5.5.1 Plant Responses

• Plants can respond to their environment in various ways
  • These plant responses have evolved because they provide the plant with some type of selective advantage (i.e. making them better adapted to survive and reproduce in their environment)

• The environmental stimuli to which plants respond can be abiotic (non-living components of the environment) or biotic (living components of the environment i.e. other organisms)
• Examples of plant responses include:
  • Tropisms
  • Responses to touch
  • Responses to herbivory
  • Responses to abiotic stress

Tropisms

• A tropism is 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)
• Plants responses include a variety of tropisms, including:
  • Phototropism
  • Geotropism
  • Hydrotropism
  • Thigmotropism
  • Chemotropism

Plant Tropisms Table

Responses to touch

• A process similar to thigmotropism occurs in Mimosa pudica, a sensitive plant
• The leaflets of the touch-sensitive leaves fold in rapidly when touched
  • This is caused by rapid water uptake (and therefore increase in volume) in cells at the base of each leaflet and rapid loss of water from (and therefore collapse of) adjacent cells

• As this response is not related to the direction of the touch stimulus it is not considered a tropism
  • Non-directional responses such as this are known as nastic movements

• The movement occurs very rapidly and is most likely caused by local bioelectrical signals (plant hormones cannot cause such a quick response)
• It is thought this movement may be an adaptation to protect the leaflets from herbivorous insects or to reduce transpiration when the leaves are no longer photosynthesising (the leaflets also fold in at night and reopen at dawn)
• The highly specialised leaves that form the 'traps' of the Venus flytrap plant show a similar response to touch, folding together rapidly when stimulated by the touch of an insect or other small organism, which is then trapped and digested to provide certain minerals (e.g. nitrogen) to the plant

Nastic movement in Mimosa pudica in response to touch

Responses to herbivory

• Herbivory is the consumption of plants by herbivores
• Just as many animals have adaptations to avoid predation by carnivores, many plants have adaptations to avoid herbivory
• For example, many plants produce herbivore-repellent chemicals (sometimes poisonous)
  • Some plants produce these chemicals in response to damage or stress caused by herbivory but in some plants they are continually present

• Examples of these chemicals include:
  • Tannins
  • Alkaloids
  • Pheromones

Chemical Defences of Plants in Response to Herbivory Table

Responses to abiotic stress

• Abiotic stress for plants can occur in various forms, including:
  • Freezing
  • Drought
  • Increased soil water salinity
  • Presence of heavy metals (e.g. lead, copper, zinc, mercury)

• Plants can respond to drought by shutting their stomata (reducing water loss through transpiration) or by dropping their leaves
• Some plants can respond to temperatures below freezing by producing an antifreeze chemical in their cells, that decreases the formation of ice crystals that can destroy plant cells if allowed to form within them