Question

Nitrogen fixation in Rhizobium is increased in the presence of
A. Potassium
B. Nitrogen
C. Phosphorus
D. None of the above

Answer 1

Hint: The chemical process by which atmospheric molecular nitrogen is converted into combined forms like ammonia or other more reactive nitrogenous compounds like nitrates and nitrites in the soil through biological or industrial processes is called Nitrogen fixation.

Complete answer:
Rhizobium belongs to the Kingdom Bacteria and is a genus of Gram-negative bacteria that live in soil and are capable of fixing nitrogen present there. Nitrogen fixation is a part of the biogeochemical nitrogen cycle where the atmospheric nitrogen- the dinitrogen molecule is converted into forms that can be used by the plants. This is done with the help of microorganisms present in the soil. Rhizobia- a group of soil-bacteria, cannot perform this function independently and thus require a host for this process. It forms a symbiotic association with the roots of the leguminous plants that have root nodules on its surface.
Root nodules- a specialized structure formed on the surface of the roots of the legumes, are the site for the reduction reaction occurring in the plants where atmospheric dinitrogen molecules are converted into ammonia and other molecules in the presence of the enzyme nitrogenase. Due to its toxicity, ammonia is absorbed by the organic compounds instantly, and the plants are provided with organic nitrogenous compounds like glutamine or ureides. This acts as a natural fertilizer for the plant.

Option A: The role of potassium is to provide proper growth and development to the plant. It helps in the movement of food and water through the plant. It also helps in the process of reproduction and photosynthesis. As it plays no direct role in the nitrogen fixation in plants, therefore, this is an incorrect option.

Option B: Nitrogen is needed by plants to produce food through the process of photosynthesis. It is also a part of the chlorophyll molecule and thus provides green color to the leaves. In the process of nitrogen fixation, no nitrogen is absorbed from the soil. As it plays no direct role in the nitrogen fixation mechanism, therefore, this is an incorrect option.

Option C: Phosphorus plays an important role in the growth and development of the plant as it is a vital part of the energy unit of the plant- ATP. It, directly and indirectly, plays an important role in various processes that require energy for their functioning. It also promotes the formation of nodules on the roots. Since it plays a significant role in nitrogen fixation by promoting nodule formation and providing the energy for the process, its presence will increase the rate of nitrogen fixation in plants. Therefore, this is the correct option.

Option D: None of the above is the incorrect option.

Hence, the correct answer is option (C)

Note: Symbiosis is a type of biological interaction where both the organisms live in a long and close association with each other, and at least one of the individuals is benefited. It can be both- positive as well as negative. In this example- the bacteria fix nitrogen for the plant, and in return, obtain organic compounds produced via photosynthesis from plants.