Chapter 16

• Chemical coordination 


  • plants are rooted - not mobile
  • however, they are coordinated organisms show clear responses to their environment, communication between cells & between diff plants
  • Have evolved a system of hormones
• Auxins 


  • made at the tip of roots and shoots, and in the meristems
  • Control cell elongation
  • Prevent leaf fall (abscission)
  • Maintain apical dominance
  • Involved in tropisms
  • Stimulate release of ethene
  • Involved in fruit ripening
• Apical dominance


  • the plant can identify the top shoot & prioritise resources to go to is so it can grow rapidly
  • apex = high auxin concentration
• Gibberellin 


  • Cause stem elongation - affect length of the internodes (regions between leaves on a stem)
  • Trigger mobilisation of food stores in a seed at germination 
  • Stimulate pollen tube growth in fertilisation
• Ethene 


  • Causes fruit ripening 
  • Promotes abscission in deciduous trees
• ABA (abscisic acid)


  • Maintains dormancy of seeds & buds
  • Stimulates cold protective responses e.g. antifreeze production
  • Stimulates stomatal closing
• Seed germination 


  • seed absorbs water - activates embryo to produce gibberellins
  • gibberellins stimulate production of enzymes (amylase/ protease) that break down food stores inside the seed
  • the food stores are used to produce ATP for building materials so the embryo can grow & break through the seed coat

  
• Evidence for action of gibberellin


  • Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins - these seeds don’t germinate - if gibberellins are then artificially added to mutant seeds they germinate 
  • If gibberellin biosynthesis inhibitors are applied to seeds they don’t germinate as cannot make the gibberellins needed for them to break dormancy - if inhibition removed or gibberellins applied the seeds germinate 
• Effect of auxins on apical shoot growth 


  • presence of auxins affect plasticity of cell wall
  • auxins bind to specific receptor sites on plant cell membrane
  • causes pH to fall to 5 - optimum pH for enzymes to keep walls flexible & plastic
  • as cells mature, auxin is destroyed, pH rises so the enzymes maintaining plasticity become inactive
  • cell wall becomes rigid & more fixed in shape & size - cells can no longer expand & grow

  
• How does a high concentration of auxin affect plants?

  • suppresses growth of lateral shoots, allowing the apical shoot to grow and dominate the stem
  • this results in apical dominance

  
• Evidence for apical dominance 


  • if apical shoot is removed, the auxin-producing cells are removed so there is no auxin - as a result the lateral shoots grow faster
  • if auxin is applied artificially to the cut apical shoot, apical dominance is reasserted & lateral shoot growth is suppressed

  
• How do low concentrations of auxin affect plants?


  • promote root growth
  • up to a given concentration, the more auxin that reaches the roots, the more they grow
  • if apical shoot is removed, the amount of auxin reaching the roots is reduced & root growth slows & stops
  • Replacing auxin artificially at the cut apical shoot restores the growth of the roots
  • (High auxin concentrations inhibit root growth)
• How were gibberellins discovered?


  • they're produced by a fungus that affects rice - the infected seedlings grew extremely tall & tin
  • scientists isolated chemicals (gibberellins) which produce the same spindly growth plants
  • conclusion = tall and spindly plants produce large amounts of gibberellins, short-stemmed plants produce little or no gibberellins
• Synergism 


  different hormones work together to produce a greater response than they would on their own
• Antagonism


  when different hormones are working in opposite directions the response will depend relative levels of each hormone