Plant defences

Plant defences

  • Plants can be attacked by insects or pathogens. If this happens to crops, crop yield can be severely reduced as the plants are eaten, which will increase the price of the crop in the long run.
  • As a result, plants can produce chemicals to protect the plant when they are attacked by pathogens. This can be done in different ways:
    • Attract parasites to kill the insects.
    • Put the insect off (doesn’t taste nice)
    • Attract predators to attack the insects eating the plants.
  • Although the plant may have to use a lot of energy to make the chemicals it is worth it.


  • Milkweed plant produces a toxic chemical that stops insects from eating due to the taste.
  • Corn plant, when attacked by caterpillars produces a chemical that attracts a parasitic wasp. The wasp lays eggs in the skin of the caterpillar. When the larvae hatch they eat the caterpillar from the inside out, preventing further attack.
  • Wheat seedlings produce a chemical when attacked by insects which attract aphids that eat the insects.
  • Young lupin leaves produce poisonous chemicals called alkaloids. These make the leaves poisonous to insect pests or larger herbivores that might want to eat them.
  • Potatoes are often attacked by a fungus-like organism called potato blight that destroys their leaves, thereby killing the plant. Some varieties of potato produce chemicals that kill it.
  • Plants are a key source of food for people. If pathogens destroy a crop our food supply is at risk. An example of this is the famine caused by potato blight in Ireland in 1845 and 1846 that killed over 1 million people.

Drugs from plants

  • Many chemicals in plants have been found to help cure diseases in people.

  • New possibilities are being researched all the time. For example, research suggests that the alkaloids found in potato plants could be developed as valuable treatments for cancer. This is one of the reasons the rainforest should be preserved as there may be many new species of plant with chemicals with curing properties we haven’t utilised yet.

Louis Pasteur

  • Louis Pasteur found microorganisms were responsible for some diseases and food going off. As a result, he proposed the idea of keeping microorganisms away from food in order to prevent diseases and preserve foods.
  • Methods of doing this are called aseptic techniques. This includes pasteurisation of milk.