In legume root nodules, what is the primary function of leghaemoglobin during biological nitrogen fixation?

Introduction / Context:
Leguminous plants, such as peas and beans, form symbiotic associations with nitrogen fixing bacteria in root nodules. Within these nodules, a special red coloured pigment called leghaemoglobin is present. Understanding the function of leghaemoglobin is important in plant physiology and agricultural science because it helps explain how nitrogen fixation can occur efficiently in the presence of oxygen sensitive enzymes.

Given Data / Assumptions:

We are dealing with nitrogen fixation in legume root nodules. Leghaemoglobin is present in the nodule tissue and gives it a pink or reddish colour. The key enzyme nitrogenase is very sensitive to high oxygen concentrations. Options offer various possible roles such as oxygen absorption, nutrition, light absorption, catalysis and ion transport.

Concept / Approach:
Nitrogenase, the enzyme responsible for converting atmospheric nitrogen gas to ammonia, is inactivated by high levels of oxygen. At the same time, the symbiotic bacteria need some oxygen for respiration to provide ATP for nitrogen fixation. Leghaemoglobin solves this problem by binding oxygen molecules inside the nodule. It acts as an oxygen buffer, keeping the free oxygen concentration very low while still delivering enough oxygen to support bacterial respiration. Thus, leghaemoglobin does not fix nitrogen directly, nor does it provide food or absorb light; its main role is to bind oxygen and create a microaerobic environment favourable for nitrogenase.

Step-by-Step Solution:
Step 1: Recognise that nitrogenase is highly oxygen sensitive and can be quickly inactivated if oxygen levels are high. Step 2: Remember that root nodules are living tissues where both plant cells and bacteria respire and therefore consume oxygen. Step 3: Recall that leghaemoglobin is structurally similar to haemoglobin and has a high affinity for oxygen. Step 4: Conclude that leghaemoglobin binds and absorbs free oxygen within the nodule, maintaining a low oxygen concentration around nitrogenase. Step 5: This controlled oxygen environment allows ATP production by respiration while protecting the nitrogen fixing machinery, making oxygen binding the primary function of leghaemoglobin.

Verification / Alternative check:
A helpful comparison is to think of haemoglobin in animals, which transports oxygen in the blood. Leghaemoglobin similarly binds oxygen, but its purpose is not long distance transport. Instead, it buffers oxygen levels locally in the nodule. Experimental observations show that nodules rich in leghaemoglobin are effective in nitrogen fixation and appear pink, whereas ineffective nodules lack this pigment and fix little nitrogen. This correlation between pigment presence, oxygen management and nitrogen fixation efficiency supports the role of leghaemoglobin as an oxygen binding molecule.

Why Other Options Are Wrong:
Leghaemoglobin does not directly feed the bacteria; plant derived carbohydrates serve as the main energy source. It does not absorb light energy, as chlorophyll fulfills that function in photosynthetic tissues. It is not the catalytic enzyme for nitrogen fixation; nitrogenase, a bacterial enzyme complex, performs that role. Transport of nitrate ions from soil to leaves occurs mainly through xylem and is unrelated to leghaemoglobin. Therefore, only the option describing oxygen binding and maintenance of a low oxygen environment correctly states its primary function.

Common Pitfalls:
Students may mistakenly think that any pigment in plant tissues absorbs light or is directly involved in photosynthesis, leading them to choose the light absorption option. Others might confuse the term haemoglobin with blood and think only about oxygen transport, not oxygen buffering in a micro environment. To avoid confusion, always associate leghaemoglobin with root nodules, oxygen buffering and protection of nitrogenase, rather than with light capture or nutrient feeding roles.

Final Answer:
In legume root nodules, leghaemoglobin primarily binds and absorbs excess oxygen to maintain a low oxygen environment, allowing efficient nitrogen fixation by oxygen sensitive nitrogenase.