Question

Which of the following nutrient helps in the nodule formation in leguminous plants

• A. Sodium
• B. Manganese
• C. Boron
• D. Zinc

Hint:

Nutrients in Legume Symbiosis:
- Boron (B): Nodule initiation, infection thread formation, and vascular integration.
- Molybdenum (Mo): Structural component of the Nitrogenase enzyme.
- Cobalt (Co): Component of Vitamin $\text{B}_{12}$ (Cobalamin), needed by Rhizobium.
- Iron (Fe): Component of Leghaemoglobin and the Fe-protein of nitrogenase.

Answer 1

The Correct Option is C

Concept:
Leguminous plants form symbiotic associations with nitrogen-fixing soil bacteria (Rhizobium) within specialized root structures called nodules.
The initiation, morphological development, and maintenance of root nodules require specific essential macro- and micronutrients.
Among micronutrients, certain elements are directly involved in cell wall integrity, infection thread formation, bacterial invasion, and vascular connection between the host plant and the developing nodule.

Step 1: Role of Boron in Root Nodule Formation:
Boron ($\text{B}$) is essential for cell wall synthesis, membrane integrity, carbohydrate transport, and cell division in root meristems.
During Rhizobium-legume symbiosis, boron is required for:
1. Proper structural development of the infection thread through which rhizobia enter host root hairs.
2. Membrane-protein interactions and development of the symbiosome membrane that encloses bacteroids.
3. Establishing the vascular bundle connection between the host root tissue and the nodule to supply photosynthates to the bacteria.
Under boron deficiency, nodule development is arrested, bacteroids fail to differentiate properly, and nitrogen fixation is significantly impaired.

Step 2: Evaluating the Other Choices:
- Sodium (A): A functional/beneficial element for halophytes and $\text{C}_4$ species; not directly required for nodule formation.
- Manganese (B): Primarily involved in the photolysis of water in Photosystem II.
- Zinc (D): Required for auxin (IAA) biosynthesis and carbonic anhydrase activation.

Step 3: Final Answer:
Among the given options, Boron is the micronutrient that directly supports nodule formation in leguminous plants, matching option (C).