Question

The fixation of nitrogen by the rhizobia in the root nodules of pulse crops generally commences at-

• A. 10 days after sowing
• B. 15 days after sowing
• C. 30 days after sowing
• D. 60 days after sowing

Hint:

In pulses, nitrogen fixation starts around 30 days and continues till pod filling stage for maximum benefit.

Answer 1

The Correct Option is C

Step 1: Role of Rhizobia.
Rhizobia are symbiotic bacteria that colonize the roots of leguminous plants and form nodules.
Inside these nodules, they fix atmospheric nitrogen into ammonia, which the plant can utilize.
Step 2: Timeline of nodule formation.
- After sowing, roots grow and rhizobia infection occurs within 7--10 days.
- Nodule initiation begins around 10--15 days after sowing.
- However, active nitrogen fixation usually does not start immediately after infection.
- It generally becomes significant only after 30 days when nodules are well developed.
Step 3: Analysis of options.
- (A) 10 days: Too early; nodules are not yet fully developed.
- (B) 15 days: Still early; nodules may appear but fixation is negligible.
- (C) 30 days: Correct; by this time, nodules are functional and nitrogen fixation is active.
- (D) 60 days: Too late; fixation begins much earlier than this.
Step 4: Conclusion.
Thus, nitrogen fixation in root nodules of pulses generally commences at around 30 days after sowing.