Nitrate deficiency stunts young leaf growth

Steps:

• Nitrate ions (NO3-) are absorbed by plant roots and reduced to ammonia for amino acid synthesis.
• Amino acids form proteins needed for cell division and expansion in growing tissues.
• Deficiency limits protein production, impairing rapid growth in meristematic regions.
• Young leaves, being actively growing, show stunted development first.

Why D is correct:

• Nitrates provide nitrogen for amino acid synthesis (via nitrogen assimilation pathway), essential for protein production driving cell growth; lack restricts new leaf expansion.

Why the others are wrong:

• A: Loss of dark green (chlorosis) results from magnesium deficiency affecting chlorophyll.
• B: Very dark green leaves indicate phosphorus deficiency reducing sugar transport.
• C: Yellowing occurs in older leaves from general nitrogen mobility, not primary nitrate shortage effect.

Final answer: D