Plant nutrition: which macronutrient is a major component of nucleic acids and proteins across plant tissues?

Introduction / Context:
Macronutrients are required in relatively large amounts and become integral parts of essential biomolecules. Identifying which element forms the backbone of amino acids and nucleotides helps diagnose deficiencies and guide fertilization strategies in crops and horticulture.

Given Data / Assumptions:

• Nitrogen is incorporated into amino groups of amino acids and nitrogenous bases of nucleotides.
• Proteins and nucleic acids are abundant in growing tissues and meristems.
• The question asks for a macronutrient, not a micronutrient.

Concept / Approach:
Nitrogen deficiency typically causes chlorosis of older leaves due to its mobility and is central to plant growth due to protein and nucleic acid synthesis. While sulphur is also present in some amino acids (cysteine, methionine), nitrogen is far more prevalent quantitatively in both proteins and nucleic acids.

Step-by-Step Solution:

Verification / Alternative check:
Standard nutrient tables list N as a primary macronutrient alongside P and K; tissue analysis reflects high nitrogen content in protein-rich tissues.

Why Other Options Are Wrong:

• A: Calcium is structural (cell walls, signaling) but not a major component of nucleic acids or most amino acids.
• C: Sulphur occurs in certain amino acids but not in nucleic acids; its total contribution is smaller.
• D: Iron is a micronutrient, essential for enzymes and electron transport, not a macronutrient component of proteins/nucleic acids.
• E: Sodium is nonessential for many plants and not a macromolecular building block.

Common Pitfalls:
Overemphasizing sulphur’s role due to its presence in some amino acids while ignoring the quantitative dominance of nitrogen.

Final Answer:
Nitrogen (N)