Plant Macro-And Micronutrient Minerals

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Plant Macro- andMicronutrient MineralsMichael A Grusak,USDA/ARS Childrens Nutrition Research Center, Texas, USA

All plants must obtain a number of inorganic mineral elements from their environment toensure successful growth and development of both vegetative and reproductive tissues. A total of fourteen mineral nutrients are required.

IntroductionAll plants must obtain a number of inorganic mineralelements from their environment to ensure successfulgrowth and development of both vegetative and reproduc-tive tissues. These minerals serve numerous functions: asstructural components in macromolecules, as cofactors inenzymatic reactions,as osmotic solutes needed to maintainproper water potential, or as ionized species to providecharge balance in cellular compartments. Minerals can bedivided into two classes, based on the relative amountsneeded for plant growth (see Table 1 ). The macronutrientsinclude nitrogen (N), potassium (K), calcium (Ca),magnesium (Mg), phosphorus (P) and sulfur (S); theseare generallyfound in plantsat concentrations greaterthan0.1% of dry tissue weight. The currently recognizedmicronutrients include iron (Fe), zinc (Zn), manganese(Mn), copper (Cu), boron (B), chlorine (Cl), molybdenum(Mo) and nickel (Ni); these generally are found atconcentrations less than 0.01% of dry tissue weight. These

14 minerals, along with the elements carbon (C), hydrogen(H) and oxygen (O), are broadly accepted as essential forthe growth of all plants. Additional minerals, such ascobalt (Co), sodium(Na), silicon (Si), selenium (Se), iodine(I) and vanadium (V), have been shown to be essential orbenecial for certain plant species, but their widespreadessentiality has yet to be established. Many other elementscan be found in plants (over half the elements in the

periodic table have been identied in some plant tissue),but these are thought to enter plants nonselectively. Mostof these nonessential elements confer no known benet tothe plant, and many, such as cadmium (Cd) or chromium(Cr), are actually detrimental to plant growth.

Criteria for EssentialityScientists have been interested in plant mineral require-mentsfor many years. As long ago as theearly 1800s, it wasrecognized that mineral fertilization of soils could stimu-

Article Contents

Secondary article

. Introduction

. Criteria for Essentiality

. Functions and Sources of Mineral Nutrients

. Nutrient Deficiencies

Table 1 Average concentrations of mineral nutrients in plant shoots (dry weight basis)considered sufficient for adequate growth a

Element mmolg2 1

mg g2 1 % Relative

number of atoms

Molybdenum 0.001 0.1 1Nickel b 0.001 0.1 1Copper 0.10 6 100Zinc 0.30 20 300Manganese 1.0 50 1000

Iron 2.0 100 2000Boron 2.0 20 2000Chlorine 3.0 100 3000Sulfur 30 0.1 30 000Phosphorus 60 0.2 60 000Magnesium 80 0.2 80 000Calcium 125 0.5 125 000Potassium 250 1.0 250 000Nitrogen 1000 1.5 1 000 000a From Epstein (1972). b Based on Brown et al . (1987).

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late growthandimprove crop yields. By theearly 1900s,thesix macronutrients N, P, K, S, Ca and Mg, along with themicronutrient Fe, were regarded as critical for plantgrowth, but the status of the other micronutrients wasuncertain and quite controversial. In 1939, three criteriawere suggested for classication of a chemical element asan essential mineral nutrient (Arnon and Stout, 1939).These can be restatedas: (1)a given plant must be unable tocomplete either the vegetative or reproductive stages of itslife cycle in the absence of the mineral nutrient; (2) thefunction of the element is specic for that element andcannot be replaced by anothermineral element; and (3) theelement must be directly involved in a plant metabolic orstructural process, rather than having an indirect effect onplant growth through the correction of some unfavourablemicrobiological or chemical condition of the soil.

