Q22.Which one of the following phytochemicals produced in plants defend against herbivores?
(A) Terpenes
(B) Lignin
(C) Flavonoids
(D) Tannins
The correct answer is (D) Tannins. Tannins are specialized phytochemicals that plants produce to chemically deter herbivores by binding to proteins in their digestive tracts, reducing nutrient absorption and causing toxicity.
This makes tannins a primary direct defense mechanism, distinct from structural or less targeted compounds.
Option Analysis
Terpenes: Volatile organic compounds that often repel herbivores through strong odors or toxicity and attract predators/parasitoids indirectly. While defensive, they function more as repellents than digestive inhibitors.
Lignin: A structural polymer reinforcing cell walls, making plant tissues tougher and harder to chew. It provides physical/mechanical defense rather than direct chemical deterrence against herbivores.
Flavonoids: Phenolic compounds with antioxidant roles that can deter feeding via bitterness or UV protection. They contribute to defense but are less specific to herbivore protein-binding than tannins.
Tannins: Polyphenolic compounds that precipitate proteins, inhibiting herbivore digestion by forming indigestible complexes in the gut. This antinutritional effect is a classic direct chemical defense.
Tannins rank among the top phytochemicals plants produce to defend against herbivores, forming protein-tannin complexes that disrupt insect and mammal digestion. These secondary metabolites evolved as a targeted chemical warfare strategy in plant-herbivore interactions.
Defense Mechanisms
Terpenes volatilize to repel or signal carnivores. Lignin toughens tissues physically. Flavonoids add bitterness and stress tolerance. Tannins excel by directly impairing herbivore nutrition.
