plant responses

• Plants, like animals, increase their chances of survival by responding to changes in their environment:

  • They sense the direction of light and grow towards it to maximise light absorption for photosynthesis.
  • They sense gravity, so their roots and shoots grow in the right direction.
  • Climbing plants have a sense of touch, so they can find things to climb and reach the sunlight.
• tropism 


  a directional growth response
• postive tropism 


  towards the stimulus
• negative tropism 


  away from the stimulus
• Abiotic stress 


  anything that's potentially harmful to a plant that's natural, but nonliving, like a drought
• Plants can respond to abiotic stress, e.g. Carrots produce antifreeze proteins at low temperatures - the proteins bind to ice crystals and lower the temperature that water freezes at, stopping more ice crystals from growing.
• Herbivory 


  when plants are eaten by animals (including insects).
• Plants have chemical defences that they can use against herbivory. For example, they can produce toxic chemicals in response to being eaten.
• Alkaloids 


  these are chemicals with bitter tastes, noxious smells or poisonous characteristics that deter or kill herbivores, e.g. tobacco plants produce the alkaloid nicotine in response to tissue damage.

  Nicotine is highly poisonous to many insects.
• Tannins 


  these taste bitter, and in some herbivores (e.g. cattle, sheep) they can bind to proteins in the gut, making the plant hard to digest.
• Pheromones 


  signalling chemicals that produce a response in other organisms.
• Some plants release alarm pheromones into the air in response to herbivore grazing. This can cause nearby plants that detect these chemicals to start making chemical defences such as tannins.
• •When corn plants are being eaten by caterpillars, they can produce pheromones which attract parasitic wasps. These wasps then lay their eggs in the caterpillars, which eventually kills them.
• If a single leaflet (a mini leaf-shaped structure that makes up part of a leaf) of the plant Mimosa pudica is touched, a signal spreads through the whole leaf, causing it to quickly fold up. It's thought that this could help protect Mimosa pudica against herbivory in a variety of ways, e.g. it may help to knock off any small insects feeding on the plant. It may also scare off animals trying to eat it.
• Phototropism 


  the growth of a plant in response to light. Shoots are positively phototropic and grow towards light. Roots are negatively phototropic and grow away from light
• Geotropism is the growth of a plant in response to gravity. Shoots are negatively geotropic and grow upwards. Roots are positively geotropic and grow downwards
• Hydrotropism 


  plant growth in response to water. Roots are positively hydrotropic.
• • Thermotropism 


  plant growth in response to temperature
• Thigmotropism 


  plant growth in response to contact with an object.
• chemotropism


  plant growth in response to chemicals
• Deciduous plants 


  plants that lose their leaves in winter. Losing their leaves helps plants to conserve water (lost from leaves) during the cold part of the year, when it might be difficult to absorb water from the soil (the soil water may be frozen), and when there's less light for photosynthesis.
• Leaf loss


  1. Triggered by shortening day length in autumn

  2. Controlled by hormones
• Auxins


  • Inhibit leaf loss
  • Produced by young leaves
  • Less auxin produced as leaf gets older, leading to leaf loss
• Ethene


  • Stimulates leaf loss
  • Produced by ageing leaves
  • More ethene produced as leaves get older
  • Stimulates cells in abscission layer to expand, breaking cell walls and causing leaf to fall off
• Abscission layer


  • Layer of cells that develops at bottom of leaf stalk (where leaf joins stem)
  • Separates leaf from rest of plant
• Auxins


  Antagonistic (work in opposition) to ethene