Plant responses

• Plant responses

  • They provide the plant with some type of selective advantage (i.e. making them better adapted to survive and reproduce in their environment)
  • They can be in response to abiotic (non-living components of the environment) or biotic (living components of the environment i.e. other organisms) stimuli
• Plant responses

  • Tropisms
  • Responses to touch
  • Responses to herbivory
  • Responses to abiotic stress
• Tropism

  A growth movement of a part of a plant in response to a directional stimulus
• Tropisms

  • Can be towards the stimulus (known as positive tropisms) or away from the stimulus (known as negative tropisms)
• Types of plant tropisms

  • Phototropism
  • Geotropism
  • Hydrotropism
  • Thigmotropism
  • Chemotropism
• Phototropism


  Response to light
• Geotropism


  Response to gravity
• Hydrotropism


  Response to moisture
• Thigmotropism


  Response to touch
• Chemotropism


  Response to chemicals
• Advantage of tropisms is that the plant gets access to as much light as possible, maximising photosynthesis
• Shoots show negative geotropism (grow away from the pull of gravity) and roots show positive geotropism (grow towards the pull of gravity) so shoots/roots free-operating seeds grow in the right directions regardless of the orientation of the seed
• Hydrotropism causes roots to grow towards damper areas of soil, increasing their access to water
• Thigmotropism allows plants adapted to climb on abiotic supports to detect living or non-living support and use it to gain greater access to light in dense rainforests
• Chemotropism causes some plants to show a tropic response to certain chemicals (e.g. pollen tubes grow down the flower's stigma towards the ovule due to chemotropism)
• Nastic movement

  A non-directional response to a stimulus, not considered a tropism
• Mimosa pudica (sensitive plant) shows nastic movement, with leaflets folding in rapidly when touched, likely an adaptation to protect the leaflets from herbivorous insects or reduce transpiration when not photosynthesising
• Venus flytrap plant also shows a similar nastic response, with the trap leaves folding together rapidly when stimulated by touch of an insect or other small organism, trapping and digesting it
• Herbivory

  The consumption of plants by herbivores
• Many plants produce herbivore-repellent chemicals (sometimes poisonous) either in response to damage/stress from herbivory or continually
• Examples of plant chemical defences against herbivory

  • Tannins
  • Alkaloids
  • Pheromones
• Tannins

  Water-soluble carbon compounds that can be fatal to herbivores
• Alkaloids

  Nitrogenous compounds derived from amino acids that can be bitter-tasting and toxic to herbivores and fungi
• Pheromones

  Chemicals released by one member of a species that affect the physiology or behaviour of other members of the same species
• Plants can respond to abiotic stress such as freezing, drought, increased soil salinity, or heavy metal presence
• Plants can respond to drought by shutting their stomata (reducing water loss through transpiration) or by dropping their leaves
• Some plants can produce an antifreeze chemical in their cells to decrease ice crystal formation and prevent cell damage in freezing temperatures