Plant Biology and Production

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• Plant Biology and Production

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Unit 3

• Seed Germination, Growth, and Development

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Lesson 5

• Examining the Roles of Plant Growth Regulators

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Interest Approach

• Several days before the introduction of this
  lesson prepare two identical plants for an
  interest approach.
• Select an upright growing potted herbaceous plant
  such as a coleus or tomato.
• Lay one of the two plants on its side.
• The plant on its side will curve upward in
  response to gravity.

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Interest Approach

• Present the two plants to the class and ask a
  series of questions.
• Ask the students why the one plant is curved.
• Is there an advantage to the plant for responding
  in this way?
• Is there a term used to describe the plant
  response?
• What caused it to curve?

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Terms

• Abscisic acid
• Apical dominance
• Auxins
• Cytokinins
• Ethylene
• Etiolation
• Indoleacetic acid (IAA)
• Indolebutyric acid (IBA)

• Gibberellins
• Gravitropism
• Phototropism
• Plant growth regulators
• Synthetic growth regulators
• Thigmotropism
• Tropisms

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Student Learning Objectives

• Identify plant growth regulators (PGR) and their
  functions.
• Explain plant tropisms.
• Discuss synthetic growth regulators.
• Describe commercial uses of plant growth
  regulators.

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What are Plant Growth Regulators and What are
Their Functions?

• I. Natural occurring or synthetic chemicals that
  regulate plant growth and development are plant
  growth regulators.

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What are Plant Growth Regulators and What are
Their Functions?

• A. Plant growth regulators promote growth,
  inhibit growth, and modify growth and
  development.
• Plants produce compounds to regulate growth.
• These natural chemicals are hormones.
• B. Plants produce five different hormones.

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What are Plant Growth Regulators and What are
Their Functions?

• 1. Auxins consist of a group of hormones that
  have a primary role in promoting cell elongation.
  
• The primary auxin is indoleacetic acid (IAA).
• Plants produce auxins in the apical meristem.
• They migrate down the stem moving through one
  cell to the next rather than being transported
  through the phloem or xylem.

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What are Plant Growth Regulators and What are
Their Functions?

• Auxins have the greatest influence on cells
  closest to the apical meristem.
• The higher concentrations of auxins give the
  apical meristem dominance over growth below.
• Apical dominance is a term used to describe the
  apical buds dominance over the lateral buds.

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What are Plant Growth Regulators and What are
Their Functions?

• 2. Gibberellins induce cell elongation and cell
  division.
• They are produced in stem and root apical
  meristems, seed embryos, and young leaves.
• Gibberellins play a key role in the development
  of flowers and in the production of enzymes
  during seed germination.

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What are Plant Growth Regulators and What are
Their Functions?

• 3. Cytokinins are hormones responsible for cell
  division and differentiation.
• They are produced in the roots and transported
  throughout the plant via the xylem.

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What are Plant Growth Regulators and What are
Their Functions?

• Cytokinins encourage the growth of lateral
  shoots.
• They inhibit the branching of the roots.
• As an ingredient in tissue culture medium,
  cytokinins are an important promoter of cell
  division.

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What are Plant Growth Regulators and What are
Their Functions?

• 4. Ethylene is a colorless gas that speeds the
  aging of plant parts, particularly fruit.
• It is produced in stems, ripening fruit, and
  dying leaves.

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What are Plant Growth Regulators and What are
Their Functions?

• 5. Abscisic acid is a growth-inhibiting hormone
  largely responsible for seed dormancy.
• It also causes stomates to close in times of
  drought in an effort to conserve water.

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What are Plant Tropisms?

• II. External stimuli and environmental conditions
  cause plants to respond in certain ways.
• Plant responses to the external stimuli are known
  as tropisms.

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What are Plant Tropisms?

• A. Phototropism is a plants response to the
  source of light.
• Auxins move down the shaded side of the plant
  stem.
• The presence of auxins causes the cells on the
  shady side of the stem to elongate more than
  cells than the bright side of the stem.

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What are Plant Tropisms?

• The result is a stem that bends towards the
  light.
• Under low light or dark conditions cells elongate
  on all sides of the stem.
• The result is an appearance of the stem
  stretching.
• Stretching due to low light is known as
  etiolation.

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What are Plant Tropisms?

• B. Gravitropism also referred to as geotropism is
  a plant response to gravity.
• The stems of plants laid on their side curve
  upward.
• In this scenario auxins settle to the bottom side
  of the stem and cause cells to elongate.

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What are Plant Tropisms?

• C. Thigmotropism is a response to mechanical
  stimuli.
• A good example is the tendrils of a cucumber
  plant.
• When the tendrils touch an object, the response
  is to curl around that object.

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What are Synthetic Growth Regulators?

• III. Other human-made chemicals that regulate
  plant growth are referred to as synthetic growth
  regulators.
• A. Many synthetic growth regulators have been
  developed to regulate plant growth.

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What are Synthetic Growth Regulators?

• 1. Two common root-promoting materials used in
  the propagation of plants are naphthaleneacetic
  acid (NAA) and indolebutyric acid (IBA).
• 2. Growth retardant chemicals inhibit cell
  elongation and keep plants compact. Many growth
  retardants are available.

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What are Synthetic Growth Regulators?

• 3. Some synthetic growth regulators serve as
  herbicides by altering growth and metabolic
  processes of weeds.

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How are Plant Growth Regulators Used Commercially?

• IV. Plant growth regulators are used in
  agricultural practices to improve the quality and
  quantity of yields.

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How are Plant Growth Regulators Used Commercially?

• A. Auxins in the forms of indoleacetic acid (IAA)
  and indolebutyric acid (IBA) are widely used to
  speed the rooting of cuttings in the horticulture
  industry.
• Naphthaleneacetic acid (NAA) is sprayed on apples
  to prevent pre-harvest drop of the fruit.

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How are Plant Growth Regulators Used Commercially?

• B. Gibberellins are used to induce flowering.
• Some plants that respond by flowering are
  carrots, endive, cabbage, turnips, and
  chrysanthemums.

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How are Plant Growth Regulators Used Commercially?

• Gibberellins serve as growth stimulants making
  plants, including sugar cane, grow larger.
• Gibberellins are used with some plant species,
  such as grapes, to produce larger fruit.

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How are Plant Growth Regulators Used Commercially?

• C. Cytokinins have been shown to extend the shelf
  life of lettuce.
• Cytokinins are also an important ingredient of
  tissue culture medium, as they promote cell
  division.

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How are Plant Growth Regulators Used Commercially?

• D. Ethylene is used in the ripening of fruits
  before being placed on grocery shelves.

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How are Plant Growth Regulators Used Commercially?

• E. Many synthetic growth regulators are used to
  regulate plant growth.
• Some promote growth.
• Others inhibit growth.
• Growth retardants are widely used in the
  horticulture industry to keep plants compact.

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Review / Summary

• 1. What are plant growth regulators and what are
  their functions?
• 2. What are plant tropisms?
• 3. What are synthetic growth regulators?
• 4. How are plant growth regulators used
  commercially?