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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.
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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
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 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
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
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
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
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
red 660 nm far-red 730 nm red light response Pr Pfr far-red light inactive activated Pfr influences gene expression Pfr reverts to Pr in darkness Fig. 31-15, p. 532
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
JANUARY dormancy FEBRUARY MARCH APRIL seed germination or renewed growth; short-day plant flowering MAY JUNE long-day plant flowering JULY AUGUST short-day plant flowering SEPTEMBER onset of dormancy OCTOBER NOVEMBER dormancy DECEMBER 14 12 10 8 Length of night (hours of darkness) Fig. 31-16, p. 532
critical night length will flower will not flower night day will not flower will flower night day 24 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. 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
Long-Day Plant: Short-Day Plant: critical night length did not flower a flowered b did not flower 0 4 8 12 16 20 24 Time being measured (hours) 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
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
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 ConceptsResponses 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
