Chapter 48 - Plant Growth and Development
Some Performance Objectives
Be able to state the name of the plant hormone(s) that is (are):
- growth inhibitors - abscisic acid, ethylene
- stress hormone - abscisic acid
- closes the stomata - abscisic acid
- growth promotors - auxin, cytokinin, giberellins
- a gas - ethylene
- produces dormancy in seeds and buds - abscisic acid
- breaks dormancy in seeds and buds - gibberellin
- stimulates the ripening of fruit - ethylene
- prevents plant tissues from senescing or aging - cytokinin
- is produced by the apical bud and inhibits the growth of lateral buds - auxin
Plant Hormones
HORMONE- a signaling molecule released by one cell and transported to TARGET CELLS (usually nonadjacent). The target cell contains receptor sites.
Hormones that Promote Growth
Gibberellins
promote growth
moves upward and downward in vascular system
regulate height
too little- dwarf
too much- long, spindly
important in bolting (sudden stem lengthening)
induce seeds of some grasses to germinate (breaks dormancy)
can break dormancy in buds
can stimulate flowering in mature plants
Auxins
Auxin moves only downward but gravity does not cause its movement.
It is generally produced by apical shoot meristems and developing leaves.
Auxin promotes growth by stimulating cell elongation.
It triggers enzyme activities that loosen the cell wall fibers (normally tightly woven).
Auxin also stimulates cell division. It activates the vascular cambium and promotes the formation of lateral roots by the pericycle.
Auxin inhibits the growth of lateral buds.
inhibits roots
small quantities promote root growth. Slight increases inhibit it.
Auxin production by seeds stimulates fruit growth. If eggs are not fertilized, ovules do not become seeds and auxin cannot be produced. The lack of auxin results in abscission of the flower.
Auxin also prevents fruits and leaves from off prematurely. It can be sprayed on to prevent from falling off.
high concentration ? uncontrolled growth, death
example: some herbicides- 2,4-D, 2,4,5-T (components of agent orange)
Apical Dominance
Apical dominance is the inhibition of lateral buds by auxin from the apical bud.
Auxin is broken down as it moves down stem, so its concentration decreases.
lowest buds- least inhibited.
Removal of apical bud releases buds farther down
Cytokinins
stimulate cell division
major source is the roots
oppose auxin by:
move upward
promote growth of lateral buds
as plant grows, lower buds are more under the influence of cytokinins
prevents leaf senescence
Senescence
Senescence refers to the aging and eventual death of a plant or plant parts.
When plant parts senesce, nutrients are withdrawn and redistributed via the phloem.
In deciduous species (species that lose their leaves), nutrients move to the stems and roots for storage.
Inhibit Growth
gas- dispersed by air
produced by ripening fruits, accelerates ripening of nearby fruits
stimulates senescence and abscission in leaves and fruits
abscission is apparently started by the relative decrease in auxin and perhaps gibberellin
once started the process is stimulated by ethylene
Ethylene released into the abscission layer causes the release of cellulase. Cellulase digests cellulose allowing them to stretch. The pressure that is normally found within the cell is sufficient to cause the cell to enlarge when the cell wall is loosened.
Abscisic Acid (ABA)
stress hormone- protects plants
closes stomata (water shortage)- causes K+ to leave guard cells
growth inhibitor- counteracts growth hormones
accelerates abscission (dropping of leaves)
moves only short distances from site of production
induces and maintains dormancy (metabolic slowdown) in seeds and buds
Some seeds will not germinate until the ABA is leached away.
Hormone Interactions
allows complex control of growth and development
Auxin and cytokinin- the 2 major hormones used in culturing plant tissues.
the ratio of auxin to cytokinin and the pH of the culture medium determine whether the plant tissue remains as an undifferentiated callus or differentiates to produce roots, vegetative shoots, leaves, or floral shoots
Plant Movements
Tropisms are growth in response to stimuli.
Positive tropisms are those in which the plant moves toward the stimulus. For example, positive phototropism refers to bending toward light.
Negative tropisms are movements away from the stimulus. Plant shoots exhibit negative gravitropism because they grow away from gravity.
