2.1 Essential elements #
2.1.1 The criterion, and the two quantity classes #
An element is essential, by the criterion associated with Arnon and Stout, if the plant cannot complete its life cycle without it, if no other element can substitute, and if it acts in metabolism itself rather than by changing the soil, for example by precipitating a toxin. A field response does not meet the test. Soils are contaminated, and a treatment can help indirectly. The test is a solution culture with one element omitted. These notes do not give recipes.
Seventeen elements meet the criterion for a typical angiosperm: C, H, and O from CO2 and water, plus N, P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B, Mo, Cl, and Ni. Nickel, required for urease, is the latest addition. Macronutrients (N, P, K, Ca, Mg, S) occur at or above about a tenth of one percent of dry matter. Micronutrients are required at much lower levels. Quantity is not importance: molybdenum shortage stops nitrate reductase and nitrogenase. Sodium for some C4 species, silicon for rice, and cobalt for rhizobia are beneficial, which is a different word from essential.
2.1.2 Six functions and six symptoms #
N, P, K, and Mg move in the phloem, so deficiency shows on old leaves. Ca and Fe do not, so deficiency shows on young leaves and meristems.
Pin Function In Auxin Transport
| Element | Main roles | One standard deficiency symptom | Phloem-mobile? |
|---|---|---|---|
| N | Amino acids, nucleic acids, chlorophyll | General chlorosis of older leaves; stunting | Yes |
| P | ATP, nucleic acids, phospholipids, sugar phosphates | Stunting; dark green or purple leaves from anthocyanin | Yes |
| K | Enzyme activation; osmotic solute in vacuoles and guard cells. Not built into organic molecules | Marginal scorch of older leaves; weak stems | Yes |
| Ca | Calcium pectate in the middle lamella; membranes; second messenger | Death or distortion of apices. Blossom-end rot of tomato is a local delivery failure, often after irregular watering | No |
| Mg | Centre of chlorophyll; enzyme activation; ribosomes | Interveinal chlorosis of older leaves | Yes |
| Fe | Cytochromes, ferredoxin, Fe–S centres. Needed to make chlorophyll, but not part of it | Interveinal chlorosis of young leaves, veins remaining green | No |
Sulphur is also a macronutrient, in cysteine, methionine, and coenzyme A. It is less mobile than nitrogen, so a shortage can look like nitrogen deficiency on younger leaves. Iron and magnesium both strip colour between veins. Leaf age separates them.
2.2 From the soil solution into the xylem #
The study of From the soil solution into the xylem provides fundamental principles governing cellular architecture, molecular interactions, and physiological regulation.
Flow And Exchange Of Nutrients In The Phloem And Xylem Of Plants
2.2.1 Availability, pH, and iron #
The root sees the soil solution, not the total inventory of the profile. Phosphate adsorbs to iron and aluminium oxides in acid soil and precipitates with calcium in alkaline soil, so it is most available near neutral pH, and it barely moves even then. Nitrate moves and is leached. Ammonium binds cation-exchange sites.
Fe3+ is very insoluble as pH rises, so calcareous soils cause lime-induced chlorosis even when total iron is high. Strategy I plants, dicots and non-grass monocots, acidify the rhizosphere, reduce Fe3+ to Fe2+ with FRO, and take Fe2+ up through IRT. Strategy II grasses release mugineic-acid phytosiderophores, and a YSL transporter takes up the Fe3+ chelate. Rice in flooded soil, where Fe2+ is abundant, also uses an IRT-type route. Acid soils add a separate problem, Al3+ toxicity. Some wheats exude malate or citrate and chelate aluminium outside the root. That is tolerance, not a use for aluminium.
2.2.2 Passive and active uptake #
Passive uptake runs down the electrochemical gradient, through a channel or a facilitator. Active uptake runs against it and costs metabolism. The primary pump of the plant plasma membrane is a P-type H+-ATPase, not the animal Na+/K+-ATPase. It acidifies the wall to about pH 5–5.5 and holds the cytosol near pH 7 and electrically negative, often about −100 to −200 mV. The tonoplast has a V-type H+-ATPase and an H+-pyrophosphatase, which acidify the vacuole. Symporters carry anions and sugars in with H+. Antiporters push Na+ or Ca2+ out of the cytosol in exchange for H+.
The Nernst relation sorts the two cases. At room temperature a ten-fold gradient of a monovalent ion corresponds to about 59 mV. A cell at −150 mV can hold K+ far above the outside concentration without spending energy on K+ itself, so ordinary uptake through channels is passive. At very low external K+, H+-coupled HAK/KUP symporters take over, and that uptake is active. Anions are the opposite: a negative cytosol pushes nitrate, phosphate, and sulphate out, so accumulating them is H+ symport. “All mineral uptake is active” is false. The tests are thermodynamic, kinetic, and metabolic. They are principles, not a protocol.
2.2.3 Apoplast, symplast, and the Casparian strip #
The apoplast is everything outside the plasma membrane: walls, intercellular spaces, and the lumens of dead xylem. The symplast is cytoplasm joined by plasmodesmata, which pass small solutes and, under regulation, some macromolecules. They are not an open door. A third, transmembrane, path crosses a membrane at every cell.
Ions can diffuse through cortical walls until the endodermis. The Casparian strip is a lignin band in the radial and transverse walls, continuous from cell to cell, sealing the apoplast. Suberin lamellae often appear later. Water and solutes still outside a protoplast must cross an endodermal plasma membrane. Selectivity is imposed again when xylem parenchyma releases ions into the dead conduits. Without the strip, transpiration would sweep soil solution into the shoot. Lateral roots interrupt the strip locally, and a small bypass, important for Na+ in rice, is a leak rather than the design. Root hairs add surface. They are still epidermal membranes.
