Nitrogen fixing bacteria make combination with cells of the roots of

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Correct — A, Pulses. Nitrogen-fixing bacteria of the Rhizobium group (Rhizobium, Bradyrhizobium, Mesorhizobium, Sinorhizobium) infect the root hairs of leguminous plants — the pulses: gram/chickpea, arhar, moong, urad, masoor, pea, plus groundnut and soybean — and multiply inside the cortical cells of the root, which swell into visible root nodules. Inside the nodule the bacteria change into bacteroids and use the enzyme nitrogenase to reduce atmospheric N2 into ammonia that the plant can use; in return the plant supplies sugars and makes leghaemoglobin, a pink oxygen-binding pigment that keeps free oxygen away from the oxygen-sensitive nitrogenase. This literal 'combination with cells of the roots' is a legume speciality, and it is why pulses leave the soil richer in nitrogen and are the standard rotation crop after a cereal.

Why the other options are wrong — (b) Rice is a cereal of the grass family (Poaceae) and forms no root nodules. Flooded paddy fields do gain biologically fixed nitrogen, but from free-living cyanobacteria (Anabaena, Nostoc) in the standing water and from the Azolla water-fern, which houses Anabaena azollae in its leaf cavities — not from bacteria living inside the root cells.  (c) Wheat is also a non-leguminous cereal with no nodule symbiosis. Its nitrogen comes from soil organic matter and applied urea/ammonium fertiliser; engineering cereals to nodulate is an active research goal, not a field reality.  (d) Sugarcane is a grass too. Some nitrogen-fixing bacteria are reported living inside sugarcane tissue as endophytes, but that is an associative relationship — sugarcane forms no root nodules, so it is not the root-cell symbiosis this question describes.

Concept: Atmospheric nitrogen is 78% of the air but is locked in an N2 triple bond that plants cannot break. Only certain prokaryotes carry nitrogenase, the enzyme that reduces N2 to ammonia — this is biological nitrogen fixation. It comes in three forms: symbiotic (Rhizobium inside legume root nodules; Frankia inside the nodules of Casuarina and Alnus), free-living (Azotobacter, Clostridium, and the cyanobacteria Anabaena and Nostoc), and associative/endophytic (Azospirillum with cereal roots). Only the symbiotic form involves the bacterium living inside the plant's own root cells.

State PCS 2019 Q11 — Science & Technology PYQ with Answer

State PCS 2019 Q11 — Science & Technology PYQ with Answer