Chapter 36

Overview of plant behavioral responses

• Behavior is a response of an organism to an internal or external stimulus
• types of plant behavior
  • movement
    • bending,twisting, or rotating
      • nutation
    • rapid movement as in sensitive plants
      • response to touch
  • growth
  • seed germination
  • seasonal production of reproductive structures
  • defensive responses to attacks
    • thorns, spines, chemicals

Responses to internal and external stimuli

Internal

• Internal biological clock
  • circadium rhythms
• chemical signals
  • transcriptions factors and other proteins or hormones
  • often interact with each other and external signals

External

• light atmospheric gases (CO2 and water vapor) temperature, touch, wind, gravity, water, rocks, and soil minerals
• Herbivors, pathogens, organic chemicals from neighboring plants, and beneficial or harmful organisms

Plant Behavior

Involves internal and external stimuli

• tropism
  • growth response that is dependent on a stimuli that occurs in a particular direction
• Reception molecules
  • located in plant cells
  • sense stimuli and cause response

Phototropism

• Growth response to light
• light causes movement of hormone auxin away from said light
• result in unequal distribution of auxin
  • causing unequal cell elongation
• positive tropism

Gravitropism

• growth response to gravity
• positive tropism
  • roots
• negative tropism
  • shoots
• columella cells in root cap/tip region sense gravity 

Thigmotropism 

• Growth response to touch
• roots
  • columella cells cause roots to grow around obstacles

Regulation of plant growth

Hormones

• chemical messengers that regulate plant growth
  • most transported in phloem tissue
  • all require an expenditure of energy on part of the plant (ATP) for transport
• interact with external environmental stimuli

Hormones control

• growth
• seed germination
• flowering
• fruiting
• shedding of leaves
• color change of leaves

Hormones of two broad categories

• growth inhibiting
  • mostly fall/winter
  • certain times of the year growth is not good
• growth promoting
  • mostly spring/summer

Auxins

• first group of plants hormones to be described
• growth promoting
• produced in 
  • shoot tips, seeds, fruits, leaves, stem
  • NOT in the roots

Effects of auxin

Promotes

• cell elongation
• shoot elongation
• production of wood
• fruit development

Inhibits

• lateral bud development
• absission (falling off) of leaves, flowers, fruits

Cytokinins

• Originally detected in coconut "milk"
• growth promoting
• prodiced in
  • seed, fruits, roots

Effects of Cytokinins

Promotes

• cellular division
  • named derived from Cytokenesis

Inhibits

• senesence
  • change of color due to breakdown of pigments

Gibberellins (giberellic acids)

• many types
  • >200
  • more than any other group
• growth promoting
• found throughout the plant but concentrated in seeds

Effects of Gibberellins

Promotes

• stem elongation by cell division and cell elongation

• intake of water causes swelling and embryo hydration
• embryo secretes gibberellins
• gibberellins transported to cells of aleurone layer to secrete enzyme
• (alpha-amaylase) for breakdown of endosperm (starchy stored food) to glucose
• embryo will respire glucose to produce ATP
• embryo is directing the timing of plant germination
• Advantage seed plants

Brassinosteriods

• growth promoting

Effects of Brassinosteriods

Promotes

• cell expansion
• shoot elongation
• xylem tissue development
• stress response

Inhibits

• leaf abscission

Abscisic Acids (ABA)

• Growth inhibiting
• found in large quantities in seeds. mature leaves, and dormant buds

Effects of ABA

Promotes

• senesence
• production of storage molecules in seeds

Inhibits

• cell elongation
• alpha-amaylase production

Ethylene

• growth inhibiting
• actually a gas produced by incomplete metabolism
• interacts with the 4 growth promoting hormones to determine cell size and shape

Effects of Ethylene

Promotes

• fruit ripening
• abscission of leaves, fruits, flowers

Seed germination

• requires breaking of dormancy
  • combination of internal and external factors

Internal

• hormones
• stored food
• H2O absorption
• embryo swelling

External

• sunlight
• temperature
• longer day light
• soil moisture

Generalized Seed

• Seed coat(s)
• as seed coat cracks
• Radical comes out first
  • then then shoot

Seedling

• result of cellular reproduction and increase size
• internal development
  • cells>tissues>organs>organism