Q50. Microbes like Rhizobium, Nitrosomonas and Nitrobacter are involved in which cycle?
[SSC CGL Tier I, 12 September 2024, Shift 1]
(a) Nitrogen cycle
(b) Carbon cycle
(c) Water cycle
(d) Sulphur cycle
Answer: (a) Nitrogen cycle
Explanation: Rhizobium fixes atmospheric nitrogen in legume nodules. Nitrosomonas oxidizes ammonia into nitrite, and Nitrobacter converts nitrite to nitrate. These activities form different stages of the nitrogen cycle that determine which nitrogen compounds become available to plants.
Q49. Mycorrhiza is a symbiotic association between roots of higher plants and ________.
[RSMSSB Agriculture Supervisor, 4 February 2024]
(a) Fungi
(b) Bacteria
(c) Nematodes
(d) Viruses
(e) Question not attempted
Answer: (a) Fungi
Explanation: Mycorrhizae consist of fungi associated with plant root systems. Fungal hyphae extend the effective absorption area, especially for phosphorus and water. In return, the plant supplies photosynthates. This beneficial interaction is the basis of several fungal biofertilizer applications.
Q48. Which of these cyanobacterial genera can fix atmospheric nitrogen: Anabaena, Nostoc and Oscillatoria?
[Bihar TRE 3.0 PGT Botany, 22 July 2024]
(a) Anabaena
(b) Nostoc
(c) Oscillatoria
(d) More than one of the above
(e) None of the above
Answer: (d) More than one of the above
Explanation: Anabaena and Nostoc are established examples of nitrogen-fixing cyanobacteria. Their nitrogenase activity allows them to convert atmospheric nitrogen into usable compounds. The official multiple-answer format therefore selects more than one; nitrogen fixation among Oscillatoria species is not universally established.
Q47. Which organism is NOT helpful in fixing atmospheric nitrogen into a plant-usable form?
[CUET PG B.Ed., 15 March 2024, Shift 2]
(a) Rhizobium
(b) Azospirillum
(c) Mycorrhiza
(d) Azotobacter
Answer: (c) Mycorrhiza
Explanation: Mycorrhizal fungi mainly help plant roots obtain phosphorus, water and other minerals. They do not themselves perform atmospheric nitrogen fixation. Rhizobium, Azospirillum and Azotobacter, in contrast, include bacteria able to fix atmospheric nitrogen and contribute to plant nutrition.
Q46. Which aquatic plant is commonly used as a biofertilizer in waterlogged rice fields?
[UPPSC PCS Prelims, 14 May 2023]
(a) Wolffia
(b) Azolla
(c) Trapa
(d) Lemna
Answer: (b) Azolla
Explanation: Azolla is a floating aquatic fern that hosts a nitrogen-fixing cyanobacterium in specialized leaf cavities. This association contributes biologically fixed nitrogen to flooded rice systems. Farmers can incorporate Azolla biomass into fields, reducing reliance on some chemical nitrogen inputs.
Q45. Which alga serves as a biofertilizer in rice fields?
[BPSC PGT Botany, 26 August 2023, Shift 2]
(a) Chlamydomonas
(b) Cladophora
(c) Nostoc
(d) More than one of the above
(e) None of the above
Answer: (c) Nostoc
Explanation: Nostoc is a nitrogen-fixing cyanobacterial genus commonly called blue-green algae. Its ability to enrich flooded soils with fixed nitrogen makes it valuable in rice cultivation. Chlamydomonas and Cladophora are green algae and are not the standard nitrogen-fixing rice biofertilizers.
Q44. Which biofertilizer is used for soybean cultivation?
[MP Police Constable, 19 August 2023, memory-based paper]
(a) Nostoc
(b) Azospirillum
(c) Rhizobium
(d) Azotobacter
Answer: (c) Rhizobium
Explanation: Soybean is a leguminous crop that develops nitrogen-fixing root nodules with compatible rhizobia. Appropriate Rhizobium or Bradyrhizobium inoculation improves nodulation and nitrogen nutrition. Free-living Azotobacter and Azospirillum are beneficial microbes but not the conventional soybean symbiotic inoculant.
Q43. Biofertilizers convert atmospheric nitrogen into which compound?
[MP Patwari, 24 March 2023, memory-based paper]
(a) Ammonia
(b) Nitrate
(c) Nitride
(d) Uric acid
Answer: (a) Ammonia
Explanation: Biological nitrogen fixation uses nitrogenase to reduce molecular nitrogen into ammonia. Ammonia subsequently forms ammonium ions in soil and may be converted into nitrite and nitrate through nitrification. Fixation and nitrification are separate steps of the soil nitrogen cycle.
Q42. Which pigment is essential for effective nitrogen fixation in the root nodules of legumes?
[Bihar TRE 2.0, 9 December 2023]
(a) Anthocyanin
(b) Leghemoglobin
(c) Phycocyanin
(d) More than one of the above
(e) None of the above
Answer: (b) Leghemoglobin
Explanation: Leghemoglobin is the oxygen-binding red pigment found in functional legume root nodules. It regulates free oxygen concentrations, enabling respiration while protecting oxygen-sensitive nitrogenase. Anthocyanins color plant tissues, whereas phycocyanin is a photosynthetic pigment of cyanobacteria.
Q41. What term denotes fungi living symbiotically with roots of higher plants?
[MP Forest Guard, 7 June 2023, Shift 3]
(a) Mycorrhiza
(b) Phycorrhiza
(c) Lichens
(d) Corals
Answer: (a) Mycorrhiza
Explanation: Mycorrhiza describes the association of fungal hyphae with plant roots. The fungi increase effective access to phosphorus and soil moisture, while the plant supplies carbohydrates. These associations improve nutrient capture and are used in sustainable agriculture and ecological restoration.
