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