dormancy
 Dormancy
• A period of arrested growth that is triggered (and
ended) by environmental cues
• Signals to begin dormancy include long nights,
cold temperatures, and dry, nitrogen-poor soil
• Favorable conditions signal to break dormancy
31.4-31.6 Key Concepts
Responses to Environmental Cues
 Plants respond to environmental cues, including
gravity, sunlight, and seasonal shifts in night
length and temperatures, by altering patterns of
growth
 Cyclic patterns of growth are responses to
changing seasons and other recurring
environmental patterns
Animation: Cell shapes
Animation: Daylength and dormancy
Animation: Gravitropism
Animation: Vernalization