Mineral Nutrition

I.  Elements in Plant Dry Matter
  A.  15 to 20% of a nonwoody plant is made from elements, the rest is water
  B.  DRY MATTER: Principal components
    1.  Cell-wall polysaccharides and lignin
    2.  Protoplasmic components:  proteins, lipids, amino acids, organic acids, elements such as 
potassium that exist as ions. C. Plants can absorb and accumulate nonessential elements from soil solutions. Research has
found 60 different elements in plants 1. Gold, lead mercury, arsenic, uranium are some of the unusual ones. D. Leaves usually contain more nitrogen, phosphorus, and potassium than do shoot systems. 1. Soils on the plant earth are mainly composed of aluminum, oxygen, silicon, and iron. 2. Plant do not reflect this soil composition. 3. Soil elements are present as insoluble minerals. 4. Plant roots exhibit substantial selection over the rates at which elements are absorbed. II. Methods of Studying Plant Nutrition: Solution Cultures A. (1860) W. Pfeffer, Julius von Sachs, W. Knop: grew plants in nutrient solutions. 1. Typically call hydroponics, soil-less culture, or solution culture B. Large seeds sometimes contain enough of some elements for a plant to grow and mature.
So during a single life cycle the element may not appear as essential. 1. Examples are: molybdenum, nickel, copper, zinc, boron C. Atomic Absorption Spectrometer: used to measure metals and some nonmetals D. Optical Emission Spectrometers: elements are vaporized at temperatures above 5,000 K.
High temperatures excite electrons from their ground-state orbits into higher orbits. As electrons
move back to original ground state electromagnetic energy is emitted at wavelengths that are
different but distinctive for each element. These wavelengths are measured and the energy
quantified by the spectrometer. E. Root aeration is essential in liquid nutrient solution culture. The solution needs to be monitored
because the composition and pH change continuously. F. Nutrient-Film Technique: washed white-quartz sand or perlite. G. Dennis Hoagland and Daniel Arnon: nutrient solution most commonly used 1. Nitrogen as nitrate: nitrate is absorbed by the plant roots so fast that there is a rapid rise in the
nutrient solution pH. The absorption of nitrate is accompanied by absorption of H+ or excretion
of OH- in order for the plant root cells to maintain charge balance. At high pH values, iron and
some other elements precipitate as hydroxides which makes them unavailable to the plant roots.
pH fluctuations can be minimized by supplying part of the nitrogen as an ammonium salt.
Absorption of NH4+ and other cations occurs concurrently with absorption of OH- or transfer of
H+ from the plant root to the surrounding solution. 2. Nutrient solutions are generally more concentrated than soil solutions. In soils many minerals
are not in solution but are absorbed on the negatively charged surfaces of clay and organic
matter, or are precipitated as insoluble salts. III. Essential Elements A. There are 13 essential elements for all angiosperms and gymnosperms: 1. C, H, O - brings the total to 16 elements 2. Nickel, is now known to be essential, but at the time of development of many of the nutrient
solution recipes it was not. 3. With these 17 elements and sunlight, most plants are able to synthesize all the compounds
they require for growth, development, and reproduction. Autotrophic: make all the organic
molecules needed for growth and reproduction. 4. Some bacteria and fungi associations are important to plants because they perform roles that
allow plants to survive in the face of competition and harsh environmental conditions. B. Principal criteria by which an element is judged essential or nonessential: 1. An element is essential if the plant cannot complete its life cycle (form viable seed) in the
absence of that element. 2. An element is essential if it forms part of any molecule or constituent of the plant that is itself
essential in the plant. 3. An element is essential if it acts directly inside the plant and does not cause some other element
to be more readily available or antagonize the effect of another element. a. Selenate inhibits the absorption of phosphate, which otherwise is absorbed by plants in toxic
amounts. C. Even if a plant can form viable seeds, an element is essential if deficiency symptoms appear on
the plant grown in the nutrient solution without addition of the element. 1. Use of this parameter has lead to evidence that sodium and silicon are essential for certain plant
species. D. Trace elements or micronutrients: needed in tissue concentrations equal to or less than
100 mg/kg of dry matter. Mo, Ni, Cu, Zn, Mn, B, Fe, Cl 1. C4 plants: require Na+ as a micronutrient. a. Chlorosis: lack of chlorophyll in leaves b. Necrosis: dead tissues 2. Species that fix CO2 in photosynthesis via the crassulacean acid metabolism pathway common
in succulents also grow faster with sodium, and for them Na+ is also essential. E. Macronutrients: needed in concentrations of 1,000 mg/kg of dry matter.
