5.1.5

• physical plant responses to herbivory

  thorns and sharp spikes

  inedible fibrous leaves

  folding in response to contact
• chemical plant responses to herbivory

  alkaloids

  pheromones
• alkaloids

  nitrogenous compounds that affect metabolism
• pheromones

  chemicals given off that affect the behaviour of other members of that species or kingdom to communicate danger
• responses to abiotic stress

  leaf loss

  daylength sensitivity

  abscission

  freeze prevention

  stomatal control
• leaf loss

  when abiotic factors turn cold and dark the rate of photosynthesis is lowered meaning it is more efficient to loose leaves
• daylength sensitivity

  photoperiodism - a plants sensitivity to lack of light

  controls when leaves and flowers bud
• abscission

  ethene triggers the breakdown of the cell wall causing leaves to drop off
• freeze prevention

  natural anti freeze stops cytoplasm from freezing meaning metabolic activities can still occur
• stomatal control

  abscisic acid causes stomata to close so water is not lost via evaporation

  stomata open to allow for evaporation of water having a cooling effect
• tropism

  when a plant responds via growth to a stimuli
• positive tropism

  growth towards the stimuli
• negative tropism

  growth away from the stimuli
• what are tropisms controlled by

  growth factors
• positive phototropism occurs in

  shoots
• how does positive phototropism work?

  1. shoot tip cells produce auxins causing the cells to elongate
  2. the auxins diffuse to the most shaded side of the shoot
  3. the shaded side elongates more causing the plant to bend towards the light
• positive phototropism

  A) light

  B) auxins

  C) shaded

  D) elongation

  E) towards

  
• how does negative phototropism work?

  1. a high concentration of auxins inhibits cell elongation on shaded side
  2. root cells elongate on the side with the most light
  3. roots bend away from light
• negative phototropism

  A) auxin

  B) elongation

  C) away

  
• negative phototropism occurs in

  roots
• negative gravitropism occurs in

  shoots
• how does negative gravitropism work?

  1. auxins diffuse to lowest side of shoot
  2. cells elongate upwards
  3. shoot bends upwards
• negative gravitropism

  A) auxin

  B) auxin

  
• positive gravitropism occurs in

  roots
• how does positive gravitropism occur?

  1. auxins diffuse to the lowest side of the root
  2. the upper side of the root elongates
  3. roots bend downwards
• positive gravitropism

  A) auxin

  B) auxin

  
• plant hormones

  auxin

  ethene

  gibberellin

  abscisic acid / ABA
• Auxin function

  cell elongation

  inhibit root growth

  prevent leaf dropping

  maintain apical dominance
• ethene function

  ripen fruit

  leaf loss
• gibberellin function

  stimulate seed germination

  stem elongation

  pollen tube growth for fertilisation
• abscisic acid function

  stomatal closing

  maintaining seed dormancy
• apical dominance

  auxins cause the growth of one main shoot which inhibits the growth of lateral shoots
• what controls the number of lateral shoots

  the concentration of auxins
• evidence of auxin controlled apical dominance

  when an apical shoot is removed lateral shoots grow

  when auxin is applied to a shoot lateral shoot growth decreases

  
• gibberellin is antagonistic to

  abscisic acid
• process of seed germination

  1. the seed absorbs water
  2. gibberellin production is stimulated
  3. gibberellins activate the production of enzymes by turning on genes
  4. enzymes break down food stores in the seed
  5. embryo plant uses food stores to respire and release ATP
• evidence of gibberellin controlled seed germination

  when removing the gene for gibberellin or adding a biosynthesis inhibitor for gibberellin the seed did not germinate

  when gibberellin was added it did
• stem length / elongation is controlled by

  gibberellins
• evidence for gibberellin controlled stem elongation

  dwarf plants have low levels of gibberellins due to a mutation

  when dwarf plants were given gibberellins they grew to the same height as non dwarf varieties
• ethene and commercial use

  means unripe fruit can be harvested and transported then ripened when ready to sell