2.3 Nitrogen and the nutritional mutualisms #
2.3.1 Nitrate and ammonium #
Nitrate is taken up by NRT H+ symporters and ammonium by AMT transporters. Nitrate moves in aerated soil. Ammonium sticks to exchange sites, moves less, and is toxic if it accumulates. Ammonium uptake acidifies the rhizosphere, because charge balance requires H+ export.
Nitrate reductase, in the cytosol, uses NADH and contains molybdenum and haem. Nitrite is toxic and moves at once into the plastid, where nitrite reductase and ferredoxin make ammonium. Eight electrons are required, so nitrate costs more than ammonium. Many herbs reduce nitrate in the shoot. Many woody plants reduce more of it in the root. Any ammonium — taken up, made from nitrite, released by photorespiration, or delivered by a nodule — is assimilated by GS–GOGAT. Glutamine synthetase adds NH4+ to glutamate, spending ATP. Glutamate synthase transfers the amide nitrogen onto 2-oxoglutarate and releases two glutamates. Glutamate dehydrogenase is not the main route: its affinity for ammonium is poor, and it mostly breaks glutamate down. Nitrate also induces its own reductase and changes root branching.
2.3.2 The nodule #
Plants have no nitrogenase. The enzyme, an Fe protein plus a MoFe protein, is bacterial. Electrons pass from ferredoxin through the Fe protein, which hydrolyses ATP, to the MoFe protein. The minimum stoichiometry is N2 + 8 H+ + 8 e− + 16 ATP → 2 NH3 + H2 + 16 ADP + 16 Pi. H2 production is obligatory. The whole nodule costs more carbon than those 16 ATP, because the bacteroids must be fed and kept short of oxygen.
O2 inactivates nitrogenase, yet the bacteroids need respiration. A diffusion barrier in the nodule cortex slows O2 entry. Leghaemoglobin, a plant protein in the infected cells, buffers free O2 at nanomolar levels and still delivers it to a high-affinity bacterial oxidase. An active nodule is pink. Other fixers solve oxygen differently. Filamentous cyanobacteria use heterocysts, which Microbiology owns.
The nodule starts with flavonoids from the root. Compatible rhizobia reply with Nod factors, lipochitooligosaccharides, which curl a root hair, start an infection thread, and set off cortical divisions. Bacteria are released inside a plant symbiosome membrane and differentiate as bacteroids. The plant supplies malate. The bacteroid returns ammonium, assimilated by plant GS–GOGAT. Pea exports amides. Soybean and cowpea export ureides. A pea strain does not nodulate soybean. Frankia with alder, and Nostoc in Azolla, are other partnerships. The chemistry of N2 does not move into the plant.
2.3.3 Ecto- and arbuscular mycorrhizas #
Phosphate is immobile, so most plants in natural soil do not forage for it with roots alone. Mycorrhizas extend the search. The fungus gains carbon. The plant gains phosphate especially, and also nitrogen, zinc, copper, and water.
Arbuscular mycorrhizas, formed by glomeromycete fungi, occur in most herbs, including cereals and legumes. Hyphae enter cortical cells and branch into an arbuscule, which remains outside the cytoplasm: the plant plasma membrane surrounds it and is the exchange surface. These fungi are obligate biotrophs. Fatty acids exported by the host are a major part of the carbon they receive, together with sugars. Strigolactones in the root exudate stimulate their hyphae.
Ectomycorrhizas, formed by basidiomycetes and some ascomycetes, wrap the roots of pine, oak, beech, birch, and eucalyptus. A mantle covers the short root tip, and a Hartig net grows between cortical cells without entering them. These fungi receive sugars, and many can also use soil organic matter. Mutualism is the right word for a green plant with either partner. An orchid seedling that depends on its fungus for carbon is a different arrangement.
| Arbuscular | Ecto | |
|---|---|---|
| Interface | Arbuscule inside the wall, outside the cytoplasm | Hartig net between cells, plus a mantle |
| Typical hosts | Herbs, cereals, legumes | Many trees |
| Carbon from the host | Fatty acids and sugars; obligate biotroph | Sugars; some saprotrophy as well |
| Main mineral gain | Phosphate | Phosphate and nitrogen |
2.4 Key terms, summary, and self-check #
Key terms. Essential element; macronutrient; micronutrient; Strategy I; Strategy II; H+-ATPase; apoplast; symplast; Casparian strip; GS–GOGAT; nitrogenase; leghaemoglobin; bacteroid; arbuscule; Hartig net.
Summary. Essentiality is a completed life cycle, no substitution, and a direct metabolic role. N, P, K, and Mg move in the phloem and their shortages show on old leaves; Ca and Fe show on young tissue. Iron fails on alkaline soil because Fe3+ is insoluble, and grasses and dicots fetch it differently. Anion uptake is active because the cytosol is negative; much K+ uptake is not. The Casparian strip forces a membrane crossing. Nitrate is reduced and assimilated by GS–GOGAT. Only bacterial nitrogenase reduces N2, leghaemoglobin buffers O2 in the nodule, and mycorrhizal fungi gather phosphate for most plants.
Self-check.
1. Why does a positive field response fail the criterion of essentiality?
2. Which leaves show iron deficiency, which show magnesium deficiency, and why does liming make iron deficiency more likely?
3. Why can a steep K+ gradient be passive in a cell at −150 mV, while nitrate accumulation cannot?
4. What does the Casparian strip force a solute to do?
5. Why does a nodule need leghaemoglobin, and how does an arbuscule differ from a Hartig net?
