Symbiotic nitrogen fixing bacteria that convert atmospheric nitrogen into usable forms are commonly found in the root nodules of which of the following crops?

Introduction / Context:
Nitrogen is an essential element for plant growth, but atmospheric nitrogen gas cannot be used directly by most plants. Certain bacteria can fix atmospheric nitrogen into ammonia and related compounds, making it available to plants. Some of these bacteria live in a symbiotic relationship with specific plants, forming root nodules. This question asks you to identify which crop among the options usually has such nitrogen fixing symbiotic bacteria in its root nodules.

Given Data / Assumptions:

• The question refers to symbiotic nitrogen fixing bacteria present in root nodules.

• The crop options include peas, wheat, corn, oats, and rice.

• We assume basic knowledge that legumes often form root nodules with Rhizobium bacteria.

• Only one of the listed crops is a typical legume known for root nodulation.

Concept / Approach:
Leguminous plants such as peas, beans, lentils, and clover form symbiotic associations with Rhizobium species. These bacteria live in nodules on the roots of the host plant, fix atmospheric nitrogen, and provide nitrogen compounds to the plant. In return, the plant supplies carbohydrates and a protected environment to the bacteria. Cereals like wheat, corn, oats, and rice do not normally form such nitrogen fixing nodules, although they may benefit from nitrogen fixed by free living or associative bacteria in the soil. Therefore, peas, which are legumes, are the correct choice.

Step-by-Step Solution:
Step 1: Recall that peas are leguminous plants with seeds in pods and the ability to form root nodules. Step 2: Recognise that Rhizobium bacteria infect pea roots and stimulate the formation of nodules, where nitrogen fixation occurs. Step 3: Understand that these bacteria convert atmospheric nitrogen into ammonia and other nitrogenous compounds that the plant can use for protein and nucleic acid synthesis. Step 4: Consider wheat, corn, oats, and rice, which are cereals belonging to the grass family and do not typically form nitrogen fixing nodules. Step 5: While cereals require nitrogen, they usually obtain it from soil nitrates and fertilisers rather than from root nodule symbiosis. Step 6: Therefore, among the options, peas are the crop known for having symbiotic nitrogen fixing bacteria in root nodules.

Verification / Alternative check:
Agricultural and biology texts list peas and other legumes as natural nitrogen enrichers of soil, because after harvesting, their roots and nodules add nitrogenous compounds back to the soil. Crop rotation systems often include legumes like peas or beans to improve soil fertility for subsequent cereal crops. Diagrams of legume roots with nodules labelled as sites of Rhizobium bacteria are common in textbooks, confirming that peas are associated with symbiotic nitrogen fixation, while wheat, corn, oats, and rice are not.

Why Other Options Are Wrong:

Wheat: A cereal crop that does not usually form root nodules containing nitrogen fixing bacteria.

Corn maize: Another cereal crop that lacks the typical legume type root nodules.

Oats: A grass similar to wheat in nutrient needs and without nitrogen fixing root nodules.

Rice: A staple cereal that depends on soil nitrogen and fertilisers rather than Rhizobium root nodules.

Common Pitfalls:
Students sometimes assume that any crop that needs nitrogen must have nitrogen fixing bacteria in its roots, which is not correct. Another confusion arises from hearing that all plants benefit from nitrogen fixation and incorrectly thinking that all have nodules. Remember that true symbiotic nitrogen fixing nodules are characteristic of legumes like peas, beans, and clover, not cereals like wheat and rice.

Final Answer:
Symbiotic nitrogen fixing bacteria in root nodules are commonly present in Peas.