Apical dominance

• Where are meristems located for plant growth?

  • Apical meristems (tips of roots/shoots)
  • Lateral bud meristems- in buds on side, could give rise to shoots but not if apical meristem is functioning
  • Intercallary meristems- between nodes and where leaves branch off stem

  
• Evidence for apical dominance?

  • Cut of terminal bud- lateral buds now grow stems
  • Stops growth upwards and causes sideways growth
  • Usually presence of the apex of the shoot inhibits sideways growth from lateral bud- apical dominance
• Describe the hormones implicated in apical dominance and state how they are implicated

  • Normal auxin- prevents growth of side buds
  • Low auxin- promotes growth
  • Absicisic acid - inhibits buds. High auxin in shoot may keep ABA high and when the tip is removed ABA drops so buds grow
  • Cytokinins- promote bud growth, can override apical dominance
  • High auxins in shoot spec- sink for cytokinins in the roots
  • When shoot apex is removed- spreads around the plant and promotes growth of the buds
• What is apical dominance?

  Apical dominance is a plant phenomenon where the main shoot of a plant inhibits the growth of other shoots. (ie. the apical bud)
• Describe how high concentrations of auxins inhibit lateral bud growth

  • Auxins produced by apical meristem travel down the stem by diffusion or active transport
  • Inhibits sideways growth from the lateral buds
  • Lower down the concentration of auxin is lower and lateral buds grow more
• Describe evidence for apical dominance

  • Removal of apical shoot means no auxin so lateral shoots are freed from dominance
  • If IAA (an auxin) applied artificially to cut shoot, apical dominance is reasserted and lateral shoot growth suppressed

  
• How can turning plant upside down be evidence for apical dominance?

  • If growing shoot is tipped upside down, apical dominance is prevented and lateral buds start to grow out sideways
  • Supports theory because auxins are transported downwards and cant be transported upward against gravity
• Describe generally how auxin affects growth

  Stimulates growth of main apical shoot

  • Produced at apex
  • Moves by diffusion or active transport to cells in zone of elongation
  • The more auxin the more elongation occurs
  • Auxin promotes AT of H+ ions by ATPase enzyme in plasma membrane into cell wall
  • Reduced pH providing optimum conditions for wall-loosening enzymes (expansions) to work
  • Expansions break bonds in cellulose, so walls become less rigid and can expand when cells take in water
• What hormone stimulates the growth of the main apical shoot?

  Auxin
• Explain the effects of pH and auxin on the length of a plant

  • Auxin binds to receptor sites on the plant membrane, causing a fall in pH to about 5
  • Optimum needed for enzymes to keep the wall flexible so it grows
  • Auxin is destroyed as cells mature
  • pH then increases so the enzymes maintaining plasticity become inactive.
  • Wall becomes more rigid and fixed in shape- cells cannot expand and grow
• What effect does auxin concentration have on the roots?

  • Low concs promote root growth
  • As auxin conc increases root growth increases, to a certain point, past that point root growth is greatly inhibited- auxin reaches the roots in low concentrations from growing shoots
  • High auxin concs inhibit root growth
  • If apical shoot is removed, auxin conc is greatly reduced and root growth slows or even stops
• Effects of auxin concs on different parts of plant?

  Effects:

  
• Do plants that have short stems produce gibberellins

  No- if they do then it’s very few
• In terms of growth, what are the effects of gibberellins?

  • Stem elongation
  • Affect internode length (regions between leaves)
• Why is it an advantage for plant stems to be shorter?

  • Reduces waste
  • Plants less vulnerable to damage by weather and harvesting