5.1.5 - plant and animal responses

• Abiotic stress is non-living environmental factors that could harm a plant
• Plants respond to abiotic stress and herbivory by

  • Containing bitter tannins
  • bitter nitrogen compounds called alkaloids
  • antifreeze enzymes
  • release cell signalling pheromones
• mimosa pudica responds to touch ( seismonasty) by causing leaves to fold
• A plant tropism is a directional growth response of plants
• Phototropism : response to light
• geotropism : response to gravity
• hydrotropism : response to water
• thermotropism : response to temperature
• thigmotropism : response to touch
• Leaf loss in deciduous plants is controlled by:

  as leaf ages cytokinin and auxin levels lower, ethene level increases

  triggers production of cellulase enzymes which breaking down cell Walls in abscission layer

  leaves break from branch. Below the abscission layer Suberin layer forms to prevent entry of pathogens
• Gibberellins are used to stimulate

  • germination
  • elongation at cell internodes
  • fruit growth
  • rapid growth / flowering
• Germination is stimulated when the seed absorbs water it activates the embryo to secrete gibberellins. The gibberellins diffuse to aleurone layer which produces amylase. Amylase diffuses to endosperm layer to hydrolyse starch. Hexose sugars act as respiratory substrate to produce ATP as energy currency.
• Auxins are used to:

  • control cell elongation
  • suppress lateral buds to maintain apical dominance
  • promote root growth
  • involved in tropic responses
• Shoots show positive phototropism because:

  auxin / indoleacetic acid diffuses to shaded side of shoot Tip. As IAA diffuses down shaded side it causes active transport of H+ ions into the cell wall.

  disruption to H-bonds between cellulose molecules and action of expansins make cell more permeable to water.

  Cells on shaded side elongate faster due to higher turgor pressure. Shoot bends towards light.
• Root show positive gravitropism
• shoots show positive phototropism
• roots show positive gravitropism because:

  gravity causes IAA to accumulate on lower side of the root. IAA inhibits elongation of root cells. Cells on the upper side of the root elongate faster so the root tip bends downwards.
• IAA = indoleacetic acid
• Hormones stimulate stomata to close by:

  abscisic acid binds to complementary receptors on guard cell membrane causing calcium ion channels on tonoplast to open. Ca 2+ ions diffuse from vacuole into cytoSol.

  Positive feedback triggers other ion channels to open. So K+ diffuses out of guard cell.

  Water potential of guard cell becomes more positive. Water diffuses out via osmosis. Guard cells become flaccid so stomata close.
• Apical dominance is during the growth of the shoot, the growth of side shoots does not take place. Maintained by the action of auxin, abscisic acid and cytokinins.
• Environmental evidence that auxins maintain apical dominance:

  auxin production in apex maintains high levels of abscisic acid. Inhibits growth of side shoots.

  When apex is removed: auxin levels drop, causing abscisic acid levels to drop. Cytokinins diffuse evenly to promot lateral bud growth.
• Experimental evidence that gibberellins control stem elongation:

  tall plants have higher gibberellin concentration than dwarf plants.
• expermental evidence that gibberellins control germination:

  Mutant seeds with non-functional gibberellin gene do not germinate unless gibberellin is applied externally. Inhibitors of gibberellin production prevent germination.
• auxins are used commercially in:

  • rooting powder
  • growing seedless fruit
  • herbicides
  • low conc prevent fruit and leaf growth
  • high conc promote fruit drop
• cytokinins are used commercially:

  • prevent yellowing of lettuce leaves
  • promotes shoot growth
• Gibberellins are used commercially:

  • delay senescence in citrus
  • elongation of apple and grape stalks
  • brewing beer for malt production
  • increase sugar cane yield
  • speed up seed formation in conifers
• ethene is used commercially:

  • speeds up ripening
  • promotes lateral growth
  • promotes fruit drop
• Plants need to respond to their environment:

  • to avoid abiotic stress
  • to avoid herbivory
  • to maximise photosynthesis
  • help ensure germination and fertilisation
• Positive phototropism is seen in the shoot tips and is useful to get maximum light for photosynthesis
• Plant roots exhibit positive hydrotropism as they grow towards water
• Some plants use thigmotropism to use other plants in order to grow.
• Pheromones are chemicals produced that affect the behaviour of other members of the same species.
• cytokinins promotes cell division in roots and shoots

  cytokinins - more mitosis - more cells - plant growth
• Abscisic acid stimulates stomata opening and closing
• in stomata closure:

  when water becomes limiting, root cells synthesise and release abscisic acid. ABA is transported to the leaves. ABA molecules bind to receptors on the plasma membrane of guard cells. Ionic concentration of guard cells is decreased. The water potential in the guard cells decreases. Water moves out of the cell via osmosis. Reduction in turgor pressure causes stomata to close.
• Auxin is made at the shoot tip and diffuses down the shoot tip.

  Sunlight breaks down auxins so the shaded area has the highest auxin concentration causing more cell division on the shaded side of the root. This leads to the shoot bending towards the light.
• Leaves are lost as there is more energy and glucose needed to keep them in the winter months. Increase in darkness means less auxin is made and auxin inhibits ethene. So more ethen is produced and causes cells in the abscission zone to produce enzymes which digest leaf.
• When the tips of a shoot is removed, no auxin is made so stems do not grow.
• Ethene is used to control fruit ripening

  eg, bananas
• Auxins are used as hormonal weed killers

  cause broad leaved weeds to grow at an extremely quick rate that the plant cannot support