Why are healthy root nodules pink in colour?

  • Healthy root nodules are pink due to the presence of leghaemoglobin. (like haemoglobin in mammalian blood).
  • This leghaemoglobin helps the plant by absorbing oxygen, which inhibit the enzymes in the nodule that 'fix' nitrogen into soluble forms. 
  • Root nodules without pink colour are likely to be parasitic on the host plant, rather than symbiotic.

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