Plant Responses (OCR A Level Biology): Revision Note

Plant responses

• Plants can respond to their environment in various ways

• The environmental stimuli to which plants respond can be abiotic or biotic

• Examples of plant responses include:

  • responses to abiotic stress

  • responses to herbivory, e.g.

    • chemical defences

    • response to touch

  • tropisms

Responses to abiotic stress

• Abiotic stress for plants can include:

  • freezing

  • drought

  • increased soil water salinity

  • presence of heavy metals, e.g. lead, copper, zinc, mercury

• Plants can respond to drought in ways that reduce water loss by transpiration, e.g.:

  • shutting stomata

  • dropping leaves

• Some plants can respond to freezing temperatures by producing an antifreeze chemical in their cells that decreases the formation of ice crystals

  • Ice crystals can destroy plant cells if allowed to form within them

Responses to herbivory

Chemical defences

• Many plants have adaptations that help them to avoid being eaten by herbivores

  • Herbivory is a biotic stress factor

• Some plants produce chemicals that provide a defence against herbivory, e.g.

  • alkaloids

  • pheromones

Chemical defences to herbivory table

Responses to touch

• Some plants are sensitive to touch, e.g. Mimosa pudica

• In Mimosa the leaflets of the touch-sensitive leaves fold rapidly when touched

  • It is thought that this movement may be an adaptation to protect the leaflets from herbivorous insects, though it could also reduce transpiration when the leaves are no longer photosynthesising (the leaflets also fold in at night and reopen at dawn)

• The response occurs very rapidly and is most likely caused by local bioelectrical signals

Mimosa pudica is an example of a plant that can respond to touch stimuli

Tropisms

• A tropism is a growth response of a part of a plant in response to a directional stimulus

• Tropisms can be towards a stimulus (positive tropisms) or away from a stimulus (negative tropisms)

• Plant responses include a variety of tropisms, including:

  • phototropism

  • geotropism, also known as gravitropism

  • hydrotropism

  • thigmotropism

  • chemotropism

Plant tropisms table