Understanding Plant Growth Regulators

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• Understanding Plant Growth Regulators

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Interest Approach

• Lets talk for a moment about yourself. What are
  some differences between your present body and
  the one you had as a child?
• What kinds of chemicals were involved in those
  changes?
• These chemicals are called hormones for example,
  estrogen, testosterone, adrenaline, etc.

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Continued

• Plants also experience growth changes with the
  help of hormones
• Sometimes these changes are more dramatic than
  what you experienced!
• Take a look at this plant to the right which fell
  over the night before. What happened to the
  plant?

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Vocabulary

• Abscisic acid
• Apical dominance
• Auxins
• Cytokinins
• Ethylene
• Gibberellins

• Phototropism
• Plant hormones
• Thigmotropism
• Geotropism

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Student Objectives

• 1. Describe the work of plant growth regulators
• 2. Explain the functions of several plant
  hormones
• 3. List several commercial uses for plant growth
  regulators

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What Do Plant Growth Regulators Do?

• Plant growth regulators are chemicals which
  affect the plant in many complex ways
• They can control such activities as cell division
  and differentiation, root and shoot growth,
  flowering and ripening
• Plant growth regulators which are made by the
  plant are called plant hormones
• They are moved around the plant in very low
  concentrations

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• Some growth regulators are man-made, or synthetic
• They can be applied to plants to obtain a wide
  variety of changes
• These changes often make the plant better or more
  saleable!

Which grapes would you probably buy?
The ones on the right were treated with
gibberellic acid to make them larger.
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What Are the Functions of Several Plant Hormones?

• Several hormones are made by tissues of the plant
• Each hormone has many different jobs, thus they
  are very much like human hormones
• There are five groups of plant regulators
  auxins, cytokinins, ethylene, gibberellins and
  abscisic acid

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Auxins

• This growth hormone is produced by the tip, or
  apical meristem, of the stem
• It causes the stem cells to elongate and divide
• They also flow down from the tip of the stem,
  preventing lateral buds from sprouting until
  needed
• There are three major affects caused by auxins on
  the plant

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Effects of Auxin on Plants

• 1. Auxin causes the tip of the middle stem to
  grow at a faster rate
• This is known as apical dominance
• Apical dominance is why many conifers have a
  pyramid shape
• It can be overcome by cutting off the dominant or
  terminal stem, losing the source of auxin

This trunk should be a single stem however, the
terminal bud was removed forcing it to branch.
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• 2. Auxins are responsible for allowing a plant
  stem to grow toward the sun
• This is known as phototropism
• Sunlight slowly breaks down auxin When the side
  not exposed to the sun grows faster, the stem
  bends towards the light

The left side of the plant was exposed to the
sun. The right side had shade and grew faster.
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• 3. Auxins allow a plant to respond to the touch
  of a person or other object
• This is known as thigmotropism
• The repeated touch of an object causes less auxin
  to remain on that side of the stem
• When the auxin side starts to grow faster, the
  plant grows towards the object and ultimately
  wraps around it

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Cytokinins

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

In tissue culture, cell division or root growth
can be encouraged by adjusting hormones in the
agar. If given higher levels of auxins in the
agar, roots are produced. If given higher levels
of cytokinins, shoots multiply.
Courtesy of McGraw Hill Publishers
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Ethylene

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

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Effects of Ethylene

• Ethylene gas is why fruit will ripen faster in a
  paper bag, than on the counter
• The bag helps to concentrate the gas in a
  specific area
• Ethylene has a negative effect on cut flowers
  foliages
• It causes them to age more quickly, reducing
  their useful life

In this experiment, two holly twigs were placed
under separate jars for a week. At the same
time, an apple was placed in the second jar. The
ethylene given off by the apple caused the holly
to lose its leaves and die.
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Gibberellins

• These hormones cause the internode of a stem to
  elongate and cell division to occur
• They are produced in the 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
• Flowering of plants and the breaking off of seed
  dormancy can also be achieved by adding
  gibberellins

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Effects of Gibberellins

• The cabbage plants on the left were grown on
  their own
• The plants on the right were given gibberellic
  acid once a week for eight weeks
• Notice the long stems and flowers at the top of
  the plants on the right
• They are a result of this hormone

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Abscisic Acid

• This hormone inhibits (prevents) growth
• It is found in seeds which are dormant and in
  dying leaves
• It also appears to help a plant prepare its buds
  for winter

The dormant bud on the left has a high
concentration of abscisic acid. As spring nears,
the acid level drops and the bud begins to
develop a new shoot.
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What are Several Commercial Uses for Plant Growth
Regulators?

• Synthetic growth regulators are very useful for
  commercial plant crops
• They can save money, time and can lead to a more
  sellable crop
• There are at least three commercial uses of
  regulators
• 1. Growth regulators are routinely sprayed on
  crops such as poinsettias, Easter lilies and mums
  to reduce size and make a shorter, bushier and
  more attractive plant
• Products such as A-rest, B-nine, Cycocel and
  Florel are commonly used
• Prevents greenhouse plants from getting leggy

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• 2. Growth regulators are commonly used to help
  plants root more completely
• These are often sold as a powder under the names
  Rootone and Hormodin
• 3. Ethylene gas is used commercially to ripen
  bananas, tomatoes once they get to market and to
  induce flowering in pineapple crops

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Summary

• What is a plant growth regulator called and what
  does it do?
• Where in a plant is auxin produced?
• Explain how apical dominance works in a plant.
• How is phototropism different from thigmotropism?

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Summary continued

• What do cytokinins control?
• How does ethylene effect plants?
• What does gibberellin cause in a plant?
• What does abscisic acid control?
• Name one way that hormones are used commercially
  to control plants.