Module 5.1.5- Plant responses

• Tropism


  A growth response of a plant toward or away from a stimulus
• Role of auxins


  Control cell elongation, prevent leaf fall, maintain apical dominance, stimulate the release of ethene, involved in fruit ripening
• Role of gibberellins


  Cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth in fertilisation
• Role of ethene


  Causes fruit ripening, promotes leaf fall (abscission) in deciduous trees
• Role of ABA


  Maintains dormancy of seeds and buds, stimulates cold protective responses, stimulates stomatal closure
• Germination of seeds (long)

  - seed absorbs water

  - embryo activates, gibberellins produced

  - stimulates production of enzymes that break down food stores found in the seed

  - embryo plant uses these food stores to produce ATP and so in turn it can grow and break out through the seed coat
• ABA in seed germination


  Evidence that ABA acts as an antagonist to gibberellins and so the relative levels of both hormones determines may determine when a seed may germinate
• Experimental evidence for gibberellins- Mutant varieties


  Mutant varieties of seeds have been bred which lack the gene that enables them to make gibberellins and so these seeds do not germinate. If gibberellins are applied to the seed externally, they then germinate normally.
• Experimental evidence for gibberellins- Biosynthesis inhibitors

  If gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate. When removed or gibberellins are applied externally they germinate.
• Auxins move _____ the stem and _____ the root both in the transport tissue and from cell to cell

  down, up
• Effect of auxins on apical shoot growth

  1) Auxins synthesised in meristem cells

  2) Auxins diffuse away away from tip and bind to receptor sites

  3) As a result vacuole forms and pH drops low (5)

  4) This is the optimum pH for enzymes to keep cell walls flexible and plastic

  5) Cell expands and more water is absorbed. Vacuoles get bigger and cell walls stretch.

  6) Large central vacuole is formed, auxins are destroyed by enzymes. Cell walls become rigid and no more cell elongation. No more growth and is fixed in shape.
• Effects of auxins on apical shoot growth diagram

  
• Apical dominance


  Concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth.
• High concentration of auxins


  suppress growth of lateral shoots -> apical dominance
• Low concentrations of auxins

  promotes root growth
• Synergism


  occurs when the effect of two hormones together is greater than the effect of either alone
• Antagonism


  occurs when the effect of two hormones together is less than the effect of either alone
• Abiotic factors (or stresses)

  Nonliving components of environment e.g. day length, cold and heat, lack or excess of water, high winds, changes in salinity.
• Why do deciduous trees lose their leaves during winter

  to save water and so also remain dormant
• Photoperiodism


  Where plants are sensitive to a lack of light in their environment
• Abscission (leaf fall)


  during winter or dry season

  falling light = lower auxins concentration

  lower auxin concentration = higher concentration of ethene

  at abscission zone, ethene initiates genes that produce new enzymes that digest and weaken the outer cell walls

  
• Midway abscission (after enzymes digest the cell wall)

  vascular bundles are sealed off to prevent the entry of pathogens as well as depositing fats in the area (protective scar)cells deep in the separation zone respond to hormonal cues by retaining water and swelling, putting more strain

  with other abiotic factors, eventually, the leaf falls off.

  
• Preventing freezing in plants adaptations

  Cytoplasm and vacuole sap contains solutes which lower the freezing point

  Some plants produce sugars etc. which act as antifreeze to prevent cytoplasm from freezing or protect the cells from damage even if they do freeze
• What hormone controls the opening and closing of the stomata?

  ABA
• How does ABA cause stomatal closure?


  ABA produced and is transported to the leaves where it binds to receptors on the plasma membrane of the stomatal guard cells.

  ABA activates changes in the ionic concentration of guard cells, reducing water potential and therefore turgor of cells.

  As a result, guard cells close the stomata and water loss by transpiration is greatly reduced.
• Physical plant defences

  thorns, spikes, spiny leaves, fibrous and inedible tissue, hairy leaves, stings etc.
• Chemical plant defence- tannins


  bitter taste

  
  

  toxic to insects (bind to saliva digestive enzymes and deactivate them)
• Chemical plant defence- alkaloids


  bitter taste

  affect metabolism of animals (maybe poison them)caffeine: toxic to insets and fungi, prevents germination of other plants (rivals)
• Pheromone chemical


  Chemical signals released by an animal that communicate information and affect the behavior of other animals of the same species.
• Pheromone examples


  If maple tree is attacked by insects, a pheromone is released that is absorbed by leaves on other branches. Callose is then made to help protect
• Volatile organic compounds (VOCs)


  compounds that contain carbon (organic) and evaporate very easily (volatile).attracts the pest's predators or acts as a pheromone
• Folding in response to touch- Mimosa pudica


  contains toxic alkaloid with sharp prickles

  when leaves are touched, they fold down and collapse

  frightens off larger herbivores and dislodges small insects which have landed on the leaf
• Phototropism


  Growth of a plant shoot toward or away from light.
• Geotropism


  response to earth's gravity, as the growing of roots downward in the ground
• Chemotropism


  plant growth in response to a chemical
• Thigmotropism


  A directional growth of a plant in response to touch.
• Shoot tip

  terminal bud: at end of each shoot; includes the SAM and other parts
• How does the plant shoot respond to unilateral light when the shoot tip is cut off


  No response

  the tip detects the stimulus or produces a response/messenger
• Mechanism of the effect of unilateral light on a plant


  Auxins move laterally (down) across the shoot so higher concentration of auxins towards the shaded sides. Cell elongation and growth is then stimulated and the shoot then grows towards the light

  
• Growing in the dark


  Plants grow faster in the dark

  Growing upwards is faster because seedlings don't have to compete with other seedlings for light

  Gibberellins are responsible for extreme elongation