Plant nutritionists have used the technique of solutionculture, or hydroponics, to assess these criteria in variousplant species. Hydroponics is a method in which plants aregrown in an aerated liquid medium of dened chemicalcomposition; individual nutrients can be selectively with-held from this medium to test the inuence of their absenceon plant growth and metabolism. All micronutrientminerals so far deemed essential have been determinedusing hydroponic culture, with experiments sometimesrequiring exhaustive removal of impurities from thechemicals used to make the hydroponic solutions, absolutecleansing of containers used to grow the plants, and/orltration of the air supply to prevent contamination fromdust. Thanks to these efforts,a clearer understandingof thecommon essential mineral nutrients has been established.

Functions and Sources of MineralNutrients

NitrogenAs a constituent of all amino acids and proteins (and thusall enzymes), nitrogen serves a central role in cellularmetabolism. Additionally, as a component of nucleotidesand nucleic acids (deoxyribonucleic acid (DNA) andribonucleic acid (RNA)), nitrogen is critical for thetranscription, translation and replication of geneticinformation. Nitrogen is obtained from the soil environ-ment either as the ammonium (N e ) or nitrate (N 4 ) ions,with nitrate being chemically reduced within the plant toammonium prior to incorporation into organic molecules.An alternative source of nitrogen for some species isatmospheric nitrogen (N 2 ), obtained through the processof nitrogen xation. Other nitrogenous compoundsinclude various secondary metabolites derived fromaminoacids; these include compounds of low relative molecularmass used in osmoregulation (e.g. betaine), stress re-sponses (e.g. plant hormones), or metal chelation (e.g.

phytosiderophores). Nitrogen is also a major structuralcomponent of chlorophyll.

PotassiumPotassium is absorbed as the cation, K 1 , which is readily

soluble in soil solutions. It is the most abundant cation inthe cytoplasm and, because it is not metabolized, K 1 andits accompanying anions contribute signicantly to theosmotic potential of cells. Thus, potassium functions inplant water relations processes and affects cell extensionand growth through the regulation of turgor, leaf gasexchange through the controlof stomatal opening/closing,and long-distance nutrient ow through pressure-drivenphloem translocation. The potassium ion also helpsestablish the electrochemical gradient across membranes,and thus contributes to the membrane transport of numerous chemical species.

CalciumThemajor functionsof calcium arerelated to itscapacity toform coordinate bonds, and its ability to establish stablebut reversible intra-and intermolecular linkages, especiallyin thecell wall and at the surface of membranes. Calcium isreadily absorbed from soils as the abundant cation, Ca 2 1

but its free ion concentration in the cytoplasm is kept lowby sequestration in the vacuoles, by complexing withcalcium-binding proteins (e.g. calmodulin), or by chemicalprecipitation as calcium oxalate crystals. Calcium plays animportant role in stimulusresponse coupling involvingsignal transduction pathways, as its release from intracel-lular pools activates various protein kinases, phospha-tases, or phospholipases, whose target moleculessubsequently regulate many cellular functions (Bush,1995).

MagnesiumMagnesium is absorbed as the highly soluble divalentcation, Mg 2 1 , and its functions in plants relate to itscapacity to interact with various ligands through ionicbonding. Numerous enzymes and enzyme reactionsrequire or are strongly promoted by magnesium, includingphosphatases, adenosine triphosphatases (ATPases), andcarboxylases (e.g. ribulose-bisphosphate carboxylase).Adenosine triphosphate (ATP) synthesis has an absoluterequirement for magnesium and Mg 2 1 plays an essentialfunction in protein synthesis, because it serves as a bridgingelement for the aggregation of ribosome subunits. Ad-ditionally, magnesiums most signicant function in greentissues is its role as the central atom in the porphyrinstructure of the chlorophyll molecule.

Plant Macro- and Micronutrient Minerals

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PhosphorusPhosphorus is a structural component of numerousmacromolecules, including nucleic acids, phospholipids,certain amino acids, and several coenzymes. It has asignicant role in energy transfer via the pyrophosphatebond in ATP, and the attachment of phosphate groups to

many different sugars provides metabolic energy inphotosynthesis and respiration. Phosphorus is absorbedby plants largely as the primary or secondary orthopho-sphate anions, H 2 PO 4

2 and HPO 42 2 .

