1.1 Ion Absorption & Radial Movement of Ions

Nutrients reach the root surface by bulk flow (transpiration-driven, carried in the water stream) and diffusion (down a concentration gradient created by uptake). Membrane crossing is passive (channels/carriers, no energy, down gradient) or active — primary active transport is coupled directly to ATP hydrolysis or light (e.g. the H⁺-ATPase), while secondary active transport uses the proton-motive force the primary pump creates (symport/antiport). Maximum absorption occurs at the root-hair zone; mycorrhizal association extends the effective absorptive surface for P, NH₄⁺ and K⁺.

Step-by-step pathway

1. Entry into the apoplast: ions diffuse freely through cell-wall microchannels of the epidermis and cortex (no membrane, no metabolic cost).
2. Optional symplastic entry: an ion may cross a cortical plasma membrane and then move symplastically cell-to-cell through plasmodesmata.
3. Casparian strip checkpoint: the endodermal wall seals the apoplastic path; any ion still travelling apoplastically must cross the endodermal plasma membrane to proceed — giving the plant selective control over ions reaching the stele.
4. Xylem loading: mature tracheary elements are dead protoplasts (no living symplast), so ions travelling symplastically must exit via efflux transporters into the apoplastic xylem lumen.

Figure 1. Radial movement of ions from soil solution to the xylem via the apoplastic vs symplastic routes.

1.2 Essential Mineral Elements

Arnon & Stout (1939) criteria for essentiality: (i) the life cycle cannot be completed without the element, (ii) its function cannot be replaced by another element, (iii) it is directly involved in metabolism. About 17 elements are essential: 3 non-mineral (C, H, O, from CO₂/H₂O) and 14 mineral elements from soil, split into macronutrients (N,P,K,Ca,Mg,S; ≥1000 mg/kg dry wt) and micronutrients (Fe,Mn,Zn,Cu,B,Mo,Cl,Ni; ≤100 mg/kg).

Figure 2. Classification of essential elements by source and quantitative requirement.

Elements, roles & deficiency symptoms

1.3 The Nitrogen Cycle

Nitrogen moves between atmosphere, soil and organisms via biological N₂ fixation, ammonification (mineralisation of organic N), two-step nitrification (NH₄⁺→NO₂⁻ by Nitrosomonas/Nitrosospira, then NO₂⁻→NO₃⁻ by Nitrobacter/Nitrospira), denitrification (anaerobic, returns N₂ to the atmosphere) and the recently discovered anammox process (anaerobic oxidation of ammonium using nitrite as the electron acceptor, N₂ produced directly).

Figure 3. The nitrogen cycle.

1.4 Nitrate & Nitrite Assimilation

1. Membrane transport: NO₃⁻ enters root cells via 2H⁺/NO₃⁻ symporters, energised by the H⁺-ATPase-generated proton-motive force. NH₄⁺ enters via ammonia channels, uniporters, or H⁺/NH₄⁺ symporters.
2. Nitrate reductase (cytosol): NO₃⁻ + NAD(P)H + H⁺ → NO₂⁻ + NAD(P)⁺ + H₂O. Electrons flow FAD → heme (cytochrome b₅) → Mo-molybdopterin. Rate-limiting; inactivated in the dark by phosphorylation + 14-3-3 protein binding.
3. Nitrite reductase (plastid): NO₂⁻ + 6 Fd(red) + 8H⁺ → NH₄⁺ + 6 Fd(ox) + 2H₂O, via Fe₄S₄ and siroheme in one active site — avoids releasing toxic intermediates to the cytosol.
4. GS–GOGAT cycle: Glutamine synthetase (Glu + NH₄⁺ + ATP → Gln + ADP + Pi) then glutamate synthase/GOGAT (Gln + α-ketoglutarate + 2e⁻ → 2 Glu; NADH- or Fd-dependent isoforms).
5. Nitrogen distribution: one glutamate regenerates the GS acceptor (cyclic); the other feeds transamination reactions that distribute the amino group across the amino acid pool.

Figure 4. Plasma-membrane transporters for nitrate/ammonium entry, and vacuolar nitrate sequestration.

Figure 5. Nitrate/nitrite assimilation cascade: cytosolic nitrate reductase → plastidial nitrite reductase → GS-GOGAT cyclic assimilation.