Gravitropism
Gravitropism refers to growth movements in response to gravity.
Shoots
Shoots are negatively gravitropic.
In stems, the gravity-sensitive cells (detectors) are in the apical meristem.
In horizontal stems, auxin accumulates on the lower surface of the stem. Auxin should cause the stems on the lower part of a horizontal stem to enlarge, bending it upward but the role of auxin is unclear. Evidence indicates that it may not be involved in dicot gravitropism and other hormones may also be involved in monocot stems.
Roots
When a plant is placed in a horizontal position, the roots respond by bending down (positive gravitropism). The Darwins discovered that if root caps are removed, the roots do not respond to gravity.
Roots seem to perceive gravity by the movement of starch granules in root cap cells. These may influence hormone distribution but the mechanism is unknown.
Auxin is one of the hormones that is redistributed within the root but the role of auxin in causing the root to bend is uncertain.
Roots are inhibited by auxin (unlike stems). Perhaps cells on the bottom of the root are inhibited while those on the top are not.
Phototropism
Phototropism refers to movement in response to light.
The Darwins’ (Charles and son Francis) experiment.
plant bends in response to light
block light in bending area (collar) ? plant bends
remove or cover tip ? plant does not bend
conclusion- tip sends signal
Frits Went
removed tip of oat seedling
placed on an agar [gelatin] block
placed agar on decapitated oat seedling (grown in dark)
bent away from side w/ agar block
conclude- substance (he called auxin) caused cells to grow.
Illumination results auxin moving to the dark side of stem tips. As this auxin diffuses down the dark side of the stem, the cells respond by enlarging. This causes the plant to bend toward the light.
Other Tropisms
Thigmotropism
pressure sensitive orientation- pea tendrils
cells touching- don't expand (shrink)
outer cells- expand
Thigmomorphogenesis
Plants can respond to external stimuli by changing their shape (also called morphology).
Plants that are exposed to wind grow shorter and thicker stems. This helps prevent them from being blown down in the wind.
Turgor Pressure
Some plants are capable of producing fast movements as a result of external stimulation.
The movement results from a rapid change in the turgor pressure of some cells.
For example, a Venus flytrap closes and traps insects that touch sensitive hairs.
Internal Stimuli
Biological clock - enables organisms to maintain rhythm in the absence of external stimuli.
circadian rhythm - a biological rhythm with a 24 hour cycle
Example - Sleep movements in pla nts occur on a 24-hour schedule even in the absence of light.
Detection of Light
phytochrome- changes shape in presence of light
2 forms
Pr - absorbs red light- becomes converted to Pfr. Pfr accumulates during the day because sunlight contains more red than far-red light.
Pfr - (active form) absorbs far red- becomes converted to Pr in dark.
Because Pfr is produced during the day, its concentration in plant tissues is highest at the end of the day. When days are longer, more Pfr accumulates.
Day: Pr ? Pfr (active form) ? physiological response
Night: Pr ? Pfr
Pr/Pfr ratio provides the plant with information on daylength.
Pfr - normal growth, many enzymes and cell types become active
Flowering
Long-day plants- flower when daylength is longer than a critical value
Spinach cannot grow in tropics- needs 14 hours
Short-day plants- daylength is shorter than a critical value
Day-neutral plants- whenever mature
Many short day plants- brief interruption at night inhibits flowering
Pfr (active form) inhibits
ex- Douglas fir- 12 h day, 12 h night; 12 h day, 11 h night w/ 1hr interruption; 20 h day, 4 h night
ex: poinsettias: Pfr inhibits flowering after a brief interruption Pr ? Pfr
Dormancy
Dormancy refers to a slowdown in an organisms metabolic rate. Because chemical reactions are occurring at a slower rate, the animal requires fewer nutrients.
Dormant plants are more tolerant to drought and lower temperatures because of their reduced need for nutrients.
Dormancy may be triggered by short daylength and by environmental factors such as temperature, moisture, and nutrient availability.
Pfr (the active form of phytochrome) prevents dormancy
Abacisic acid induces and maintains dormancy, gibberellins can break dormancy in buds and some seeds.
In some species low temperatures for hundreds of hours may be needed to break dormancy.