The Rhizobium or Bradyrhizobium bacteria colonize the host plant's root system and cause the roots to form nodules to house the bacteria. The bacteria then begin to fix the nitrogen required by the plant.

The correct option for the bacteria present in root nodules of leguminous plants that fix atmospheric nitrogen is 'A) Rhizobium'. Rhizobium is a type of soil bacteria that forms a symbiotic relationship with leguminous plants, such as peas, beans, and clover. This symbiotic association between Rhizobium and leguminous plants is known as nitrogen fixation.

Rhizobium bacteria have the unique ability to convert atmospheric nitrogen (N2) into a usable form for plants, such as ammonia (NH3) or nitrate (NO3-). This process is essential for plants because nitrogen is an essential nutrient required for their growth and development.

Here is a detailed explanation of why Rhizobium is the correct answer:

1. Symbiotic relationship: Leguminous plants have specialized structures called root nodules in which Rhizobium bacteria reside. These root nodules provide a suitable environment for bacteria to live and multiply.

2. Nitrogen fixation: Rhizobium bacteria possess an enzyme called nitrogenase, which enables them to convert atmospheric nitrogen into ammonia. This process is known as nitrogen fixation. The ammonia produced by Rhizobium is then used by the plant to synthesize proteins, nucleic acids, and other essential compounds.

3. Nodulation process: The process of nodulation begins when the roots of leguminous plants release specific chemical signals, called flavonoids. These flavonoids attract and stimulate the Rhizobium bacteria in the soil.

4. Infection and nodule formation: Once attracted, Rhizobium bacteria enter the root hairs of the plant through infection threads. These infection threads grow towards the inner tissues of the root, forming nodules. Inside the nodules, Rhizobium bacteria multiply and establish a symbiotic relationship with the plant.

5. Exchange of nutrients: Within the nodules, the plant provides carbohydrates and other organic compounds to the Rhizobium bacteria. In return, the bacteria supply the plant with fixed nitrogen in the form of ammonia or nitrate.

6. Benefits of nitrogen fixation: The ability of Rhizobium bacteria to fix atmospheric nitrogen provides several benefits to leguminous plants. It reduces the plant's dependence on external nitrogen sources, such as fertilizers. It also improves soil fertility by enriching it with nitrogen, which can be utilized by other plants in the ecosystem.

In summary, Rhizobium bacteria are the specific bacteria that form a symbiotic relationship with leguminous plants and fix atmospheric nitrogen. This relationship is crucial for the growth and development of leguminous plants and contributes to the overall nitrogen balance in the ecosystem.

Rhizobium