Horticulture Science Lesson 13 Understanding Plant Growth Regulators

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Horticulture Science Lesson 13Understanding
Plant Growth Regulators
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Interest Approach
Start a discussion on the obvious effects of
human hormones. Ask what the students know about
estrogen, testosterone, adrenaline, etc. Ask
students to think about differences between
little boys and men and between little girls and
women. Reinforce the concepts that human hormones
are very potent, have a lot of individual jobs,
and are found in extremely small amounts. Now ask
about plant hormones. Note the silence. Tell the
students that plants do indeed have hormones and
that they may cause even more dramatic changes in
plants than humans experience.
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Student Learning Objectives

• Describe the work of plant growth regulators.
• Compare the functions of plant hormones.
• Examine commercial uses for plant growth
  regulators.

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Terms

• abscisic acid
• abscission layer
• apical dominance
• auxins
• cytokinins
• ethylene
• etiolation
• gibberellins

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Terms

• gravitropism
• growth retardants
• phototropism
• plant growth regulators
• plant hormones
• rooting compounds
• synthetic growth regulators
• thigmotropism
• tropism

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What do plant growth regulators do?

• Plant growth regulators are chemicals that affect
  the plant in many complex ways.
• They can control such activities as cell division
  and differentiation, root and shoot growth,
  flowering, and ripening.

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What do plant growth regulators do?

• Naturally occurring chemicals that regulate all
  growth and development of plants are called plant
  hormones.
• Hormones are moved around the plant in extremely
  low concentrations.
• Some plant growth regulators are man-made
  synthetic growth regulators.
• These synthetic growth regulators can be applied
  to plants to get a myriad of changes.

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What are the functions of several plant hormones?

• Several hormones are made by tissues of the
  plant.
• Each plant hormone has many different jobs.
• Auxins are growth hormones that are produced by
  the tip or apical meristem of the stem.
• Auxins cause stem cells to elongate and divide.
• They also flow down from the tip of the stem,
  preventing lateral buds from sprouting.

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What are the functions of several plant hormones?

• Auxins set up a situation in which the middle
  stem grows at a greater rate, suppressing the
  side growth.
• This is called apical dominance.
• Apical dominance is why many conifers are
  pyramidal in shape.
• Apical dominance can be overcome by simply
  cutting off the dominant stem, losing the source
  of the auxin.

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What are the functions of several plant hormones?

• Auxins are responsible for a response called
  phototropism.
• Phototropism is a response by a plant to grow
  toward a light source while tropism is a growth
  response to an external stimulus.
• Sunlight slowly breaks down auxins.

• When the side with more auxins grows faster, the
  stem starts to bend toward the sun.
• The earths gravity elicits a plant response
  known as gravitropism in this situation, a plant
  placed on its side will still result in downward
  root growth and upward stem growth.

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What are the functions of several plant hormones?

• Auxins are involved in a behavior called
  thigmotropism.
• Thigmotropism is a plants response to a hard
  object.

• The repeated touch of an object causes less
  auxin to remain on that side of the stem.
• When the auxin side starts to grow at a greater
  rate, the plant grows toward the plant.

• This continues to happen until the plant is
  actually wound around the object.

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What are the functions of several plant hormones?

• Cytokinins are hormones that are mostly
  responsible for cell division and
  differentiation.
• Cytokinins are produced in the root tips and in
  seeds.
• They tend to travel up the stem.

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What are the functions of several plant hormones?

• Ethylene is a gas that affects the plant like a
  hormone.
• Ethylene is produced by ripening fruit and dying
  plant materials.
• Ethylene stimulates flowering in some plants and
  causes other fruits and flowers to ripen more
  quickly and evenly.

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What are the functions of several plant hormones?

• It is also associated with the abscission, or
  dropping, of leaves. (The abscission layer is a
  specialized layer of cells that extends across
  the petiole near its attachment to the stem.)
• Ethylene gas is why fruit will ripen faster in a
  paper bag than out on a counter.
• The bag concentrates the ethylene gas.
• Ethylene has a negative effect on cut flowers.
• It causes them to age more quickly, reducing
  their useful life.

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What are the functions of several plant hormones?

• Gibberellins are hormones that cause internodal
  elongation and cell division.
• Gibberellins are produced in stems, roots, and
  young leaves.
• Gibberellins are commonly used on commercially
  grown dessert grapes to spread the fruits out and
  cause them to be bigger.

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What are the functions of several plant hormones?

• However, if light levels are low, gibberellins
  cause the internodes to lengthen.
• Stretching of internodes raises leaves to a
  position where they can capture light.

• Plants grown in low light or darkness develop a
  severe case of stretching known as etiolation.

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What are the functions of several plant hormones?

• Abscisic acid is a hormone that inhibits growth.
• It is found in seeds that are dormant and in
  dying leaves.
• It also appears to help a plant prepare its buds
  for the winter.

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What are several commercial uses for plant growth
regulators?

• Plant growth regulators are very useful for
  commercial plant crops.
• They can save money and time while leading to a
  better crop.
• These are some commercial uses for growth
  regulators.

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What are several commercial uses for plant growth
regulators?

• Growth regulators (growth retardants) are
  routinely sprayed on crops such as poinsettias,
  Easter lilies, and chrysanthemums to make
  shorter, bushier, and more attractive plants.
• Products such as A-rest, B-nine, Cycocel, and
  Florel are commonly used.

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What are several commercial uses for plant growth
regulators?

• Synthetic auxins (rooting compounds) are used in
  the horticulture industry to promote rooting of
  cuttings.
• Cuttings are treated with synthetic growth
  regulators to increase the number of cuttings
  that form roots, to speed rooting, to increase
  the number and quality of roots, and to increase
  the uniformity of the roots.

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What are several commercial uses for plant growth
regulators?

• The two synthetic root-promoting materials most
  widely used are naphthaleneacetic acid (NAA) and
  indolebutyric acid (IBA).
• IBA is regarded as the best material for general
  use.
• It can be used with a wide variety of plants and
  is nontoxic over a wide range of concentrations.
• These materials are available in a liquid
  formulation or mixed with talc.
• Two commercially available products are Rootone
  and Hormodin.

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What are several commercial uses for plant growth
regulators?

• Ethylene gas is used commercially to ripen
  bananas once they get to market and to induce
  flowering in pineapple crops.

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Review/Summary

• What do plant growth regulators do?
• What are the functions of several plant hormones?
• What are several commercial uses for plant growth
  regulators?