Mineral Nutrition

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A farmer notices that the older leaves of a crop turn yellow first while younger leaves stay green, suggesting a deficiency of which highly mobile macronutrient?

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Mineral deficiency symptoms depend on whether an element is mobile within the plant, a concept covered in NCERT Class 11, Chapter 12 (Mineral Nutrition). Nitrogen is a highly mobile nutrient: when supply is short, the plant remobilises it from older, mature leaves and redirects it to actively growing young tissues. Because nitrogen is central to chlorophyll and protein synthesis, its withdrawal from older leaves causes them to lose chlorophyll and turn yellow first, a symptom called chlorosis, while younger leaves remain green. Calcium is largely immobile, so its deficiency appears in young tissues and growing tips, not old leaves. Boron is also relatively immobile and its deficiency damages meristems and young parts. Iron is immobile too, producing interveinal chlorosis in young leaves rather than old ones. The pattern of symptoms appearing first in older leaves therefore points specifically to a mobile element. A logical check reinforces this: only nutrients that can be relocated from old to new tissue, such as nitrogen, potassium and magnesium, show their deficiency symptoms initially in the older leaves.

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