S, P, Mg, Ca, K, N, O, C, H. F. Silicon increases growth of some plants 1. Maize accumulates silicon to the extent of 1-4 % of its dry weight 2. Rice and Equisetum contain up to 16% silicon 3. Dicots grown with limited silicon do not fail to produce viable seeds. No essential role for silicon
in plants has been found. Soybean plants grown without silicon accumulated unusually high
concentrations of phosphorus. Symptoms of silicon deficiency sometime represent phosphate
toxicity. 4. Functions of silicon a. When accumulated in epidermal cell walls silicon seems to cause less transpiration and fewer
fungal infections. b. In xylem cells, silicon provides rigidity and limits compression c. Silicon reduces lodging in monocot crop plants. d. Silicates present in grass leaves and inflorescence reduce grazing (herbivory) by animals and
insects. e. There may be an ecological requirement for silicon rather than a physiological or biochemical
requirement. f. Silicates cause: (1) the formation of kidney stones, (2) excessive wear on sheep's teeth, and
(3) throat cancers of humans. IV. Quantitative Requirements and Tissue Analysis A. Deficient Zone: growth of a plant increases as more of an element is provided and the element
concentration in the plant increases. B. Critical concentration: minimal tissue concentration of the element that gives maximum growth,
(90% of maximum), increases in concentration do not substantially affect growt (adequate zone). C. Luxury Consumption: storage of excess element in vacuoles. D. Toxic Zone: continued increases of any element will lead to toxicity and reduced plant growth. E. Originally in agricultural settings cost prevented fertilizing soils with nitrogen, phosphorus, or
potassium beyond the critical plant-tissue concentrations. Once mass production of artificial
fertilizers became available, and high usage of fertilizers became common, it was noted that
excess nitrate and phosphate not absorbed by plants were leached from soils and appeared
in lakes and streams. This increase in nutirients in aquatic systems caused eutropication.
Eutrophication is the nutrient enrichment of aquatic systems that leads to growth of algae and
other plants. Once these algae and plants die the microorganisms decomposing them uses so
much dissolved oxygen that fish and other animals in the lakes and streams die. Manufacturing
of nitrogen fertilizers is one of the most energy-expensive aspects of modern agriculture. F. One of the determining factors controlling where plants grow is their ability to obtain essential
nutrients from soils. Native trees, grasses, and herbaceous dicots have lower nutrient
requirements that result form their ability to absorb nutrients faster than selected crops at low
nutrient concentrations. This makes the native plants good competitors. It is also one of the
reasons why native (or wild) plants are bread with cultivated plants, to make them stronger
growers in minimal soils. G. Before leaf fall a significant amount of nitrogen, phosphorus, potassium and magnesium move out
of deciduose tree leaves into twigs and branches. These nutrients are used in new growth the
next season. In late summer perennial range grasses conserve minerals by translocating the
elements to roots and to lower stem tissues that make up the crown of the plant. V. Chelating Agents A. Micronutrient cations: iron, zinc, manganese, and copper are comparatively insoluble. Insolubility
is extensive at pH's above 5 as found in most soils in western U.S. and many other low rainfall
regions. B. Ligan (chelating agent or chelator): a chelate is formed by the reaction of a divalent or trivalent
metal ion with a ligand. A chelate is the soluble product formed when atoms in an organic ligand
donate electrons to a cation. An example would be negatively charged carboxyl groups on a
ligand and nitrogen atoms. In calcareous soils (soils rich in Ca2+ with a pH of 7 or higher) greater
than 90% of the copper and manganese and 50% or more of the zinc are chelated with organic
compounds produced by microbs. Exactly what these ligands are is not well known. C. Iron Deficiency: chlorosis. Worldwide problem in calcareous soils in both monocots and dicots.
