External Factors and Plant Growth

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External Factors and Plant Growth
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Plant Growth Regulators

• The word hormone comes from the Greek word To
  excite and hormones stimulate biochemical
  activities that cause observable physiological
  responses by organisms
• Plant hormones or plant growth regulators are
  rather different from animal hormones in chemical
  structure, mode of synthesis, and function.
  Higher animals possess glands that are part of
  their endocrine system that are specialized
  organs for production of hormones
• Plant hormones on the other hand are synthesized
  in the cells of general organs the stems,
  leaves, roots, and flowers

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Tropisms

• A tropism is a growth response involving bending
  or curving of a plant part toward or away from an
  external stimulus that determines the direction
  of movement
• Response toward the stimulus is positive, and
  response away from the stimulus is negative

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Phototropism

• Phototropism is the growth of stems of plants
  toward light - it is probably the best known of
  the plant tropisms - phototropism is caused by
  elongation of the cells on the shaded part of the
  plant - so that entire plant bends or curves
  toward the light
• This growth pattern is caused by the hormone
  auxin - auxin migrates to the shaded part of the
  plant and stimulates increased cell growth and
  elongation on the shaded part of the plant

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a, Grass seedlings have a sheath called the
coleoptile that surrounds the first set of
leaves. Growth of the coleoptile depends on the
tip, and removal of the tip stops growth. Adding
the tip back asymmetrically illustrates that the
growth-promoting effect travels downward and not
laterally, causing the seedling to bend because
one side is growing faster than the other. Auxin
can replace the tip for this effect. b, Stem
cuttings can be induced by auxin to produce
roots. c, Strawberries depend on auxin produced
by their developing seeds for expansion and
maturation. If the seeds are removed, little
growth occurs. Normal growth can be restored with
auxin.
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Gravitropism

• Gravitropism is a response to gravity - if a
  seedling is placed on its side, the root will
  curve to grow downward and the shoot will curve
  to grow upward
• Auxin may play a role in the response of shoots -
  auxin-inducible genes exist and auxin induces
  increased growth in cells in which the gene is
  activated - auxin activates the transcription of
  certain genes on the side of the shoot showing
  increased growth - it is still not clear whether
  this transcriptional activation is due to an
  increase in auxin concentration or to an increase
  in sensitivity to auxin already present in the
  shoot

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Gravitropism in plants
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More Gravitropism

• Calcium also plays a role in gravitropism -
  calcium movements are mediated by the calcium
  binding protein calmodulin - calcium moves to
  upper surfaces of shoot cells before the shoot
  actually curves upward and calcium moves to
  bottom surfaces of root cells before the root
  actually curves downward

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More Gravitropism

• The perception of gravity is correlated with
  sedimentation of amyloplasts (starch containing
  plastids) within certain cells of the shoot and
  root - such cells are found near the vascular
  bundles in shoots and in the root cap of roots,
  particularly the central column of the root cap
• Amyloplasts are sedimented to the transverse
  walls (perpendicular to root surface) in the root
  cells - if the root is placed in a horizontal
  position, these amyloplasts slide downward and
  come to rest on what were once vertically
  oriented walls - within hours the root curves
  downward through growth of upper cells and the
  amyloplasts return to their previous position

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Thigmotropism

• Thigmotropism is response to touch or contact
  with a solid object - this is most commonly seen
  in tendrils, modified leaves or stems depending
  on the species - the tendrils wrap around any
  object they come in contact with and so enable
  the plant to cling and climb - the response can
  be rapid, a tendril can wrap around a support one
  or more times in less than an hour

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Circadian Rhythms

• Many plant activities seem to occur on a daily
  schedule - some plants open their leaves at dawn
  and shut them at dusk - some plants may open and
  shut flowers as the day changes from dawn to dusk
• Photosynthesis, auxin production, and the rate of
  cell division all have regular daily rhythms and
  these rhythms continue even when environmental
  conditions are kept constant

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More Circadian Rhythms

• Entrainment occurs when a periodic repetition of
  light and dark (or some other external cycle)
  causes a biological clock to remain synchronized
  with the same cycle as the entraining factor
• Light-dark cycles and temperature cycles are the
  principle sources of entrainment
• The main advantage of a biological clock is that
  it allows the plant to respond to the changes in
  seasons by accurately measuring changing day
  length - thus changes in the environment trigger
  responses that result in adjustments of growth,
  reproduction, and other activities of the organism

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Photoperiodism

• Photoperiodism is a biological response to a
  change in the proportions of light and dark in a
  24-hour daily cycle - photoperiodism was first
  discribed in plants and has since been found in
  insects, worms, fish, birds and mammals

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Plants have three basic patterns of flowering

• Short-day plants will only flower if the light
  period is shorter than a critical length - they
  typically flower in spring or fall
• Long-day plants will only flower if the light
  period is longer than a critical length - they
  typically flower in the summer
• Day-neutral plants flower without respect to day
  length

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Temperature Control

• Often photoperiodism works in concert with
  temperature to control plant growth and
  development
• For millennia, variations in day length with each
  passing month have been as regular as the suns
  shifting position in the sky and in temperate
  climates, an annual winters chill can be
  reasonably expected to occur.
• The consistent occurrence of such events makes
  them conditions to which the physiology of plants
  has become adapted, using them as external cues
  to turn on internal processes

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California redbud in bloom
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Aging and Ripening

• When fruits ripen and when leaves are prepared
  for abscission (separation from stems) they
  undergo a process of aging or senescence a
  process directed by hormones, often in response
  to external cues

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Plant Hormones and Senescence

• Once senescence is begun it is irreversible so
  plants must be able to tightly control when
  senescence begins. Some plant hormones inhibit
  senescence cytokinin (mainly promotes cell
  division), auxin, and gibberellin all function to
  maintain normal function and cell structure in
  plants.
• Two other plant growth regulators act to promote
  senescence ethylene a gas and abscisic acid
  named because it was thought to promote leaf
  abscission in all plants. But now it appears
  that ethylene is more commonly used for that.

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Fall Foliage
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Dormancy

• Dormancy is a special condition of arrested
  growth
• Plants do not grow at the same rate all the time
  - during unfavorable seasons, they limit their
  growth or cease to grow altogether - this allows
  plants to survive water scarcity or low
  temperatures
• Normally, after a period of rest (no growth),
  growth resumes when the temperature becomes
  milder or water becomes available
• In contrast, a dormant seed or bud can be
  activated only by a precise environmental cue or
  set of cues

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Dormancy