16) Plant responses

• cotyleden - embryonic leaf
• monocotyledon - 1 leaf
• dicotyledon - 2 leaf
• endosperm - energy source in seed
• plants respond to:

  • light
  • gravity
  • water
  • chemical - minerals
  • touch
• plants communicate by plant growth communicators (hormones)
• hormones

  • auxins
  • gibberellins
  • ethene
  • ABA (Abscisic acid)
• auxins

  • control cell elongation
  • prevent leaf fall (abscission)
  • maintain apical dominance
  • involved in tropisms
  • stimulate release of ethene
  • involved in fruit ripening
• gibberellins

  • cause stem elongation
  • cause mobilisation of food stores in a seed at germination
  • stimulate pollen tube growth in fertilisation
• ethene

  • cause fruit ripening
  • promotes abscission in decidious trees
• ADA (abscisic acid)

  • maintains dormancy of seeds and buds
  • stimulates cold protective responses e.g antifreeze production
  • stimulates stomatal close
• process of gibberellins:

  1. water enters seed
  2. absorbtion of water causes embryo to synthesise gibberellins
  3. stimulates the production of enzymes amylase and maltase
  4. amylase hydrolyses starch into maltose
  5. can be broken down into glucose, providing sugar essential for growth
  6. suggests gibberellins switch on genes which code for amylase and maltase - digestive enzymes requires in germination
• evidence of gibberellins

  • mutant varieties without the gene for gibberellins have been produced that cannot germinate
  • however, when gibberellins are applied external seeds germinates
  • when biosynthesis/protein synthesis inhibitors are added, the seed is unable to germinate when removed or when more GIB is added it will
• auxins

  • made in cells at the tip of the roots and shoots, and in the meristem
  • apical meristems (tip)
  • lateral bud meristems (side)
  • intercallary meristems
• auxins process

  1. auxins synthesised in meristem cells
  2. auxins diffuse down the shoot away from the tip
  3. auxin binds to receptor on cell surface membrane
  4. vacuole forms and low pH develops
  5. low pH of cell walls keeps them flexible allowing cells to expand as they absorb water. Vacuoles get bigger and cell walls stretch
  6. larger central vacuole formed. Auxins destroyed by enzymes. Cella walls become rigid. No further cell elongation.
• nastic movements

  • usually brought about by changed in turgidity in cells
  • rapid responses
  • e.g venus fly trap
• troposisms

  • slower responses resulting in directional growth
• phototropism

  • response of the plant organs to the direction of light
  • a shoots shows positive troposism (towards)
• auxin is actively transported to the shaded side
• types of hormones

  • auxins
  • gibberellins
  • cytokinins
  • abscisic acid
  • ethene
• synergism - 2 or more hormones acting together to reinforce an effect
• antagonist - having opposing actions and inhibit (diminish) each others effect
• auxins

  • synthesised in the shoot or root tips (meristems)
  • most common is IAA (indoleacetic acid)
  • promotes cell elongation
  • stimulate cell division
  • prevent leaf fall (abscission)
  • maintain apical dominance (lateral bud growth)
• apical dominance

  • auxins produced by apical meristem and travels down by diffusion or active transport
  • it inhibits the growth of sideways growth from lateral buds (widens branches as the bottom as there is less auxin there)
  • auxin accumulates in the nodes beside lateral buds
  • presence inhibits their activity
  • the removal of the apical bud (auxin source), releases the inhibition on lateral bud growth - bushier plant development
• apical dominance - evidence

  if tip is cut off of two shoots

  • IAA (auxin) is applied to one of them, it continues to show apical dominance
  • untreated shoot branches out sideways

  if growing shoot is tipped upside down

  • apical dominance is prevented
  • lateral buds grow out sideways
  • supports auxins are transported downwards, and cannot be transport upwards against gravity
• Gibberellins and stem elongation

  • gibberellins increase stem length
  • increases the length of the inter nodes
  • stimulate cell divisions and cell elongation
• evidence for gibberellins and stem elongation

  • dwarf beans are dwarf because they lack the gene producing gibberellins
  • mendel's short pea plants lack the dominant allele that encodes for gibberellins
  • plants with high gibberellin concentration are taller
• leaf abscission

  • trees in temperate countries shed their leaves in autumn
  • advantage:
  • reduces water loss through leaf surfaces
  • avoids frost damage
  • avoid fungal infections through damp, cold leaf surfaces
  • plants have limited photosynthesis in winter
• hormones involved in leaf abscission

  • auxins - inhibits abscission
  • ethene (gas) - increase in ethene production inhibits auxin production
  • abscisic acid
• abscisic acid

  • inhibits growth (antagonist to gibberellins and IAA)
  • controls stomatal closure
  • role in leaf abscission (falling of leaves or fruits from plants)
• leaf abscission

  • as leaf ages, rate of auxin production declines
  • leaf is more sensitive to ethene production
  • more ethene produced, inhibits auxin production
  • abscission (hardened) layer begins to grow at the base of the leaf stalk
  • environmental factors such as wind separate the leaf from the plant
  • protective layer that remains forms a scar to prevent the entry of pathogens
• abscission layer

  • made of thin walled cells
  • weakened by enzymes that hydrolyse polysaccharides in their walls
  • leaf falls off
  • tree grows protective layer where the leaf will break off
  • cell walls contain suberin
  • leaves a scar which prevents entry of pathogens
• geotropism - growth towards or away from gravity
• chemotropism - growth towards or away from certain chemicals
• positive phototropism - occurs when shoot grows towards light

  negative phototropism - occurs when shoot grows away from light
• phototropism

  • materials e.g mica which are not permeable to water can stop the curvature response in some circumstances when placed on the shaded side
  • materials e.g gelatin which are permeable to water do not interfere with the curvature response
• commercial use of auxins

  • sprayed onto developing fruits to prevent abscission
  • sprayed onto flowers to initiate fruit growth without fertilisation
  • applied to the cut end of a shoot to stimulate root production
  • synthetic auxins are used as selective herbicides
• commercial use of ethene

  • fruits harvested before they are ripe allows them to be transported without deteriorating, these are sprayed with ethene to promote ripening at sale point
  • e.g bananas
• commercial use of gibberellins

  • sprayed onto fruit crops to promote growth
  • sprayed onto citrus trees to allow fruit to stay on trees for longer
  • sprayed onto sugar cane to increase the yield of sucrose
  • used in brewing, where gibberellins are sprayed onto bailey seeds to make them germinate, amylase is produced, starch is broken down into maltose, the action of yeast on maltose produces alcohol
• commercial use of cytokinins

  • delay leaf senescence - can be sprayed on lettuce leaves to prevent them turning yellow
  • used in tissue culture to mass produce plants