Plant Defence Responses (AQA GCSE Biology): Revision Note

Plants have a range of defence mechanisms against infection and infestations

These include defences that are:

• physical

• chemical

Plants have adaptations to deter herbivores

The cellulose cell wall provides support for the plant, and protection from microorganisms

The waxy cuticle of the leaf is a barrier to microbes

• Microorganisms can still enter a leaf through the stomata or through wounds in the surface

Bark provides a tough layer around the stem of the plant to prevent pathogens from entering

As deciduous trees lose leaves in the winter, any infection present in the leaves is shed with them

Many plants produce chemicals that have antibacterial properties, e.g.:

• tea tree oil

• mint

• witch hazel

Poisons to deter herbivores, e.g. foxgloves, deadly nightshade and yew

Herbivores eating a plant can seriously damage plant structures

• E.g. consuming leaves will reduce the plant's rate of photosynthesis

Smaller insect infestations can remove essential nutrients from the plant and act as vectors for pathogens

Adaptations to deter herbivores include:

• poisons

  • Some plants produce poisons to deter herbivores – this is a chemical defence, not a mechanical adaptation

  • Animals will quickly learn to avoid eating plants that make them feel unwell

  • Examples: foxgloves, deadly nightshade and yew

• thorns

  • These make it unpleasant or painful for herbivores to bite plant structures

  • Examples: brambles, cacti and gorse

• hairy stems

  • Hairy stems or leaves deter insects and larger animals from feeding, or laying eggs on the stem or leaves

  • Examples: poppies, tomatoes and sunflowers

• drooping or curling when touched

  • When touched the leaves will automatically curl to dislodge insects and frighten off larger species

  • Example: Mimosa pudica

• mimicry

  • Plants can droop to look as though they are diseased

  • They can also resemble larger insects to scare away any potential threats

  • Example: white dead nettle does not sting, but looks very similar to a stinging nettle