The correct option is (c) 1 and 3 only.
Explanation
Rhizobium is a genus of soil bacteria known for establishing a symbiotic relationship with leguminous plants. This association is critical for the biological fixation of atmospheric nitrogen, converting it into forms that plants can assimilate for growth.
Statement-wise Analysis
- Statement 1 is Correct. Rhizobium bacteria contain the enzyme nitrogenase, which facilitates the conversion of atmospheric nitrogen ($N_2$) into ammonia ($NH_3$). This soluble form of nitrogen is readily utilized by plants for the synthesis of amino acids and proteins.
- Statement 2 is Incorrect. Rhizobium is a heterotrophic bacterium, meaning it cannot synthesize its own food. It does not possess chlorophyll and does not perform photosynthesis. Instead, it relies on the host plant to provide carbohydrates (sugars) for energy.
- Statement 3 is Correct. These bacteria inhabit the root nodules of leguminous plants such as gram, peas, beans, and lentils. Inside these nodules, the bacteria live in a symbiotic relationship, providing nitrogen to the plant while receiving shelter and nutrients in return.
Key Takeaway: Rhizobium is a symbiotic, nitrogen-fixing bacterium found in the root nodules of legumes. It is heterotrophic and depends on the host plant for food, distinguishing it from photosynthetic autotrophs like cyanobacteria.