REGULATING GROWTH

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CHAPTER 26.2

• REGULATING GROWTH DEVELOPMENT IN PLANTS

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Plant Hormones

• The growth and development of a plant are
  influenced by genetic factors, external
  environmental factors and chemicals inside the
  plant.
• Plant growth and development are regulated by
  hormones, which are produced in one part of the
  organism and are transported to another part to
  exert control.

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Plant Hormones

• Plant hormones are
• organic compounds
• effective at very low concentrations
• able to interact with specific tissues to cause
  physiological responses
• stimulate or inhibit plant growth
• Plant hormones are produced in response to
  stimuli from the environment.
• It is the target organ that effects the action of
  the hormone not the hormone itself.

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Discovery of Plant Hormones

• Charles Darwin and his son studied plant
  hormones.
• They observed that grass seedlings would bend
  toward the light while growing.
• Results of their experiments suggested that
  something in the tip of a shoot is responsible
  for sensing light.

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Groups of Hormones

• There are five major groups of hormones
• Auxins
• Gibberellins
• Ethylene
• Cytokinins
• Abscisic acid

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• Auxins
• produced in apical
• meristems
• cause elongation of
• cells by ?? the plasticity
• of the cell wall allowing
• them to stretch
• promote apical dominance inhibit lateral growth
• - Christmas trees
• Other effects
• increase number of fruits
• prevent sprouting of stored potatoes
• promote root formation on stem cuttings

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• Gibberellins
• produced in developing shoots and
• seeds

• promote abnormal stem elongation - bolting to
  produce flower heads
• Other effects
• increase size of fruit
• overcome bud dormancy
• cause seed germination

Unsprayed
Sprayed with hormone
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• Ethylene (gas)
• produced in most plant tissues
• stimulates fruit ripening One bad apple
• Other effects
• promotes abscission the detachment of leaves,
  flowers
• or fruits from a plant

Campbell p. 759 760
Tomato untreated treated
with ethylene
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• Cytokinins
• produced in root tips
• stimulate cell division
• promote lateral growth
• auxin-cytokinin ratio important for normal growth
  and development
• used to keep cut flowers fresh
• Abscisic Acid
• Promotes dormancy in plants seeds

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Plant Movements

• Plants are anchored ? growth must be adjusted to
  changes in environment.
• A tropism is a growth response in which the
  direction of growth is determined by the
  direction from which the stimulus comes. It is
  usually triggered by hormones.
• Movement toward a stimulus is a positive tropism.
• Growth away is a negative tropism.

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• Phototropism
• growth response to light
• caused by auxin migrating to dark side of stem
  causing cell elongation on that side
• growth toward
• light phototropism
• growth away from light - phototropism

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• Gravitropism
• growth response to gravity
• shoot - gravitropism
• root gravitropism
• caused by auxin migrating to lower area
  stimulating stem growth inhibiting root growth
• Thigmotropism
• growth response to touch
• ex. pea tendrils wrapping around a wire
• Hydrotropism
• growth response to water
• roots of trees can grow into water pipes

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Seasonal Responses

• Plants mark the seasons primarily by sensing
  changes in night length because these stimuli are
  more reliable than temperature or precipitation
  cues.
• Photoperiodism is the response of plants to
  length of days and nights.

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Seasonal Responses

• Plants can be categorized as one of three types,
  depending upon how they respond to day/night
  length.
• short day plant - flowers when days are shorter
  (spring and fall) and nights are longer than
  critical period
• long day plant - flowers when days are longer
  (usually summer) and nights are shorter than
  critical period
• day neutral plants - flower when mature,
  regardless of day length

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• It is the length of night which is crucial not
  the day length

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Flowering, etc

• Changing fall colors are caused mainly by a
  photoperiodic but also a temperature response.
• Chlorophyll production stops and as it degrades,
  other pigments, present in the leaf all summer,
  appear. Depending upon the leaf red, orange,
  and/or yellow might become visible.

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Flowering, etc.

• Florigen is a hormone produced in the leaves,
  suspected of influencing the flowering process.
  It has yet to be isolated but experimental
  evidence indicates its probable existence.
• Depending on how long they live, plants can be
  classified as one of three types
• annuals complete their life cycle (grow, flower,
  produce fruit seeds) within one growing season.
• biennials take two years to complete their life
  cycle, producing fruit seeds the second year.

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Flowering

• perennials live for more than one growing season,
  reproducing many times.
• Some perennials drop their leaves at the end of
  each growing season every year deciduous
• Some perennials drop a few leaves at a time
  throughout the growing season evergreen