Plant Responses

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Plant Responses
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Apical dominance
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Plant Hormones

• Hormones are defined as chemical messengers that
  coordinate the different parts of a multicellular
  organism. They are produced by one part of the
  body and transported to another.
• Signal transduction is the series of steps
  between a plant's perception of an environmental
  change and the plant's response to that change
• Plant hormones help coordinate growth,
  development, and response to stimuli.
• There are 5 main plant hormone classes auxins
  (IAA), cytokinins, gibberellins, abscisic acid
  and ethylene.
• See p.759

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Hormone Where Found in Plant Major
Functions Auxin Seed embryo, young leaves,
Stimulates cell elongation involved in
meristems of apical buds phototropism,
gravitropism, apical dominance, and vascular
differentiation stimulates ethylene
synthesis regulates fruit
development Cytokinin Synthesized in roots and
Stimulates cell division, stimulates
germination, transported to other organs
growth, and flowering, delays leaf senescence
Gibberellin Meristems of apical buds
and Stimulates shoot elongation, stimulates
roots, young leaves, embryo flowering and
development of fruit, promotes seed and bud
germination Ethylene Tissues of ripening
fruits, Stimulates fruit ripening, leaf and
flower nodes of stems, senescent senescence,
opposes some auxin effects leaves and
flowers Abscisic Acid Leaves, stems, green
fruit Stimulates stomate closure during water
stress, inhibits growth, promotes seed
dormancy
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The effect of gibberellin on Thompson seedless
grapes
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Light Responses

• A tropism is a plant growth response that results
  in the plant growing either toward or away from a
  stimulus.
• Phototropism is the growth of a shoot in a
  certain direction in response to light.
• Positive phototropism is the growth of a plant
  toward light negative phototropism is the growth
  away from light.
• The direction of growth is due to the fact that
  cells on one side of the plant elongate more
  quickly than the cells on the other side
  (fig.35.2)
• Responses to light are critical for plant success
• Photomorphogenesis is the term used to describe
  the effects of light on plant morphology
• Blue light has the greatest effect on plant
  growth and movement.
• Phytochromes are pigments that are involved in
  many of a plants responses to light.

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Grass seedling growing toward light
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Photoperiods

• A physiological response to a photoperiod (the
  relative lengths of night and day), such as
  flowering, is called photoperiodism.
• Note It is night length - NOT day length - that
  controls flowering and other responses to
  photoperiod
• Short-day plants require a period of light
  shorter than a certain critical length in order
  to flower.
• Long-day plants flower in the late spring or
  early summer they require the most daylight to
  flower.
• Day-neutral plants can flower in days of any
  length.

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Sleep movements of a bean plant (Phaseolus
vulgaris)
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Other Stimuli

• Gravitropism is a plants response to gravity.
• Roots show positive gravitropism and shoots show
  negative gravitropism
• Thigmomorphogenesis is the change in form of a
  plant that results from mechanical disturbance.
  Even simply repeatedly touching a plant with a
  ruler to measure its height can affect its growth
  pattern
• Thigmotropism is directional growth in a plant as
  a response to a touch.
• This phenomenon is clearly illustrated by the
  climbing tendrils of some plants, such as the
  sweet pea. The tendrils actually "feel" the solid
  object, which results in the coiling response.
• Human skin can minimally detect a thread weighing
  0.002mg being drawn across it, but a feeding
  tentacle of the insectivorous sundew plant
  responds to a thread of 0.0008mg. Some plants
  have a more sensitive sense of touch than humans!
• Thigmonasty is touch induced movement rather than
  growth (mimosa leaf)
• Mimosa Video Clip

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Rapid turgor movements by the sensitive plant
(Mimosa pudica)
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Other Responses

• In times of drought, the guard cells lose turgor.
  This causes the stomata to close. Young leaves
  stop growing, and they will roll into a shape
  that slows transpiration rates. Deep roots
  continue to grow, while those near the surface
  grow slowly, if at all.
• In times of flooding, certain cells in the root
  cortex die, which creates air tubes that bring in
  oxygen and enable the plant to continue cellular
  respiration.
• Plants respond to salt stress by producing
  organic compounds that keep the water potential
  of cells more negative than that of the soil
  solution, which encourages water to flow into the
  roots. Most plants cannot survive salt stress for
  long periods of time.
• In heat stress environments, plants produce
  heat-shock proteins, which which act as molecular
  chaperones to ensure that newly formed proteins
  are folded properly and mis-folded proteins are
  removed from the cell.
• In cold stress environments, plants respond by
  altering the lipid and protein composition of
  their cell membranes.