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- tropisms are growth responses to environmental
factors
1. Phototropism [Fig. 32.5]
- phototropism is a growth response toward light caused by auxin stimulation
of cells not exposed to light
- auxin concentration high in cells that are shaded --> cells on shaded side
of coleptile elongate --> coleptile bends toward light
2. Gravitropism
- gravitropism is the growth response to gravity -- shoots grow up, roots grow
down
- auxins and other (undefined) hormones may play a role in promoting, inhibiting
grown in strategic regions
- statoliths (=unbound starch grains in plastids) respond to gravity and may
trigger redistribution of auxin
3. Thigmotropism
- thigmotropism is unequal growth triggered by physical contact with surrounding
supports; exhibited by vines and tendrils
- process involves auxin and ethylene (details unknown)
- cells on contact side stop elongating w/in minutes; tendril curls around support
structure (stem of other plant); then cells on both sides resume growth at same
rate
1. Flowering is controlled by daylength
- oaks flower in spring, lettuce flowers in summer, asters flower in autumn;
production of seeds, fruit must be properly timed to physiology of plant and
rigors of environment
- most environmental cues are variable (e.g., temperature in October may be
quite warm, the summer may be unusually wet and cool, a late snow may fall in
May)
- the only reliable cue is daylength: increasing daylength
means summer is coming; decreasing daylength means fall and winter are on the
way
2. Long-day, short-day and day-neutral plants
- corn is a day-neutral plant: it develops flowers when it has grown and developed
enough, regardless of daylength
- spinach is a long-day plant: it develops flowers when the daylength is longer
than 14 hours (its "critical daylength")
- cocklebur is a short-day plant: it develops flowers when the day is shorter
than 15.5 hours (its "critical daylength")
[What will happen if cocklebur and spinach are both grown on 14.5 hours of light
per day? 16 hours of light/day? 12 hours of light/day?]
3. Phytochromes measure daylength by resetting biological clock
- phytochrome is a light-detecting pigment in the leaves that is thought to
be involved in resetting the internal, biological clock of many plants
- the biological clock has a period of approx. 24 hours and can control plant
circadian rhythms (e.g., leaf movement)
- phytochrome occurs in two, interchangeable forms: PR strongly
absorbs red light; PFR absorbs far-red light (almost infrared)
- in most plants PRF is the active form of phytochrome; i.e., a sutable concentration
of PFR stimulates or inhibits physiological processes such as flowering or setting
the biological clock
- phytochrome flips back and forth from one form to the other when it absorbs
light of the appropriate color [Fig. 32.10]; during the day, a leaf contains
both forms (since sunlight consists of all wavelengths); in the dark, PFR breaks
down to PR
- "night-interruption" experiments with a brief flash of red or far-red
light suggest that PFR can reset the clock:
Example: Cocklebur flowers on a schedule of 8 h light:16 h dark. If the long night is interrupted by a flash of red light at midnight, flowering is inhibited. No effect is seen if the light flash is far-red. This and other experiments suggest that the biological clock normally measures "night length," and that the clock is reset by PFR
- vernalization is a response to low temperature
- exposure of buds of some perennials to low winter temperature stimultes flowering
- senescence = processes leading to the death of plant
parts or whole plant; often occurs when nutrients are funneled into reproductive
parts of plant
- plants withdraw nutrients from leaves, roots, stems and redistribute them
to form new flowers, fruits and seeds
- as an outcome of this redistribution, leaves generally wither and die (Note:
abscission triggered by ethylene, not abscissic acid)
