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The nitrogen-fixing bacterium, Rhizobium leguminosarum, isolated from the root nodules of garden pea (Pisum sativum), is cultured in a petri plate containing appropriate nutrient agar medium. A bacterial colony was picked and inoculated into a liquid growth medium to scale up the culture for the production of biofertilizer.

Which one of the following statements is correct?

Correct option is C

​Rhizobium leguminosarum is a symbiotic nitrogen-fixing bacterium that forms nodules on the roots of leguminous plants (e.g., peas, beans, and clovers). It plays a crucial role in biological nitrogen fixation (BNF) by converting atmospheric nitrogen (N₂) into ammonia (NH₃) in a low-oxygen environment inside root nodules.

Analysis of the Given Options:

1. "The liquid culture will be red/pink in color due to the accumulation of the pigment leghaemoglobin." -Incorrect
   • leghaemoglobin is an oxygen-binding pigment found inside root nodules, where it maintains low oxygen levels for nitrogenase activity.
   • Rhizobium itself does not produce leghaemoglobin; it is synthesized by the plant in response to bacterial infection.
   • In a liquid culture, Rhizobium would appear milky white or light yellow, NOT red/pink.
2. "The rhizobial cells when reinoculated into the rhizosphere of soybean plants will effectively nodulate its roots to fix atmospheric nitrogen."-Incorrect
   • Rhizobium species exhibit host specificity, meaning each species associates with a specific group of legumes.
   • Rhizobium leguminosarum is specific to peas (Pisum sativum) and will NOT effectively nodulate soybean roots.
   • Soybean (Glycine max) requires Bradyrhizobium japonicum for effective nodulation and nitrogen fixation.
3. The rhizobial cells cannot fix nitrogen when exposed to atmospheric air."-Correct
   • Rhizobia require a low-oxygen environment to fix nitrogen.
   • In root nodules, leghaemoglobin maintains microaerophilic conditions that allow the nitrogenase enzyme to function.
   • In a liquid culture or when exposed to atmospheric air, nitrogenase is inactivated due to high oxygen levels.
   • Thus, free-living Rhizobium in liquid culture cannot fix nitrogen.
4. The rhizobial cells get transformed into bacteroids when grown in liquid media." - Incorrect
   • Bacteroids are the differentiated, nitrogen-fixing form of Rhizobium that develop only inside root nodules in response to plant signals.
   • In liquid culture, Rhizobium remains in its free-living, undifferentiated state and does NOT transform into bacteroids.

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