Question

Rhizobium japonicum fixes N in symbiotic relationship with:

• A. Pea group
• B. Lupin group
• C. Soybean group
• D. Phaseolus group

Hint:

Always associate the suffix "japonicum" with the soybean cross-inoculation group.
Inoculating soybean seeds with *Bradyrhizobium japonicum* before sowing is a standard practice to boost nitrogen fixation in soils where soybeans have not been grown recently.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The symbiotic relationship between legumes and nitrogen-fixing bacteria (*Rhizobium* or *Bradyrhizobium*) is highly host-specific.
Legumes are grouped into cross-inoculation groups, where a specific species of bacteria can form functional nodules and fix nitrogen only with a designated group of host plants.

Step 2: Detailed Explanation:
Let us review the cross-inoculation groups and their specific bacterial partners:
1.Pea group (A): Nodulated by *Rhizobium leguminosarum* bv. *viciae* (includes peas, lentils, and vetch).
2.Lupin group (B): Nodulated by *Bradyrhizobium lupini* (includes lupines).
3.Phaseolus group (D): Nodulated by *Rhizobium leguminosarum* bv. *phaseoli* (includes common beans like kidney beans).
4.Soybean group (C): Symbiotically nodulated by *Rhizobium japonicum* (now primarily classified as *Bradyrhizobium japonicum*).
This bacterium is highly specific to the soybean (*Glycine max*) group, where it forms effective nodules to convert atmospheric nitrogen (\( N_2 \)) into plant-available ammonia.

Step 2: Final Answer:
*Rhizobium japonicum* fixes nitrogen in a symbiotic relationship with the Soybean group.