[Campbell Biology P.865] Nitrogen fixation by Rhizobium bacteria within root nodules require... | Practice Question

Nitrogen fixation by Rhizobium bacteria within root nodules requires an anaerobic environment. Which of the following mechanisms contributes most directly to maintaining this critical environment?

  • A: The presence of vascular tissue to supply nutrients.
  • B: The sloughing off of root hairs after infection.
  • C: The formation of a layer of lignin-rich sclerenchyma cells in the mature nodule.
  • D: The branching of the infection thread into cortical cells.

Explanation

The text states that 'The woody external layers of root nodules also help to limit gas exchange' and Figure 37.13, step 5, explicitly mentions that 'A layer of lignin-rich sclerenchyma cells forms, reducing absorption of oxygen and thereby helping maintain the anaerobic environment needed for nitrogen fixation.' This directly addresses maintaining the anaerobic environment.