1.5 Biological Nitrogen Fixation

Only prokaryotes (diazotrophs) fix N₂, catalysed by the nitrogenase complex = dinitrogenase reductase (Fe protein, homodimer, one 4Fe-4S cluster) + dinitrogenase (MoFe protein, α₂β₂ tetramer with P-clusters, Fe₈S₉, and the FeMo-cofactor/M-cluster — the catalytic site). Stoichiometry: N₂ + 8e⁻ + 8H⁺ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi (an obligatory 2 of the 8 electrons reduce H⁺ to H₂).

1. Electron loading: reduced ferredoxin donates an electron to the Fe₄S₄ cluster of the Fe protein, which binds 2 MgATP.
2. Complex formation & ATP hydrolysis: the Fe protein docks onto the MoFe protein; ATP hydrolysis drives single-electron transfer via the P-cluster to the FeMo-cofactor.
3. Dissociation & re-reduction: the oxidised Fe protein dissociates (rate-limiting), is re-reduced, rebinds ATP, and repeats — 8 cycles per N₂.
4. Substrate reduction: N≡N → HN=NH → H₂N–NH₂ → 2NH₃ at the FeMo-cofactor.
5. Oxygen protection: nitrogenase is irreversibly O₂-inactivated, so fixation occurs in anaerobic microsites — heterocysts (cyanobacteria) or leghemoglobin-buffered nodule cytosol (legume symbioses).

Figure 6. The nitrogenase catalytic cycle.

1.6 Rhizobium–Legume Symbiosis & Nodule Formation

1. Flavonoid signalling: legume roots exude flavonoids that attract and activate compatible Rhizobium strains.
2. NodD activation: flavonoid binding activates bacterial NodD, which drives transcription of common (nodA,B,C) and host-specific (nodP,Q,H) nod genes.
3. Nod factor biosynthesis: NodC builds a chitin-oligosaccharide backbone, NodB deacetylates it, NodA attaches a fatty acyl chain — producing the lipochitooligosaccharide Nod factor.
4. Perception: root-hair LysM receptor kinases perceive Nod factor → root-hair curling, Ca₂⁺ spiking, cortical cell division, nodulin gene induction.
5. Infection thread: a tubular plasma-membrane invagination guides bacteria through the root hair and cortex toward the dividing cortical cells (nodule primordium).
6. Bacteroid differentiation: released bacteria stop dividing, enlarge, and differentiate into N₂-fixing bacteroids inside the host-derived peribacteroid membrane.
7. Nitrogen exchange: bacteroid NH₃ is assimilated by host GS-GOGAT and shipped to the shoot as amides or ureides; the host supplies photosynthate in return.

Figure 7. Stages of Rhizobium-induced root-nodule formation.

2. Experimental & Analytical Significance (Part C)

• Hydroponics/aeroponics essentiality assays: withhold a candidate element from a complete nutrient solution; a reproducible, non-substitutable deficiency reversed by re-supplementation confirms essentiality (Arnon & Stout criteria). Accumulation alone (e.g. Se, Au, Sr) does not prove essentiality — a classic trap.
• Acetylene reduction assay (ARA): nitrogenase also reduces C₂H₂ to C₂H₄, the standard indirect assay for nitrogenase activity (detected by gas chromatography); the theoretical 3:1 stoichiometry with N₂ reduction is not always observed due to competing H₂ evolution.
• Nod/nif/fix mutant analysis: nodA/B/C mutants cannot make Nod factor → no nodulation (Nod⁻). nif/fix mutants form nodules but cannot fix nitrogen (Nod⁺Fix⁻), often visibly white/green rather than pink due to failed leghemoglobin/nitrogenase function.
• Leghemoglobin knockdown: lowers O₂ buffering → nitrogenase inactivated → poor fixation despite normal nodule morphology — separates "nodule formation" from "fixation capacity".
• MSX (GS inhibitor) feeding: blocking glutamine synthetase causes ammonia to accumulate toxically, demonstrating that GS-GOGAT (not GDH) is the primary in-vivo ammonia assimilation route.
• Casparian strip tracer assays: apoplastic tracer dyes fail to bypass the endodermis in wild-type roots but leak into the stele in Casparian-strip-defective mutants — proof that the strip enforces a symplastic checkpoint.

Typical "mismatch" statement pairs