Plant defence responses

  • Students should be able to describe physical and chemical plant defence responses.

Physical defence responses to resist invasion of microorganisms.

  • Cellulose cell walls.
  • Tough waxy cuticle on leaves.
  • Layers of dead cells around stems (bark on trees) which fall off.
  • Chemical plant defence responses.
  • Antibacterial chemicals.
  • Poisons to deter herbivores.
  • Mechanical adaptations.
  • Thorns and hairs deter animals.
  • Leaves which droop or curl when touched.
  • Mimicry to trick animals.


  1. (HT only) Plant diseases can be detected by:
  • stunted growth
  • spots on leaves
  • areas of decay (rot)
  • growths
  • malformed stems or leaves
  • discolouration
  • the presence of pests.
  • (HT only) Identification can be made by:
  • reference to a gardening manual or website
  • taking infected plants to a laboratory to identify the pathogen
  • using testing kits that contain monoclonal antibodies
  1. Plants can be infected by a range of viral, bacterial and fungal pathogens as well as by insects.
  2. Knowledge of plant diseases is restricted to tobacco mosaic virus as a viral disease, black spot as a fungal disease and aphids as insects.
  3. Plants can be damaged by a range of ion deficiency conditions:
    • stunted growth caused by nitrate deficiency
    • chlorosis caused by magnesium deficiency.
    • Knowledge of ions is limited to nitrate ions needed for protein synthesis and therefore growth, and magnesium ions needed to make chlorophyll.

WS 1.4

The everyday application of scientific knowledge to detect and identify plant disease.