9.3 plant chemical control

• 3 examples of plant growth substances? role?

  • bring about chemical control
  • Auxins
  • Cytokinins
  • Gibberellins
• functions of auxins?

  • trophic responses
  • controls cell elongation
  • suppress lateral buds to maintain apical dominance
  • promote root growth
• How do auxins cause cell elongation?

  • auxins bind to surface proteins
  • activates H+ pumps
  • lowers the pH of cell wall
  • activates enzymes to weaken or break hydrogen bonding
  • between cellulose fibres
  • more water taken in for growth
• what is the meristem?

  • region where most dividing cells are
  • in tip of roots and shoots
• phototropism

  POSITIVE = growth towards the light

  NEGATIVE = growth away from the light
• gravitropism

  POSITIVE = growth towards gravity

  NEGATIVE = growth away from gravity
• why auxins suppress growth of lateral buds?

  
  

  • auxin released from terminal bud
  • Auxin in the main shoot inhibits development of lateral buds.​
  • The inhibition decreases as the shoot grows further away​ (effect reduced)
  • the inhibition is removed if the apical shoot is lost.
• function of gibberellins?


  • germination
  • cell elongation
  • reinforce effect of auxin - suppress lateral buds
• how are gibberellins stimulated?

  • seeds absorb water
  • activates embryo - secretes gibberellins
  • diffuses through aleurone layer - produces amylase
  • diffuses through endosperm layer - hydrolyses starch to glucose
  • glucose used to make ATP
• how do auxins effect root growth?

  • Positive geotropism​
  • Auxin released in tip of root and Diffuses to underside of root.​
  • Inhibits cell elongation on underside.​
  • Elongation continues on upper side.​
  • Root grows towards gravity.
• function of cytokinins?


  • promote growth of lateral buds
  • promote cell division
• how do cytokinins affect apical dominance?

  • antagonistic ​
  • cause the opposite effect to​ Auxins
  • stimulates lateral buds and shoots ​
  • Oppose apical dominance (terminal buds)​
• what are the 2 ways plant hormones interact?

  • synergistically - work together. combined effect larger than alone
  • antagonistically - have opposing effects
• what is a phytochrome?

  • photoreceptor pigment
  • 2 forms - Pr and Pfr

  Pr - inactive form (absorbs red light)

  Pfr - active form. formed when Pr absorbs red light (absorbs far red)
• times when each phytochrome is abundant?

  darkness - Pr abundant

  light - Pfr abundant
• How do phytochromes control flowering?

  • sunlight absorbed by pR, forming pFR
  • (the longer the day, the more pFR in plant tissue)
  • pFR transported from cytoplasm to nucleus
  • activates transcription factors
  • stimulates transcription of genes coding for photomorphogenesis
  • leads to flowering
• short day plants?

  • Pfr inhibits flowering
  • inhibits florigen production. long nights needed to convert pfr back to pr
  • levels must be low (in order to flower)
  • exceed critical night length
• long day plants?

  • Pfr activates flowering
  • short night required to make sure some pfr remains
  • levels must be high (in order to flower)
  • lower than critical night length
• way of remembering long day/ short day plants?

  SID the LAD:

  Short (pfr Inhibits) Day

  the

  Long (pfr Activates) Day
• what is photomorphogenesis?

  effect of light on growth and development of plants
• day neutral plants?

  • not affected by photoperiod (length of period of light and dark)