Fe-EDDHA [Fe-ethylenediamine di(o-hydroxyphenyl) acetic acid] [Sequestrene] and Fe-EDTA
(Fe-ethylene-diaminetetracetic acid) [Varsenate]. Fe3+ is less soluble than Fe2+. Fe2+ is easily
absorbed by roots. Ligands for iron are synthesized by microbes and plant roots. Those produced
by roots are execreated into the soil near the roots (rhizosphere). 1. Two general strategies for iron acquisition by angiosperms: a. Strategy-I plants (dicots and some monocots): release phenol based ligands such as caffeic
acid. These ligands chelate Fe3+ which moves to the root surface, where it is reduced to Fe2+
while still chelated. Roots of iron-stressed strategy-I plants form reducing agents (like NADPH)
that carry out the reduction process more readily. Upon reduction the Fe2+ is lost from the
ligand, and immediately absorbed by the plant root. Stressed strategy-I plants release H+ ions
that incrrease solubility of both forms of iron (especially Fe3+). This defense mechanism
usually fails in calcareous soils because the soil pH is high and well buffered with bicarbonate
ions (HCO3-) . This failure contributes to the physiological disease called lime-induced
chlorosis. b. Strategy-II (grasses): respond to iron-deficiency stress by forming and releasing ligands
(siderophores or phytosiderophores) that chelate Fe3+. Two examples are avenic acid and
mugineic acid which are both iminocarboxylic acids that bind to Fe3+ by oxygen and nitrogen
atoms. These and other siderophores are absorbed with the iron still attached to them. The
roots must absorb the phytosiderophores and then reduce the iron to Fe2+. The Fe2+ is
immediately released and used by the plant. The siderophore may then be degraded
chemically or released from the root to continue the transport of iron. D. Once divalent metals are absorbed they are kept soluble by chelation with cellular ligands.
Anions of organic acids (for example citric acid) are important as ligands for transport of iron,
zinc, and manganese through the xylem. Amino acids are important for the transport of copper.
Proteins are usually bound to iron, zinc, manganese, nickel, and copper. Many of these elements
increase the catalytic activity of enzymes involved in electron-transport processes of
photosynthesis and respiration. Monovalent cations (K+ and Na+) do not form stable chelates,
they associated by ionic interactions with both inorganic and organic acid anions, including
proteins. VI. Functions of Essential Elements: Some Principles A. Essential elements have been classified functionally into two groups: (1) those having a role in the
structure or important compound and, (2) those having an enzyme-activating role. Many times
there is no sharp distinction between these two groups. Most of the micronutirents are essential
because they activate enzymes. B. All elements in soluble form contribute to the osmotic potential of the cell and aid in build-up of
cellular turgor pressure to maintain form, speed growth, and allow certain pressure-dependent
movements. Non bound potassium ions are dominate in this regard (stomata). VII. Nutrient Deficiency: Symptoms and Functions of Elements A. Deficiency symptoms: characteristic symptoms often are used to determine the function of the
element in the plant. B. Root deficiency symptoms have been less well described. Symptoms differ according to the
species and growth stage. C. Deficiency symptoms for an element depend on two factors: 1. The function or functions of the element 2. Ability of the element to be translocated from old leaves to young leaves. D. Good example: chlorosis that results from magnesium deficiency. Magnesium part of chlorophyll.
No chlorophyll is formed in its absence. Chlorosis of lower, older leaves becomes more severs.
Younger parts of a plant have a pronounced ability to withdraw mobile nutrients form older parts.
