Question

What will happen if a wheat field is inoculated with the bacterium Rhizobium?
A. The soil will become rich in calcium
B. The soil will become rich in nitrogen
C. There will be no effect on soil nitrogen
D. The soil will be rich in oxygen

Answer 1

Hint: Rhizobium is a heterotrophic bacteria. Rhizobium is found in the nodules of the leguminous plants. Therefore, it is a symbiotic bacteria. It helps in fixing the atmospheric nitrogen for plants.

Complete answer:
Rhizobium is a proteobacterium. It is a genus of gram-negative bacteria. Gram-negative bacteria are those bacteria that do not retain the color of the crystal violet stain, that is used for the Gram staining of the bacteria.
-Rhizobium is found within the root nodules of the leguminous plants in a symbiotic relationship. The relation between the plant and Rhizobium is mutualistic. This means that both organisms are benefited from this association.
-These bacteria fix the free atmospheric nitrogen. They convert the free atmospheric nitrogen into ammonia using the nitrogenase enzyme. Thus, the plants receive atmospheric nitrogen in the form of ammonia.
-Nitrogen is required by the plants to perform the basic metabolic processes. Hence, Rhizobium is diazotrophic bacteria. The wheat is a cereal plant. And cereal plants do not have legumes in their roots. -The Rhizobium is found exclusively in the nodules of the leguminous plants only. Also, Rhizobium can fix nitrogen only when they are within the root nodules. In the free-living form, they cannot fix nitrogen.
-So, if we inoculate the wheat field with bacterium Rhizobium, then it will have no effect on soil nitrogen, because wheat will not have legumes, and without legumes, Rhizobium will not be able to fix nitrogen.
Hence, the correct answer is option (C).

Note: Rhizobium is found within the root nodules of the leguminous plants. It is an endosymbiotic bacterium. The word endosymbiotic is made up of two words, Endo meaning inside, and Symbiotic meaning the symbiosis relationship. Thus, the endosymbiotic bacteria are those bacteria that live within the body of the host in a symbiotic relationship with them.