Final answer:
In legume root nodules, nitrogen-fixing enzymes function at low oxygen concentration, which is enabled by leghemoglobin. The low oxygen concentration is necessary for nitrogen fixation to occur. The correct answer is option a.
Step-by-step explanation:
In legume root nodules, nitrogen-fixing enzymes function at low oxygen concentration, and this is enabled by leghemoglobin. The low oxygen concentration in the nodules allows the nitrogen-fixing enzymes to operate efficiently. Leghemoglobin, which is similar to hemoglobin in animals, binds to oxygen and reduces the oxygen concentration in the nodule, creating a favorable environment for nitrogen fixation to occur.
When legume plants form root nodules, they enter into a symbiotic relationship with nitrogen-fixing bacteria known as rhizobia. The rhizobia inside the nodules convert atmospheric nitrogen into a form that can be used by the plant. However, nitrogen fixation requires a low or limited oxygen environment. Oxygen can inhibit the activity of the nitrogen-fixing enzymes, which is why legumes have evolved the mechanism of leghemoglobin to maintain low oxygen levels within the nodule.
Therefore, the correct option for the given assertion and reason is Option A: Both Assertion and Reason are correct, and Reason is the correct explanation for Assertion. The low oxygen concentration provided by leghemoglobin is necessary for the functioning of nitrogen-fixing enzymes in legume root nodules.