Reproductive organs, flowers, and seeds are especially good at withdrawing nutrients from older
tissue. E. Movement of an element depends on the element's mobility in the phloem. Mobility is determined
by the solubility of the element, and how well it can enter the sieve tubes of the phloem. Some
elements move readily through the phloem from older leaves to younger leaves, and then to
storage organs. Examples of highly mobile elements are nitrogen, phosphorus, potassium,
magnesium, and chlorine. Examples of less mobile elements are boron, iron, and calcium.
The mobility of sulfur, zinc, manganese, copper, and molybdenum is intermediate. If the element
is soluble and can also be loaded into translocating phloem cells, the deficiency symptoms will
appear earliest and most pronounced in older leaves. Symptoms resulting form lack of a relatively
immobile element (calcium or iron) appear first in younger leaves. VIII. Nutrient Deficiency: Selected Elements A. Nitrogen 1. Soils are more deficient in nitrogen than any other element. 2. Forms: Two major ionic forms of nitrogen that are absorbed from soils are nitrate (NO3-) and
ammonium (NH4+). 3. Symptoms: general chlorosis in older leaves. Younger leaves remain green longer because
they receive soluble forms of nitrogen transported from older leaves. Excess nitrogen causes
a high shoot-to-root ration (reversed in nitrogen deficiency). Excess nitrogen causes tomato fruit
to split as they ripen (so that has been my problem!). 4. Usage: proteins, nucleic acids, lots of other macromolecules. B. Phosphorus 1. Second to nitrogen, phosphorus is most often the limiting element in soils. 2. Forms: Absorbed as the monovalent phosphate anion (H2PO4-) or less often as the divalent
anion (HPO42-). pH controls the relative abundance of these two forms. H2PO4- favored below
pH 7 and HPO42- favored above pH 7. Phosphorus never undergoes reduction, remains as
phosphate. 3. Symptoms: stunted growth. Dark green in color. Anthocyanin pigments sometimes
accumulate. Oldest leaves become dark brown as they die. Excess phosphorus causes root
growth, decreasing shoot to root ratio. Easily redistributed from older leaves to younger leaves. 4. Usage: essential part of man sugar phosphates involved in photosynthesis and respiration.
Part of nucleotides, phospholipids, and energy forms (ATP. GTP...). C. Potassium 1. After nitrogen and phosphorus, soils are usually most deficient in potassium 2. Forms: P2O5 3. Symptoms: K+ is easily redistributed from mature to younger organs. Weak stalks (lodgeing) 4. Usage: Activator of many enzymes. This element is also so abundant that it is a major
contributor of the osmotic potential of cells and therefore to their turgor pressure. D. Sulfur 1. Sulfate is present in most soils. Sulfur deficiency is uncommon. Sulfur can be absorbed by
leaves through stomates as gaseous sulfur dioxide (SO2). 2. Forms: absorbed from soils as divalent sulfate anion (SO42-) 3. Symptoms: general chlorosis. Sulfur is not easily redistributed from mature tissues in some
species, deficiencies are usually noted first in younger leaves. 4. Usage: Part of amino acids cysteine and methionine. Also part of vitamins thiamine and biotin.
