Plant Chemical Defenses Against Herbivores — NEET
✅ Asked in NEET 2023For plants, herbivores are the predators. Nearly 25 per cent of all insects are known to be phytophagous (feeding on plant sap and other parts of plants). The problem is particularly severe for plants because, unlike animals, they cannot run away from their predators. Plants therefore have evolved an astonishing variety of morphological and chemical defences against herbivores. Thorns (Acacia, Cactus) are the most common morphological means of defence. Many plants produce and store chemicals that make the herbivore sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction or even kill it. You must have seen the weed Calotropis growing in abandoned fields. The plant produces highly poisonous cardiac glycosides and that is why you never see any cattle or goats browsing on this plant. A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced by them actually as defences against grazers and browsers.
NTA tests whether students understand that plants produce chemical compounds (like cardiac glycosides in Calotropis, nicotine, caffeine, etc.) as defense mechanisms against herbivores. The core trap is confusing these compounds as primary metabolites—they're actually secondary metabolites evolved purely for protection. Students mistake them for food storage or energy compounds. Remember: these chemicals make herbivores sick, inhibit feeding/digestion, disrupt reproduction, or cause death. Examples like Calotropis producing poisonous cardiac glycosides, preventing cattle from grazing, directly illustrate this defense strategy. This concept connects morphological (thorns) and chemical defenses, testing ecological adaptation understanding—fundamental to predator-prey relationships in ecosystems.
For chemical defense against herbivores, Calotropis has: (NEET 2023)
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