Plant Defence Responses (AQA GCSE Biology): Revision Note

Physical & chemical defences

• Plants have a range of defence mechanisms against infections and infestations

• These can be split into physical and chemical defences

• Plants have adaptations to prevent herbivores from eating them (see Adaptations)

Physical defences

• The cellulose cell wall not only provides support for the plant but also protection from microorganisms

• The waxy cuticle of the leaf is a barrier to microbes from entering the plant. The only place that they can enter in the leaf is through the stomata

• 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

Chemical defences

• Many plants produce chemicals that have antimicrobial properties

• Many herbal face creams can use plant extracts such as tea tree oil, mint and witch hazel to have an antibacterial effect

• More plants are being studied for their potential source of antibiotics as more strains are becoming resistant to current antibiotics

Adaptations against herbivores

• Herbivores eating a plant can seriously damage the plant structures and they will not flower and reproduce

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

Poisons

• Some plants produce poisons to deter herbivores from eating them. Animals will quickly learn to avoid eating plants that make them feel unwell

  • Example: foxgloves, deadly nightshade and yew

Thorns

• Make it unpleasant or painful for large herbivores to eat them

  • Example: brambles, cacti and gorse

Hairy stems

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

  • Example: poppies can be combined with poisons such as nettles

Drooping or curling when touched

• This is a rare but effective mechanism, when touched the leaves will automatically curl which can dislodge insects and frighten off larger species.

  • Example: Mimosa pudica will curl and point leaves to the ground when an insect touches it.

Mimicry

• Plants can droop to look as though they are diseased and trick animals into not eating them. They can also resemble larger insects to scare away any potential threats

  • Example: passion flowers have evolved to mimic having butterfly eggs on its leaves so that butterflies do not lay their eggs.

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