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
Fig. 31-10a, p. 530
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. 31-10 (b-c), 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.
Statoliths and Auxin
Statoliths and Auxin
Fig. 31-11a, p. 530
statoliths
A Heavy, starch-packed statoliths are settled on the bottom of gravity-sensing cells in a corn root cap.
Fig. 31-11b, 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.
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
Fig. 31-12 (a-b), p. 531
light
B Auxin is transported to the shaded side, where it causes cells to lengthen.
A Sunlight strikes only one side of a coleoptile.
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
Fig. 31-15, p. 532
red 660 nm
far-red 730 nm
red light
Pr Pfr response
inactive far-red light activated Pfr influences gene expression
Pfr reverts to Pr in darkness
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
Fig. 31-16, p. 532
JANUARY
dormancy FEBRUARY
MARCH
APRILseed germination or renewed growth; short-day plant flowering MAY
JUNE
JULYlong-day plant flowering
short-day plant flowering AUGUST
SEPTEMBERonset of dormancy
OCTOBER
dormancy NOVEMBER
DECEMBER
Length of night (hours of darkness)
14 12 10 8
Flowering and Night Length
Fig. 31-17, p. 533
critical night length
will flowernight day will not flower
will not flower night day will flower
0 4 8 12 16 20 24
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.
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.
Time being measured (hours)
Phytochromes and Photoperiodism
Fig. 31-18, p. 533
Long-Day Plant: Short-Day Plant:
critical night length
did not flower
flowered
did not flower
0 4 8 12 16 20 24
floweredTime being measured (hours)
a
b
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
Fig. 31-21, p. 534
control (pods not removed)
experimental plant (pods removed)
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