Diagnosis of Plant Nutrient Deficiency Symptoms:

• Nitrogen matches with (III) Pale yellow to brown necrosis: Mobile in plants, deficiency induces generalized chlorosis starting from older leaves progressing to pale yellowing and V-shaped brown necrosis.
• Phosphorus matches with (IV) Bronzing of leaf margin: Accumulation of anthocyanins causes bronze, purple, or reddish discoloration along margins of older leaves.
• Potassium matches with (I) Leaf scorching/tip burning: Marginal chlorosis followed by dry marginal necrosis and tip scorching (marginal burn) on older leaves.
• Calcium matches with (II) Failure of terminal bud formation: Completely immobile in plants, deficiency directly arrests growing tips, causing death or distortion of terminal growing buds and hooked young leaves.

Physiological Roles of Potassium ($K^+$) in Crop Plants:

• Non-Structural Nature: Potassium is unique among major macronutrients because it does NOT enter into the permanent covalent structure of organic molecules (unlike N in proteins or Mg and N in chlorophyll). It functions primarily as a free univalent osmoticum and catalytic cofactor.
• Valid Roles:
• Activates over 60 enzyme systems (e.g., pyruvate kinase, starch synthase).
• Drives osmoregulation and guard cell turgor mechanism for stomatal opening and closing (Levitt $K^+ / H^+$ pump theory).
• Facilitates phloem loading and translocation of photosynthates/sugars.
• Statement 4 is completely false because nitrogen (not potassium) is the structural backbone of proteins, and magnesium with nitrogen forms the chlorophyll porphyrin ring.