Chapter 32 Plant Growth and Development

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Chapter 32 Plant Growth and Development
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How Do Seeds Germinate?

• Germination is the resumption of growth after a
  time of arrested embryonic development
• Environmental factors influence germination
• Spring rains provide the water amounts necessary
  to swell and rupture the seed coat (taking in
  water is imbibition)
• Oxygen moves in and allows the embryo to switch
  to aerobic metabolism
• Increase temperatures and number of daylight hours

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• Repeated cell divisions produce a seedling with a
  primary root.

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Genetic Programs, Environmental Cues

• Patterns of germination and development have a
  heritable basis dictated by a plants genes
• Early cell divisions may result in unequal
  distribution of cytoplasm
• Cytoplasmic differences trigger variable gene
  expression, which may results in variations in
  hormone synthesis
• Even though all cells have the same genes, it is
  the selective expression of those genes that
  results in cell differentiation.

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Growth and Development

• Growth and development are necessary for plants
  to survive
• Growth is defined as an increase in the number,
  size, and volume of cells
• Development is the emergence of specialized,
  morphologically different body parts

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Plant Hormones

• Plant hormones have central roles in the
  selective gene expression underlying cell
  differentiation and patterns of development.

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Types of Plant Hormones

• Gibberellins Promote stem elongation
• Help buds and seeds break dormancy and resume
  growth in the spring.
• In some species, they influence the flowering
  process.
• Cytokinins stimulate cell division in root and
  shoot meristems, where they are most abundant
  they are used commercially to prolong the life of
  stored vegetables and cut flowers

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Grapes (Gibberellins)
With
Without
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• Auxins affect lengthening of stems and
  coleoptiles (the protective cylinder that covers
  and protects the tender leaves during
  germination)
• May participate in growth responses to light and
  gravity.
• Indoleacetic acid (IAA) is applied to fruit trees
  to promote uniform flowering, set the fruit, and
  encourage synchronous development of fruit.
• Synthetic auxins (such as 2,4-D) are used as
  herbicides

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Auxins Picture
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• Abscisic Acid (ABA)inhibits cell growth, helps
  prevent water loss (by promoting stomata
  closure), and promoting seed and bud dormancy.
• Ethylene stimulates the ripening of fruit and is
  used commercially for this purpose.
• Other less well known hormones trigger flowering
  and inhibit the growth of lateral buds (apical
  dominance)

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What are Tropisms?

• A plant tropisms is a growth response
• Evidenced by a turning of a root or shoot toward
  or away from an environmental stimulus
• Hormones mediate the shifts in rates at which
  different cells grow and elongate to cause the
  overall response

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Types of Tropisms

• Gravitropisms is the growth response to gravity
  shoots grow up, roots grow down.
• Auxins, together with a growthinhibiting
  hormone, may play role in promoting, or
  inhibiting, growth in strategic regions
• Statoliths, which are unbound starch grains in
  the plastids, respond to gravity and may trigger
  the redistribution of auxin

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Roots moving down toward gravity
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• Phototropisms is a growth response to light
• Bending toward the light is caused by elongation
  of cells (auxin stimulation on the side of the
  palnt not exposed to light).
• Flavoprotein, a pigment molecule probably plays a
  role because of its capacity to absorb blue
  wavelengths of light

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• Thigmotropism is shift in growth triggered by
  physical contact with surrounding objects.
• Prevalent in climbing vines and in the tendrils
  that support some plants
• Auxin and ethylene may have roles in the response

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Response to Mechanical Stress

• Response to the mechanical stress of strong winds
  explain why plants grown at higher mountain
  elevations are more stubby than their
  counterparts at lower elevations
• Human interventions such as shaking can inhibit
  plant growth.

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How Do Plants Known When to Flower?

• Phytochrome Alarm button for plants
• Biological Clocks are internal time-measuring
  mechanisms that adjust daily and seasonal
  patterns of growth, development, and reproduction
• Phytochrome a blue-green pigment, is alarm
  button for some biological clocks in plants
• Phytochrome- can absorb both read and far-red
  wavelengths with different results.
• When is the pigment activated?
• When is the pigment inactive?

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• Some plants activities occur regularly in cycles
  of 24 hours (circadian rhythms) even when
  environmental conditions remain constant

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Flowering A case of photoperiodism

• Photoperiodism is a biological response to a
  change in relative length of daylight and
  darkness in a 24-hour cycle this resetting of
  the biological clocks is necessary to make
  seasonal adjustments

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• The flowering process is keyed to changes in
  daylength throughout the year.
• Short-day plants flower in late summer or early
  autumn when daylength becomes shorter
• Example Poinsettias
• Long-day plants flower in the spring as
  daylength becomes longer
• Example Spinach
• Day-Neutral Plants flower when they are mature
  enough to do so
• Example Roses

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Senescence

• The dropping of leaves, flowers, fruits is called
  abscission
• Senescence is the sum total of the processes
  leading to the death of plant parts or the whole
  plant
• Decrease of daylight hours trigger the reduction
  of auxin
• Cells in abscission zones produce ethylene which
  causes cells to deposit suberin in their walls

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Entering and Breaking Dormancy

• Dormancy occurs in autumn when daylight shortens
  and growth stops in many trees and nonwoody
  perennials it will not resume until spring
• Strong cues for dormancy include in short days,
  cold nights, and dry, nitrogen-deficient soil.
• Dormancy has great adaptive value in preventing
  plant growth on occasional warm autumn days only
  to be killed by later frost.

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Vernalization

• Vernalization is the stimulation of flowering
  only after plants have been exposed to lower
  temperatures (winter).
• Deliberately exposing seeds to lower temperature
  to stimulate flowering the next season is common
  agricultural practice.

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Breaking Dormancy

• Dormancy is broken by milder temperatures, rains,
  and nutrients.
• It probably involves gibberellins and abscisiic
  acid, and require exposure to specific periods of
  low temperatures.