Abscisic Acid Slows or inhibits plant growth.

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Homework

• Read Chapter 39
• Exam 3 will be moved back to April 17.
• Lab worksheets - today
• Chapter 39 today
• Chapter 36 today

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Chapter 39

• Plant Responses to Internal and External Signals

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Question

• Do plants sense and respond to their environment
  ?
• Yes - By adjusting their pattern of growth and
  development.

In Dark
In Light
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Comment

• Plants cant move away from a stimulus, but can
  change their growth response.
• Result plant bodies are more flexible in
  morphology than animals.

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Cell Signaling

• Comment most of the items in this chapter are
  examples of cell signaling. Review chapter 11
  for details if necessary.

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Classical Example

• Phototropism - plant growth response to
  unilateral light.
• Observation plants bend or grow towards the
  light.

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Phototropism Experimenters

• Darwins late 1800's.
• Boysen Jenson early 1900's.
• F.W. Went 1926

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Mechanism of Phototropism

• Cells on the dark side elongate faster than the
  cells on the light side.
• The uneven growth rate causes the bending of the
  stem toward the light.

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Question

• What is the adaptive value of phototropism ?
• It tilts the leaves toward the light source for
  more efficient photosynthesis.

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Cause of Phototropism

• Chemical messenger from the tip caused the growth
  response in the stem.
• The distribution of the chemical changes in the
  unequal light, resulting in unequal cell
  elongation.

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Went Experiments
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Hormone

• Chemical signal produced in one location,
  transported, has effect in another location.
• Phototropism is caused by a plant hormone.

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Plant Hormones

• Are produced in small quantities.
• Effects may reflect balance between several
  hormones.

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Classic Plant Hormones

• 1. Auxins
• 2. Cytokinins
• 3. Gibberellins
• 4. Abscisic Acid
• 5. Ethylene

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Auxins

• Named by Went in 1926.
• First plant hormone described.
• Ex IAA (natural) 2,4-D (synthetic)

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Major Functions

• Stimulates cell elongation.
• Fruit development.
• Apical Dominance.
• Tropism responses.
• Stimulates root formation in cuttings.
• Broadleaf plant killer.

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Apical Dominance
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Where Produced

• Apical Meristems.
• Young leaves.
• Embryos.

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Cytokinins

• Isolated from coconut "milk" (endosperm) in the
  1940s.
• Named because they stimulate cell division.
• Ex Zeatin

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Major Effects

• Stimulates cell division.
• Delays senescence.
• Root growth and differentiation.
• Where Produced - roots

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Auxin/Cytokinin Ratios

• Control shoot or root differentiation in tissue
  cultures.

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Gibberellins

• Found from the "Foolish Seedling" disease in
  rice.
• Ex GA3 70 types known

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Major Effects
Extra GA3

• Internode elongation.
• Seed/Bud germination.
• Flowering (some species).
• Fruit development.

No GA3
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Have GA3
Lack GA3
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Where Produced

• Apical Meristems.
• Young leaves.
• Embryos.

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Abscisic Acid

• Slows or inhibits plant growth.
• "Stress" hormone produced under unfavorable
  conditions.

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Major Effects

• Inhibits growth
• Seed/Bud dormancy.
• Stomata closure.
• Leaf drop produces abscission layer.

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Where Produced

• Leaves
• Stems
• Green fruit

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Ethylene

• Gaseous hormone (fast diffusion
  rates).
• Often interacts with Auxin.

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Major Effects

• Fruit ripening.
• Accelerates Senescence.
• Stem/Root Elongation ( or -).

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Where Produced

• Ripening fruits.
• Senescent tissue.
• Nodes.

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New Hormone

• Brassinosteroids steroid hormones similar to
  animal sex hormones.
• Function
• Cell elongation and division in stems and
  seedlings.

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Commercial Applications of Plant Hormones

• Weed killers
• Seedless fruit
• Rooting of cuttings
• Tissue culture

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Plants and Light

• Responses to light are critical for plant
  success.
• Plants can detect not only the presence of light,
  but its direction, intensity and wavelength.

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Two major light receptors

• 1. Response to blue light as in phototropism

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• photoperiodism a physiological response to
  changing day lengths.
• Advantages matches growth response to proper
  season. Ex leaf drop in fall or flowering

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Flowering Types

• 1. Short - Day Plants
• 2. Long - Day Plants
• 3. Day - Neutral Plants

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Short-Day Plants

• Flower when days are shorter than a critical
  period (long nights).
• Ex Mums Poinsettias

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Long-Day Plants

• Flower when days are longer than a critical
  period (short nights).
• Ex Spinach Iris Lettuce

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Day-Neutral Plants

• Flower whenever they have enough energy.
• Ex Roses African Violets

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Night Length

• Actually controls flowering response, not day
  length.
• Proof experiments show that if you interrupt
  the dark period, you reset the clock.

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Comment

• Length of night not absolute, but relative for
  the response to be triggered.

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Question

• What detects day/night length changes?
• Phytochrome - plant pigment involved with
  photoperiodism.
• Phytochrome is a red light receptor.

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Phytochrome Forms

• Pr - responds to 660nm (red light).
• Pfr - responds to 730nm (far red).

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Phytochrome

• Changes between the two forms.
• Ratio or accumulation of enough Pfr triggers the
  responses

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• In Red light Pr ?
  Pfr
• Far-red light or darkness
  Pfr ? Pr

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Photoperiodism

• Very sensitive (1 minute
  difference).
• Sets clocks for plant responses.

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Other Effects

• Seed Germination see pg 836 in text
• Stomatal Opening
• Leaf Drop

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Tropisms

• Growth responses in response to external stimuli.
• toward a stimulus
• - away from stimulus
• Ex 1. Phototropism
• 2. Gravitropism
• 3. Thigmotropism

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Gravitropism

• Response to gravity.
• Stems are gravitropic and roots are
  gravitropic.

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Gravitropism - mechanism

• Statolith movement may be the receptor for the
  stimulus.

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Thigmotropism

• Response to touch.
• A series of 5 genes are involved.
• Ex Tendrils Climbing stems Wind
  direction response of stems.

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Plants need to be touched to stay short

• Which one was touched?

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Responses to Stress

• Stress an environmental condition that can have
  an adverse effect on a plants growth,
  reproduction and survival.

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Plant Response

• 1. Developmental changes
• 2. Physiological changes
• Study the next few slides and have an example
  ready for testing.

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Water Deficit or Drought

• During high Ts, guard cells may close.
• Young leaves may slow expansion.
• Leaves may roll to reduce surface area.

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Oxygen Deprivation

• Common in roots in water-logged soils.
• Air tubes in roots may bring oxygen to the cells.

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Salt Stress

• Damages the plant through unfavorable soil water
  potentials and toxic ions.
• Some plants can concentrate and excrete salt
  through salt glands (ex. halophytes).

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Heat and Cold Stress

• Heat - use heat-shock proteins to protect other
  proteins from denaturing.
• Cold lipid shifts to keep lipid bilayers
  liquid.
• Cold solute changes to lower freezing point.

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Herbivores

• Plants have many physical and chemical defenses
  against herbivores.
• Physical thorns
• Chemical crystals, tannins and other toxic
  compounds.

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Herbivores

• Often trigger a plant to release chemicals to
  attract predators or to warn other plants to
  increase their production of toxins.

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Pathogens

• First Defense epidermis
• Second Defense chemical events to restrict or
  kill the invader.

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Summary

• Know the general plant hormones and their
  effects.
• Know photoperiodism.
• Know tropisms.
• Know general ideas about how plants respond to
  stress.