• The capability of mineral nutrients to be mobilized and redistributed via the phloem from mature tissues to young growing points under deficiency conditions varies widely:
• Nitrogen (N): Readily hydrolyzed from proteins/chlorophyll and highly mobile; redistributed extensively to younger leaves.
• Iron (Fe): Intermediate to low mobility; poorly redistributed once incorporated into chloroplast cytochromes.
• Boron (B): Strongly complexed into cell wall pectin lattices (rhamnogalacturonan-II) and essentially completely immobile in most angiosperms; cannot be redistributed from older to younger tissues.
• Therefore, the correct descending order of remobilization capability under deficiency conditions is:
  $$\text{N (Highly Mobile)} \gt \text{Fe (Intermediate/Poorly Mobile)} \gt \text{B (Immobile)}$$
• Hence, Option 4 (N > Fe > B) is the correct answer.

Plant diagnostic symptoms of iron deficiency:

• Iron (Fe) is structurally immobile within plant phloem tissue and cannot be readily remobilized from older leaves to growing shoot apices.
• Consequently, deficiency symptoms invariably develop first on the youngest expanding leaves, presenting as distinct interveinal chlorosis (the leaf lamina turns pale yellow/white while the veins remain distinctly green).
• In contrast, mobile nutrients (like Mg) show interveinal chlorosis on older leaves first.