Part of Coenzyme A E. Magnesium 1. Usually never limiting to plants grown in soil 2. Forms: absorbed as divalent Mg2+ 3. Symptoms: Chlorosis of the older leaves is the first symptom. Chlorosis is usually interveinal. 4. Usage: chlorophyll. Important in reactions with ATP. Activation of enzymes important in
photosynthesis and respiration. Used in formation of DNA and RNA. F. Calcium 1. Most soils contain enough Ca2+ for adequate plant growth, except in acidic soils with high
rain fall. Lime is added in these areas to raise the soil pH. 2. Forms: absorbed as divalent Ca2+ 3. Symptoms: deficiency symptoms more pronounced in young tissues. Meristematic zones
are most susceptible. Calcium is required to form new middle lamella in the cell plate that
arises between daughter cells. Meristematic zones die early. 4. Usage: Calcium pectates of the middle lamella. Calmodulin activation of many enzymes. G. Iron 1. Usually bound to soil particles unless chelated by microorganisms or plants 2. Forms: chelated forms 3. Symptoms: Pronounced interveinal chlorosis occurring first on young leaves. Iron deficiency
results in a rapid inhibition of chlorophyll formation. Iron in older leaves is relatively immobile
in the phloem. Abundant and stable form of iron in leaves is stored in chloroplasts (phytoferritin). 4. Usage: Forms parts of certain enzymes and numerous proteins that carry electrons during
photosynthesis and respiration. H. Chlorine 1. Absorbed from soils as the chloride ion (Cl-), and most of it remains in this form. Rarely deficient
in nature because of its high solubility and availability in soils. 2. Forms: Cl- 3. Symptoms: Reduced growth, wilting, development of chlorotic and necrotic spots. Roots
become stunted. 4. Usage: 4-chloroindoleacetic acid is a natural auxin hormone. Chlorine also stimulates the
splitting of H2O during photosynthesis. I. Manganese 1. Manganese exists in 3 oxidation states (Mn2+, Mn3+, and Mn4+) as insoluble oxides in soils,
and it also exists in chelated form 2. Forms: Absorbed largely as divalent manganous cation (Mn2+) 3. Symptoms: Initial symptoms are interveinal chlorosis on younger or older leaves.
Disorganization of thylakoid membranes but has little effect on the structure of nuclei and
mitochondria. 4. Usage: Structural role in the chloroplast membrane system. Also activated numerous enzymes. J. Boron 1. Soil form? 2. Forms: absorbed from soils as undissociated boric acid (H3BO3) Slowly translocated out of
organs in the phloem of many species once it arrives there in the xylem 3. Symptoms: Deficiencies related to disintegration of internal tissues. Failure of root tips to
elongate. Cell division in the shoot apex is inhibited 4. Usage: Biochemical functions in vascular plants is unclear. Bound in cell-wall
polysaccharides. No specific function is yet certain, but evidence favors special involvement
of boron in nucleic acid synthesis that is so essential to cell division in apical meristems. K. Zinc 1. Soil form? 2. Forms: absorbed as divalent Zn2+ as a chelated ion 3. Symptoms: Growth reduction of young leaves and stem internodes. 4. Usage: Zinc participates in chlorophyll formation or prevents chlorophyll destruction.
Requirement for the production of a growth hormone, indoleacetic acid (auxin). L. Copper 1. Rarely deficient in soils 2. Forms: absorbed both as the divalent cupric (Cu2+) or as the monovalent cuprous ion in wet
soils. Divalent Cu2+ is chelated. 3. Symptoms: Young leaves become dark green and are twisted or otherwise misshapen, often
exhibiting necrotic spots. 4. Usage: Present in several enzymes or proteins involved in oxidation and reduction.
Cytochrome oxidase and plastocyanin. M. Molybdenum 1. Exists as molybdate (MoO42-) salt or MoS2 in soils. 2. Forms: because only trace amounts are required by plants, virtually nothing is known about
the forms in which it is absorbed and the ways it is changed in plant cells. 3. Symptoms: Interveinal chlorosis occurring first on the older or midstem leaves, then
progressing to the youngest leaves. Whiptail disease, plants do not become chlorotic but
develop severely twisted young leaves, which eventually die. 4. Usage: Part of enzyme: Nitrate reductase. Role in breakdown of purines such as adenine
and guanine because of its essentially as part of the enzyme xanthine dehydrogenase.
Forms an essential part of an oxidase that converts absicisic acid aldehyde to the
plant hormone Abscisic Acid (ABA). N. Nickel 1. First element to be added to the list of essential elements since chlorine in 1954. 2. Forms: absorbed as Ni2+ 3. Symptoms: Legumes of tropical origin, including cowpea and soybean form ureides in
root nodules during nitrogen fixation; ureides are then transported via the xylem to leaves.
Transfer ureides from old, senescing leaves to developing seeds and younger leaves via the
phloem. When nickel removed from nutrient solution, plants accumulated so much urea in
their leaf tips that necrotic spots appeared. 4. Usage: Part of the enzyme urease which catalyzes hydrolysis of urea to CO2 and NH4+