Question

Which of the following is a stem nodulating green manure crop?

• A. Sesbania aculeata
• B. Crotalaria juncea
• C. Sesbania rostrata
• D. Vigna unguiculata

Hint:

Remember that the specific bacterium associated with the stem nodules of Sesbania rostrata is ______Azorhizobium caulinodans______, which is highly tolerant to flooded environments, unlike the typical root-nodulating ______Rhizobium______ species.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Green manuring is the practice of turning under undecomposed green plant tissues into the soil to improve physical structure and fertility.
Leguminous crops are preferred because they form a symbiotic association with Rhizobium bacteria to fix atmospheric dinitrogen (N$_2$).
While most legumes fix nitrogen exclusively through root nodules, a few rare species can form nodules on both roots and stems.

Step 2: Detailed Explanation:
Let us analyze the biological properties of the listed crops:
- Sesbania aculeata (Dhaincha): This is a widely used green manure crop in India. It possesses root nodules but lacks stem nodules. (Root nodulating)
- Crotalaria juncea (Sunnhemp): This is a fast-growing fiber and green manure crop that only forms nodules on its roots. (Root nodulating)
- Vigna unguiculata (Cowpea): This is a common pulse and fodder crop with nitrogen-fixing root nodules. (Root nodulating)
- Sesbania rostrata: Originating from West Africa, this species is highly unique because it forms symbiotic nodules on both its ______roots and stems______ with the bacterium Azorhizobium caulinodans.
The stem nodules appear as vertical rows of green bumps along the stem.
Because it nodulates on the stem, it can fix nitrogen under flooded or waterlogged conditions where soil root nodulation is inhibited due to anaerobic conditions.
This makes it an exceptionally valuable green manure crop for wet rice soils.

Step 3: Final Answer:
The stem-nodulating green manure crop is Sesbania rostrata, which corresponds to option (C).