Plant Science

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Unit

• Plant Science

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Problem Area

• Managing Plant Growth

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Lesson

• Regulating Plant Growth

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Student Learning Objectives

• 1. Understand plant growth regulators and plant
  hormones.
• 2. Identify different types of plant hormones.
• 3. Identify the uses of growth regulators on
  plants.
• 4. Define allelopathy.

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Terms

• Abscissic acid
• Abscission
• Allelopathy
• Alleurone layer
• Apical dominance
• Auxins
• Chlormequat
• Creosote

• Cuticle
• Cytokinins
• Daminozide
• Ethylene
• Exudation
• Gibberellins
• Internode
• Leaching

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Terms cont.

• Plant Growth Regulators
• Plant hormones
• Senescence
• Tannic acid
• Tensile strength
• Volatilization

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What are plant growth regulators and plant
hormones?

• Plant growth regulators are organic compounds,
  either natural, or synthetic, that modify or
  control one or more specific physiological
  processes with a plant.
• A. Natural plant growth regulators are produced
  by plants are plant hormones. Plant hormones are
  naturally occurring compounds produced by the
  plant to accelerate or retard the rate of growth
  or maturation.

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• 1. Plant hormones are produced in minute
  quantities in one part of a plant and then
  translocated to another part of the plant where
  growth and development are modified.
• 2. Plant hormones have many different effects on
  plant growth and development.
• B. Scientists simulate the naturally occurring
  plant hormones to artificially produce synthetic
  plant growth regulators.

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What are the different types and effects of plant
hormones?

• A. Auxins are produced in the apical meristem of
  a plants stem and migrate down stem.
• 1. Auxins cause cells to elongate.
• 2. Auxins are responsible for phototropic
  responses or the ability of a plant to bend
  toward a light source.
• 3. Auxin plays a role in apical dominance. Auxins
  will move down the stem and inhibit the growth of
  side shoots. Pinching off the apical meristem
  stops the flow of auxins and side shoots are free
  to develop.

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• 4. Auxins attach to cell walls and activate a
  series of processes that lower the tensile
  strength, resistance to lengthwise stress, of the
  cell membrane. This allows the cell to enlarge.
  Auxins, being acidic in nature, increase the
  acidity in the cell membranes making the cell
  wall less resistant to stretching from osmotic
  forces.
• 5. Auxins are commonly used to promote root
  growth. The faster a cutting develops roots, the
  better chance for survival. Plant propagators
  will apply auxins to the base of cuttings to
  promote root growth.
• 6. Auxins delay abscission (shedding) of leaves
  and maturing fruits. Auxins will work to
  breakdown the thin layer of cells in the
  abscission zone.

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• B. Gibberellins (gibberellic acid) are produced
  in stem and root apical meristems, in seed
  embryos, and in young leaves.
• 1. Gibberellins stimulate stem growth. They
  induce stem cell elongation, cell division, and
  control enzyme release.
• 2. Gibberellins produced in the embryo during the
  germination of cereal seeds moves to the aleurone
  layer, the outer layer of the endosperm,
  activating the synthesis of enzymes.
• 3. Seventy different gibberellins have been
  discovered to exist. A plant species will respond
  to only certain types of gibberellin.

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• C. Cytokinins are responsible for cell division
  and differentiation. Roots supply cytokinins
  upward toward the shoots.
• 1. Cytokinins cause cell enlargement, tissue
  differentiation, dormancy, and retardation of
  leaf senescence.
• 2. Cytokinins are most abundant in seeds, fruits,
  and roots.
• 3. The balances of auxins and cytokinins dictate
  whether cells will develop shoots, roots, or
  remain undifferentiated.
• 4. Cytokinins slow the process of senescence, or
  aging, by preventing the breakdown of chlorophyll
  in leaves.

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• D. Abscissic Acid is a growth inhibitor within
  the category of plant growth regulators.
• 1. Abscissic acid promotes dormancy in seeds and
  buds, and flowering in some short-day plants.
• 2. Abscissic acid prevents seeds from germinating
  in fruit.
• 3. Abscissic acid assists in maintaining water
  supplies within the plant. When a plant becomes
  stressed, greater amounts of abscissic acid are
  produced. Abscissic acid interferes with the
  availability of potassium in the guard cells,
  causing the stomata to close. Increased water
  supplies will breakdown the abscissic acid
  causing the stomata reopen.
• 4. Abscissic acid will slow or shutdown the
  metabolism in plants. When abscissic acid is
  reduced, metabolic processes will resume.

