Plant Science

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Unit

• Plant Science

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Problem Area

• Managing Plant Growth

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Lesson

• Homeostatic Responses of Seedling Plants The
  Tropisms

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Student Learning Objectives

• 1. Explain plant tropisms.
• 2. Identify the different tropisms affecting
  plant growth.

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Terms

• Negative tropism
• Photropism
• Positive tropism
• Riboflavin
• Statocytes
• Thermotropism
• Thigmotropism
• Traumatropism
• Tropism
• Skototropism

• Aerotropism
• Amyloplasts
• Auxins
• Chemotropism
• Coleoptile
• Electrotropism
• Geomagnetotropism
• Geotropism
• Hydrotropism

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What are plant tropisms?

• Plants demonstrate sensitivity to stimulation.
  Environmental conditions and external stimuli
  cause plant growth to be altered. Plant responses
  to the external stimuli are known as tropisms.
• A. A positive tropism is movement or growth
  toward the stimulus
• B. A negative tropism is movement or growth away
  from a stimulus

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• C. Under ideal environmental conditions, the
  external stimuli of light and gravity will
  equally affect the shoot and root development
  allowing the plant to grow vertically.
• With equal stimuli, growth regulators are
  produced in equal proportions allowing for the
  cell elongation of the plant to occur at an equal
  rate. A tropic response results in an imbalance
  of growth regulators produced due to external
  stimuli.

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How do tropisms affect plant growth?

• Tropisma growth response (toward or away from)
  an external stimulus that determines the
  direction of growth.
• A. Phototropismgrowth toward a light source due
  to unequal cell elongation. Plants have at least
  three different photo-receptor mechanisms
  involving three separate groups of pigments.
  These include phytochrome, which controls the
  plants developmental sequence. Plant response to
  lights depends on the exposure time and intensity
  of light.

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• Auxins, a plant hormone that promotes cell
  elongation produced naturally by plants or
  synthetically, will accumulate on the side of the
  stem in shade. Light decreases the auxin
  sensitivity of cells on the lighted side.
  Photoreceptors allow a growth response toward
  light. Using monocotoleydons, research has shown
  the coleoptile tip to contain auxin.
• Riboflavin, a photoreceptor and yellow pigment,
  is also found in the tip of the coleoptile, a
  protective sheath surrounding the emerging shoot
  of grass seedlings. Riboflavin is the catalyst
  which aides in the transportation of the auxin
  away from the lighted side of the stem. With the
  light source driving auxin to the shaded side,
  the abundance of auxins will cause the cells to
  elongate. Due to this elongation, the stem will
  bend toward the light source.

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• B. Geotropism (Gravitropism)plant growth
  response to gravity. Plants demonstrate positive
  geotropism through their root development and
  negative geotropism in shoot development.
• Statocytes, specialized cells that function to
  control the direction of growth by perceiving
  gravitational forces and containing amyloplasts,
  are found in the root cap and in the plants
  stem.
• Amyloplasts, colorless subcellular particles
  containing starch grains that influence the
  direction of plant growth, due to gravity
  position themselves on the lower side of the
  statocytes. When the root cap is removed,
  response to gravitational forces is negated.

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• When centrifugal forces are applied, horizontal
  plant growth is demonstrated. Stem nodes can also
  exhibit differential growth to regain vertical
  position of the plant stem when plants are
  knocked down by external forces.
• As soon as the root or shoot of a plant detects
  that it is not in a vertical position,
  growth-regulating processes restore it to a
  vertical course. In vertically positioned shoots,
  auxin is equally distributed to the sub-apical
  cells. In horizontal positions, auxin is found in
  greater concentrations on the lower side of the
  shoot, thus creating faster growth on the lower
  side and a return to vertical growth.

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• C. Thigmotropismplant response to touch.
  Diverting the direction of a plant, mechanical
  stimuli can cause plant growth patterns to be
  altered. Coiling in vine plants around an
  external object can also occur due to
  thigmotropism. Example Cucumber plant tendrils
  will wrap around an object.
• D. Hydrotropismplant response to water. Roots
  will exhibit a positive tropic response in
  relation to higher concentrations of water. While
  plant roots have been shown to apparently seek
  out and grow toward water, some plant
  physiologists today doubt that responses to water
  are true tropisms. The response of roots to water
  sources is not clearly understood.

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• E. Thermotropismplant response to temperature.
  Weeds will demonstrate horizontal growth in lower
  temperatures and vertical growth with an increase
  in temperature.
• F. Electrotropism is a response to electricity.
• G. Chemotropism is a response to chemicals
• H. Traumatropism a response to wounding
• I. Aerotropism is a response to oxygen
• J. Skototropism is a response to dark
• K. Geomagnetotropism is a response to magnetic
  fields

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Review/Summary

• What are plant tropisms?
• How do tropisms affect plant growth?