Q20. What is the function of leghemoglobin in root nodules of leguminous plants?
[Chhattisgarh Teacher SETE, 11 August 2019]
(a) Nodule differentiation
(b) Expression of nif genes
(c) Removal or buffering of free oxygen
(d) Inhibition of nitrogenase activity
Answer: (c) Removal or buffering of free oxygen
Explanation: Leghemoglobin controls free oxygen concentration inside active root nodules. Oxygen is required for bacterial respiration but can inactivate nitrogenase when freely abundant. By binding oxygen, leghemoglobin sustains metabolism while protecting the nitrogen-fixing enzyme system.
Q19. Which of the following can fix atmospheric nitrogen?
1. Azolla;
2. E. coli;
3. Cyanobacteria;
4. Azospirillum.
[UP LT Grade Teacher Biology, 2018]
(a) 1, 2 and 4
(b) 2 and 3
(c) 1, 3 and 4
(d) 1, 2, 3 and 4
Answer: (c) 1, 3 and 4
Explanation: Azolla harbours nitrogen-fixing cyanobacteria; many cyanobacteria and Azospirillum bacteria also fix atmospheric nitrogen. E. coli is not generally nitrogen-fixing. Because the first, third and fourth entries qualify in this examination, the correct combination is 1, 3 and 4.
Q18. Which of the following does NOT fix nitrogen?
[UP LT Grade Teacher Biology, 2018]
(a) Mycorrhiza
(b) Frankia
(c) Rhizobia
(d) Cyanobacteria
Answer: (a) Mycorrhiza
Explanation: Mycorrhizal fungi chiefly facilitate phosphorus, water and mineral absorption rather than atmospheric nitrogen fixation. Frankia and rhizobia contain nitrogen-fixing bacteria in plant associations; certain cyanobacteria also use nitrogenase to fix nitrogen. Hence mycorrhiza is the exception.
Q17. Which nitrogen-fixing agent and crop pair is NOT correctly matched?
[UPPSC PCS Prelims, 2017]
(a) Blue-green algae — paddy
(b) Rhizobium leguminosarum — pea
(c) Azotobacter — wheat
(d) Azolla — maize
Answer: (d) Azolla — maize
Explanation: Azolla is strongly associated with flooded rice cultivation through its symbiotic nitrogen-fixing cyanobacteria, rather than maize biofertilization. Blue-green algae support paddy, Rhizobium leguminosarum associates with peas, and free-living Azotobacter can be applied to wheat.
Q16. Which is NOT a nitrogen-fixing organism?
[UPPSC PCS Mains, 2017]
(a) Nostoc
(b) Anabaena
(c) Azotobacter
(d) Mycorrhiza
Answer: (d) Mycorrhiza
Explanation: Nostoc and Anabaena include nitrogen-fixing cyanobacteria, while Azotobacter is a nitrogen-fixing soil bacterium. Mycorrhiza is the association of fungi with plant roots; its best-known nutrient function is improved phosphorus uptake, not atmospheric nitrogen fixation.
Q15. Which free-living bacterium can fix atmospheric nitrogen?
[MP Lab Assistant, 15 April 2017, Shift 1]
(a) Azospirillum
(b) Spirulina
(c) Rhizobium
(d) Nitrosomonas
Answer: (a) Azospirillum
Explanation: Azospirillum includes nitrogen-fixing bacteria frequently living in the rhizosphere or close association with cereal roots. Rhizobium is best known for symbiotic legume nodules, while Nitrosomonas oxidizes ammonia. The paper selects Azospirillum among the given alternatives.
Q14. Which biofertilizer is also used as supplementary feed for milking cattle?
[RPSC RAS/RTS Prelims, 2016]
(a) Azospirillum
(b) Azolla
(c) Azotobacter
(d) Rhizobium
Answer: (b) Azolla
Explanation: Azolla is a rapidly growing aquatic fern used as a biological nitrogen source in rice fields. Its protein-rich biomass can also supplement livestock feed. Azospirillum, Azotobacter and Rhizobium are microbial inoculants, not conventional leafy cattle-feed supplements.
Q13. In which plant are root nodules NOT formed by Rhizobium?
[UP TGT Biology, 2016]
(a) Cajanus cajan
(b) Cicer arietinum
(c) Glycine max
(d) Casuarina glauca
Answer: (d) Casuarina glauca
Explanation: Casuarina is not a leguminous host of Rhizobium; its nitrogen-fixing root nodules are associated with Frankia actinobacteria. Pigeon pea, chickpea and soybean are legumes and establish rhizobial symbioses. The question distinguishes two major kinds of nitrogen-fixing plant partnerships.
Q12. Which aquatic fern is used as a biofertilizer?
[UP Review Officer/Assistant Review Officer Mains, 2014]
(a) Salvinia
(b) Azolla
(c) Marsilea
(d) Pteridium
Answer: (b) Azolla
Explanation: Azolla is a floating water fern that contains nitrogen-fixing cyanobacteria within specialized leaf structures. The symbiosis contributes nitrogen to flooded rice systems and allows Azolla biomass to serve as green manure. Other listed ferns are not the conventional answer.
Q11. Consider: 1. Agaricus; 2. Nostoc; 3. Spirogyra. Which are used as biofertilizers?
[UPSC Civil Services Prelims, 2013]
(a) 1 and 2
(b) 2 only
(c) 2 and 3
(d) 3 only
Answer: (b) 2 only
Explanation: Nostoc is a nitrogen-fixing cyanobacterium used to increase soil nitrogen availability. Agaricus is a mushroom genus, while Spirogyra is a green alga that is not conventionally used as a nitrogen-fixing inoculant. Therefore only the second organism qualifies.