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• E. Ethylene is a water-soluable gas that moves
  readily throughout the plant. The cuticle, a
  thin, waxy layer of cutin that covers the
  epidermis, prevents loss of ethylene from plant
  tissue.
• 1. Ethylene is produced in ripening fruits,
  senescent flowers, plant meristems and at sites
  where plant or fruit injury occurs.
• 2. Ethylene is commonly used to promote fruit
  ripening and flower initiation.

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What are the uses of growth regulators on plant
tissues?

• Each hormone promotes many different responses,
  and each is effective in very low concentrations.
• A. There are many practical applications of
  auxins in the agricultural industry.
• 1. Auxins are used in the horticultural industry
  to promote rooting of cuttings.
• 2. Auxins are used to thin fruit blossoms,
  resulting in larger fruit, and in growing
  seed-less grapes.

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• B. There are numerous applications of
  gibberellins.
• 1. Gibberellins play an important role in the
  expression of hybrid vigor in corn plants. Larger
  growing hybrids contain higher concentrations of
  gibberellins than inbred plants.
• 2. Gibberellins stimulate the development of
  flowers.
• 3. Gibberellins are applied to grape vineyards
  increasing the berry size by thinning clusters
  and increasing internode, stem region between
  nodes, length, allowing for more space for the
  fruit to grow.
• 4. Gibberellins used on citrus crops assist in
  keeping the peels tough and resistant to molds.
  Gibberellins keep the peel green, and growers
  could use the ripening hormone ethylene to bring
  fruit to its normal color.

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• C. Cytokinins have numerous applications in
  todays agricultural industry.
• 1. Cytokinins are added to media for cell
  division to occur.
• 2. Cut flowers that lose their source of
  cytokinins are sprayed with cytokinins to extend
  their vase life.
• D. Ethylene has many applications for use in the
  agriculture industry.
• 1. Bananas are picked green for shipment to
  prevent bruising. They are treated with ethylene
  to promote ripening.
• 2. Cut flowers are never stored with ripening
  fruit or decaying leaves that might give off
  ethylene and shorten the life of the flowers.

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• E. Abscissic Acid has many uses in the
  agricultural industry.
• 1. Numerous synthetic growth inhibitors have been
  produced and used commercially in the
  horticulture industry. These synthetic regulators
  were developed from abscissic acid or the way it
  functions from the plant.
• 2. Daminozide retards growth and stimulates
  flowering in some plants, including
  chrysanthemums, bedding plants and azaleas.
• 3. Chlormequat (Cyclocel, CCC) is used to reduce
  the height of poinsettias and prevent lodging in
  wheat.

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What is allelopathy?

• Allelopathy is the release of chemicals by
  certain plants that inhibit the growth of
  competing plants.
• A. Plants release chemicals into the soil, water
  or air that have specific effects on neighboring
  plants (i.e. inhibiting growth or germination).
  Two examples of familiar chemicals that may be
  used for allelopathy by plants include tannic
  acid and creosote.

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• 1. Tannic acid is found in the leaves and bark of
  oak and sumac trees and is used to tan leather.
• 2. Creosote is the black tar-like substance put
  on telephone poles, railroad crossties and pier
  pilings to slow decay and rot. Creosote bushes
  are the dominant plant in the deserts of Texas.
  Scientists around the world are studying
  allelopathy as a natural way to inhibit weeds in
  crop fields and reduce chemical herbicide use.

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• B. There are several ways in which an
  allelopathic plant can release its protective
  chemicals
• 1. Volatilization Allelopathic trees release a
  chemical in the form of a gas through small
  openings in their leaves. Other plants absorb the
  toxic chemical and die.
• 2. Leaching All plants lose leaves. Some plants
  store protective chemicals in the leaves they
  drop. When the leaves fall to the ground, they
  decompose. As this happens, the leaves give off
  chemicals that protect the plant.
• 3. Exudation Some plants release defensive
  chemicals into the soil through their roots.
  Those chemicals are absorbed by the roots of
  other trees near the allelopathic one. As a
  result, the non-allelopathic tree is damaged.

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Review/Summary

• What are plant growth regulators and plant
  hormones?
• What are the different types and effects of plant
  hormones?
• What are the uses of growth regulators on plant
  tissues?
• What is allelopathy?