Mineral Nutrition

Iron: It activates catalase enzyme, and is essential for the formation of chlorophyll. Zinc: It is needed in the synthesis of auxin. Boron: It is required for uptake and utilisation of $Ca^{2+}$, cell elongation, cell differentiation, membrane functioning, pollen germination and carbohydrate translocation. Molybdenum: It is a component of several enzymes, including nitrogenase and nitrate reductase.

Iron (Fe) is a constituent of cytochromes and ferredoxin (iron-sulphur proteins), which carry electrons in both the photosynthetic and the mitochondrial electron transport chains.
Copper (Cu) is a constituent of plastocyanin in the photosynthetic chain and of cytochrome oxidase (complex IV) in the mitochondrial chain.
Manganese (Mn) is essential for the splitting of water (photolysis) in photosystem II, which supplies electrons to the photosynthetic chain, and it activates several respiratory enzymes.
So a deficiency of Cu, Mn and Fe affects both photosynthetic and mitochondrial electron flow the most.
Ca, K and Na are not electron carriers, and Co, Ni and Mo have other roles (for example Mo in nitrogenase and nitrate reductase).
So the group is Cu, Mn, Fe.

$N_2 + 8e^- + 8H^+ + 16ATP \xrightarrow{nitrogenase,\ Mg^{2+}} 2NH_3 + H_2 + 16ADP + 16P_i$ So the products are ammonia and hydrogen.

In Glycine max (soybean), the fixed nitrogen is transported from the root nodules to the rest of the plant in the form of ureides.

Magnesium plays a role in maintaining the structural integrity of ribosomes (holding ribosomal subunits together), in addition to being a component of chlorophyll and an activator of many enzymes.

Calcium is a relatively immobile mineral element in plants and is generally not remobilized from older to younger tissues, unlike potassium, nitrogen, and phosphorus, which are readily mobilized.

Nitrococcus converts ammonia to nitrite (A-II); Rhizobium converts atmospheric nitrogen to ammonia (B-IV); Thiobacillus is involved in denitrification (C-I); Nitrobacter converts nitrite to nitrate (D-III).

Iron is absorbed by plants in the form of ferric ions (according to NCERT). Note: the source key marks 'Ferric' as per NCERT but notes that plants actually absorb iron in both forms, $Fe^{2+}$ and $Fe^{3+}$ (preferably $Fe^{2+}$).

$Rhodospirillum$ is an anaerobic, free-living nitrogen fixer. Mycorrhiza is a symbiotic association between fungi and roots of higher plants. So '$Rhodospirillum$ – Mycorrhiza' is the mismatch.

$Frankia$ is a nitrogen-fixing microbe (an actinomycete) that produces nitrogen-fixing nodules on the roots of non-leguminous plants such as $Alnus$; it is a symbiotic nitrogen fixer.
$Azotobacter$ is a free-living aerobic nitrogen fixer of the soil.
$Glomus$ is a mycorrhizal fungus that helps in phosphorus absorption; it does not fix nitrogen.
$Azolla$ is a water fern; it does not fix nitrogen itself (the cyanobacterium $Anabaena$ living in its leaves does).
Hence the symbiotic nitrogen fixer is $Frankia$.

Casuarina equisetifolia is a flowering plant (angiosperm), although its needle-like branchlets with scale leaves make it look like a pine; it is often called the Australian pine, she-oak or horsetail tree.
Its roots bear nodules that contain Frankia, a filamentous nitrogen-fixing actinomycete.
So it is a flowering plant with nodules containing a filamentous nitrogen-fixing micro-organism.
Crotalaria juncea (sunn hemp) and Cicer arietinum (gram) are legumes; their root nodules contain Rhizobium, which is a rod-shaped bacterium and not filamentous.
Cycas revoluta has coralloid roots containing the filamentous cyanobacteria Anabaena and Nostoc, but Cycas is a gymnosperm and not a flowering plant.
So the answer is Casuarina equisetifolia.

Nitrogenase is the enzyme that reduces atmospheric nitrogen to ammonia in biological nitrogen fixation:
$N_2 + 8e^- + 8H^+ + 16ATP \xrightarrow{nitrogenase} 2NH_3 + H_2 + 16ADP + 16P_i$
In legumes, nitrogen fixation takes place in the root nodules formed by Rhizobium, where nitrogenase is protected from oxygen by leghaemoglobin.
If all nitrogenase is inactivated, the fixation of nitrogen in legumes will stop.
Conversion of nitrate to nitrite is done by nitrate reductase, and conversion of ammonium to nitrate in soil (nitrification) is done by Nitrosomonas and Nitrobacter. These processes do not need nitrogenase, so they will continue.
So, as per the answer key, there will be no fixation of nitrogen in legumes. (Biological fixation of atmospheric nitrogen by free-living microbes would also stop, but nitrogen is still fixed by lightning and industrial processes, which do not use nitrogenase.)

Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides.
The statement reverses this (certain nucleic acids and all proteins), so it is wrong.
The other three statements are correct.

Bakanae (foolish seedling) disease of rice was studied by E. Kurosawa; Skoog worked on cytokinins.
Passive transport does not use ATP.
The apoplast is the system of cell walls and intercellular spaces; plasmodesmata belong to the symplast.
Potassium is a readily mobilised element in plants, so this is the pair accepted as correctly matched.

The figure is the nitrogen cycle, showing atmospheric $N_2$, the soil nitrogen pool ($NH_3 \rightarrow NO_2^- \rightarrow NO_3^-$) and the biomass.
A is the arrow from nitrate in the soil back to atmospheric $N_2$: reduction of nitrate to nitrogen by bacteria such as $Pseudomonas$ and $Thiobacillus$ is denitrification.
B is the arrow from decaying biomass to $NH_3$: decomposition of the organic nitrogen of dead plants and animals into ammonia is ammonification.
C receives nitrate/ammonia from the soil by 'uptake': these are the plants.
D is the group that obtains its nitrogen from plants and returns it to the decaying biomass: the animals.
Hence A – denitrification, B – ammonification, C – plants, D – animals.

Premature yellowing of leaves is chlorosis, i.e. the loss or non-formation of chlorophyll.
Magnesium is a constituent of the chlorophyll molecule, and iron is essential for the formation of chlorophyll (it activates the enzymes of chlorophyll synthesis).
Deficiency of either element causes chlorosis, which lowers photosynthesis and hence the seed yield.
Applying iron and magnesium promotes the synthesis of chlorophyll, keeps the leaves green and photosynthetically active, and so gives the maximum seed yield.
A pulse crop fixes its own nitrogen through root nodules, so nitrogenous fertiliser is not the main need.
Frequent irrigation does not correct chlorosis, and 2,4,5-T is a herbicide (defoliant) that would damage the crop.
So the most beneficial treatment, as given in the answer key, is the application of iron and magnesium to promote synthesis of chlorophyll.

Magnesium is the central atom of the chlorophyll molecule.
Even if nitrogen (from urea) is plentiful, chlorophyll cannot be made without magnesium. So the leaves lose their green colour and turn yellow (chlorosis), first in the older leaves, because Mg is mobile and moves to the young leaves.
So the plants show yellowish leaves.

Plants obtain sulphur in the form of sulphate ($SO_4^{2-}$).
Sulphur is present in the amino acids cysteine and methionine, and is a constituent of several coenzymes, vitamins (thiamine, biotin, coenzyme A) and ferredoxin.
Oilseed crops have a particularly high requirement of sulphur, because it is needed for the synthesis of oil and of sulphur-containing compounds.
For example, the pungent oils of mustard and other crucifers (glucosinolates) contain sulphur.
An adequate supply of sulphur increases both the seed yield and the oil content of oilseed crops such as mustard, groundnut and sunflower.
Pulses also respond to sulphur, but its importance for productivity is greatest in oilseeds.
So sulphur is an important nutrient for optimum growth and productivity in oilseed crops.

Bacteriochlorophyll, like chlorophyll, has a porphyrin ring made of four pyrrole rings, each containing nitrogen.
Invertase and pepsin are enzymes, i.e. proteins, built of amino acids, which contain nitrogen.
An idioblast is not a molecule but a type of cell: an isolated cell that differs from the surrounding cells in shape, size or content (e.g. cells containing crystals of calcium oxalate, tannins or oil).
So idioblast is the item in which nitrogen is not listed as a constituent (it is a cell, not a nitrogen-containing compound).

Grey speck (gray spot) disease of oats is a physiological disorder caused by manganese deficiency. Grey-brown spots and streaks appear on the leaves.
Manganese activates many enzymes of photosynthesis, respiration and nitrogen metabolism, and is needed for the splitting of water in photosynthesis.
Other examples: marsh spot of peas is also due to Mn deficiency; little leaf is due to Zn deficiency; reclamation disease is due to Cu deficiency; and chlorosis of young leaves is due to Fe deficiency.
So gray spots of oat are caused by deficiency of Mn.