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Agronomy QUESTION #2101
Question 1
Leghemoglobin in legume root nodules serves the critical function of:
  • Providing carbon skeleton for amide synthesis and nitrogen export to shoots
  • Maintaining microaerophilic conditions within the infected nodule cells ā€” binding Oā‚‚ at high affinity to supply it to bacteroid respiration while keeping free Oā‚‚ concentration low enough to protect the oxygen-sensitive nitrogenase enzyme from irreversible inactivationāœ”ļø
  • Transporting fixed nitrogen (as ureides or amides) from nodule to xylem
  • Synthesizing ATP required for the high energy demand of the nitrogenase reaction
Correct Answer Explanation
Leghemoglobin (a plant-encoded heme protein, heme supplied by bacteroid) maintains the paradox of nodule metabolism: bacteroids need Oā‚‚ for ATP production (nitrogenase is highly energy-demanding: 16 ATP/Nā‚‚) but nitrogenase is irreversibly destroyed by Oā‚‚. Leghemoglobin buffers Oā‚‚ at ~10ā»āø M (vs atmospheric ~0.2 mM), supplying Oā‚‚ for respiration while keeping free Oā‚‚ below inhibitory levels ā€” the pink nodule color indicates active BNF.