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Chapter 48 - Plant Growth and Development

Some Performance Objectives

Be able to state the name of the plant hormone(s) that is (are):

  1. growth inhibitors - abscisic acid, ethylene
  2. stress hormone - abscisic acid
  3. closes the stomata - abscisic acid
  4. growth promotors - auxin, cytokinin, giberellins
  5. a gas - ethylene
  6. produces dormancy in seeds and buds - abscisic acid
  7. breaks dormancy in seeds and buds - gibberellin
  8. stimulates the ripening of fruit - ethylene
  9. prevents plant tissues from senescing or aging - cytokinin
  10. 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

Ethylene

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.