Problem 110 - Objective Biology for NEET

Question

Assertion: Rhizobium fixes nitrogen in the symbiotic association with root nodules of leguminous plants only. Reason: Biological nitrogen fixation is anaerobic process.

Step-by-Step Solution

Answer

Although Rhizobium is known for its symbiotic relationship with leguminous plants, there are some strains that can interact with non-leguminous plants. Nitrogen fixation can work in both aerobic and anaerobic conditions.

Step 1: Understand Nitrogen Fixation

Nitrogen fixation is the process by which nitrogen in the atmosphere is converted into a form that plants can use. In the given assertion, Rhizobium is a type of bacteria that is known to fix nitrogen when it forms a symbiotic association with root nodules of leguminous plants.

Step 2: Evaluate the Assertion

The assertion states that Rhizobium fixes nitrogen only in the root nodules of leguminous plants. While it's true that Rhizobium is most known for its symbiotic relationship with leguminous plants, it is not entirely accurate to say that Rhizobium can only fix nitrogen within these plants. Some strains of Rhizobium have also been discovered that can form symbiotic relationships with non-leguminous plants. Therefore, the assertion is not entirely correct.

Step 3: Understanding the nature of the process

The reason states that biological nitrogen fixation is an anaerobic process. Nitrogen fixation is a process that can work both in aerobic (oxygen presence) and anaerobic (lack of oxygen) conditions. While many nitrogen-fixing bacteria, like Rhizobium, prefer low-oxygen environments, they can also function in the presence of oxygen. Therefore, that part of the reason is not entirely correct.

Step 4: Evaluate reason based on Assertion

In this case, while both the assertion and reason contain elements of truth, neither are entirely accurate. Furthermore, the reason does not explain or substantiate the assertion, as the aerobic or anaerobic nature of nitrogen fixation does not determine whether Rhizobium bacteria can form a symbiotic relationship with non-leguminous plants too.

Key Concepts

Rhizobium

**Rhizobium** is a type of soil bacteria renowned for its role in facilitating biological nitrogen fixation. These bacteria are crucial for the nitrogen cycle, as they convert atmospheric nitrogen (\(N_2\)) into ammonia (\(NH_3\)), a form that plants can utilize for growth and development. Rhizobium is predominantly associated with leguminous plants, where they colonize plant roots, forming nodules that house the bacteria. Inside these nodules, Rhizobium converts the atmospheric nitrogen into a usable form, significantly enhancing soil fertility. However, it is interesting to note that some Rhizobium strains can associate with non-leguminous plants, although they primarily form symbiotic relationships with legumes.

Symbiotic Association

A **symbiotic association** is a mutually beneficial relationship between two different organisms. In the case of Rhizobium and leguminous plants, each party gains something: Rhizobium gets shelter and essential nutrients from the plant, while the plant receives ammonia that it can use to synthesize essential proteins. During this interaction, the plant creates nodules around Rhizobium, allowing these bacteria to thrive in a low-oxygen environment which is crucial for the nitrogen fixation process. This relationship is vital for improving soil nitrogen levels, which reduces the need for synthetic fertilizers and promotes sustainable agriculture practices.

Leguminous Plants

**Leguminous plants** belong to the family Leguminosae, and they have a unique ability: forming symbiotic relationships with Rhizobium bacteria. These plants, such as beans, peas, lentils, and clovers, are significant in agriculture because they naturally enhance soil fertility through nitrogen fixation. They are often used in crop rotations and green manure to replenish soil nitrogen levels. The nodules formed by Rhizobium on their roots act as tiny factories for nitrogen conversion, making these plants critical in ecosystems for maintaining the nitrogen balance.

Biological Nitrogen Fixation

**Biological nitrogen fixation** is a natural process carried out by certain microorganisms, including Rhizobium, through which atmospheric nitrogen is converted into ammonia. This process is fundamental for life on Earth as it makes nitrogen accessible to plants, which do not have the capability to absorb nitrogen directly from the atmosphere. While it often occurs in anaerobic conditions within soil nodules, Rhizobium bacteria can also operate in low-oxygen zones. This ability to fix nitrogen underlies many agricultural practices aimed at maintaining and enhancing soil productivity without relying heavily on chemical fertilizers.