Plant growth and development II

X. Plant Growth and Development (continued)

D. Tropisms

- 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

E. The Flowering Process

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

F. Vernalization

- vernalization is a response to low temperature
- exposure of buds of some perennials to low winter temperature stimultes flowering

G. Senescence

- 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)