Describe any three chemical defence mechanisms in plants against herbivores in population interaction.

Answer: Plants cannot run away from herbivores, so many produce harmful or distasteful chemicals (secondary metabolites):

1. Alkaloids and other toxic compounds: nicotine (tobacco), caffeine (coffee), quinine (Cinchona), strychnine and opium are poisonous or bitter and discourage grazing animals and insects.
2. Cardiac glycosides: Calotropis produces highly poisonous cardiac glycosides, so cattle and goats do not browse on it.
3. Tannins, resins and latex: make the plant tissue distasteful and hard to digest; sticky latex of plants such as Euphorbia gums up the mouthparts of insects.
4. Some plants produce chemicals that, after ingestion, cause nausea, vomiting or interfere with the growth and reproduction of herbivores (e.g. thorny Acacia and cactus use physical defence in addition).