Sulfur Although atmospheric sulfur dioxide SO 2 , can betakenupand utilized by the aerial parts of plants, most sulfur isabsorbed by plant roots as the divalent sulfate anion,SO 4

2 2 . Sulfatereduction then is required for the incorpora-tion of sulfur into organic compounds. Sulfur is aconstituent of the amino acids cysteine and methionine,

and thus is a component of proteins, as well as severalsulfur-containing coenzymes and secondary plant pro-ducts derived from these amino acids. Because sulfurgroups (e.g. thioethers, R-C-S-C-R or thiols, R-S-H; withR representing various chemical groups) can be reversiblyoxidized and reduced, they impart protein stability via theformation of -SS- bridges and play central roles inenzymes that regulate reductionoxidation (redox) oracidbase reactions. Important sulfur compounds are thetripeptide glutathione, involved in detoxication of oxygenradicals; phytochelatins and metallothioneins, involved inheavy metal detoxication; and the proteins thioredoxinand ferrodoxin, involved in redox chemistry.

IronIron is a transition metal characterized by its ability toreadily change its oxidation state from Fe 3 1 to Fe 2 1 , andby its effectiveness in forming octahedral complexes withvarious ligands. When incorporated into proteins, theseattributes allow for controlled reversible redox reactions.Iron is found in haem proteins (which contain iron porphyrin complexes), such as cytochromes, peroxidasesand catalases; in ironsulfur proteins, such as ferrodoxin,aconitase and superoxide dismutase; and in other iron-containing enzymes, such as lipoxygenase. Iron also isrequired for various reaction steps in the biosynthesis of chlorophyll. Iron is found in soils predominantly as theferric ion, Fe 3 1 , but absorption mechanisms exist indifferent species for either the trivalent or divalent forms of iron.

ZincZinc is taken up predominantly as a divalent cation, Zn 2 1 ,and it exists only in the Zn( ii ) oxidation state when

complexed with macromolecules. The metabolic functionsof zinc are based on its ability to form tetrahedralcomplexes with N-, O-, and S-ligands (Vallee and Auld,1990), thereby inuencing both the tertiary structure of proteins (e.g. via the formation of zinc ngers) and enzymecatalytic activity. Important zinc-containing enzymes arealcohol dehydrogenase, carbonic anhydrase, superoxidedismutase and RNA polymerase.

ManganeseThe predominant source of manganese in soils is Mn 2 1

but manganese can exist in the oxidation states ii , ii i andiv , within biological systems. Because Mn( ii ) can boxidized readily to Mn( iv ), manganese plays an importantrole in redox reactions. Of prominence are its inclusion inthe manganese-containing superoxide dismutase, and inthe water-splitting protein complex associated with photo-system II. Manganese also serves as a cofactor, activatingnumerous enzymes involved in the catalysis of oxidation reduction, decarboxylation and hydrolytic reactions.

Copper Copper is a transition metal found in soils either as thedivalent (Cu 2 1 ) or monovalent (Cu 1 ) cation. It com-plexes readily with many organic molecules, includingproteins, and its strong electron affinity in the monovalentform makes it well suited for numerous redox reactions.Copper-containing proteins can be grouped into three

types (Sandmann and Bo ger, 1983): blue proteins, whichfunction in one-electron transfer, such as plastocyanin, acomponent of the electron transport chain in photosystemI; non-blue proteins, which catalyse peroxidation reac-tions, such as CuZn-superoxide dismutase; and multi-copper proteins, which contain at least four copper atomsper molecule and act as oxidases, such as cytochromeoxidase in the mitochondria.

BoronThe chemical species of boron that is absorbed by plantshas not been established, but it is clear that boric acid,B(OH) 3 , predominates in acidic to neutral biological andsoil solutions, and at alkaline pH, the borate anion[B(OH) 4

2 ] is formed. Boric acid forms complexes withdiols and polyols, such as sugar alcohols and uronic acids,and thus boron plays an important role in the bridging of cell wall polymers (Blevins and Lukaszewski, 1998). Boronalso contributes to the integrity and functioning of theplasma membrane, probably by affecting physical proper-ties of membrane proteins.




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ChlorineChlorine is present in aqueous solutions as the monovalentchloride ion, Cl 2 , and it is readily absorbed by plants inthis form. Although numerous chlorinated organic com-pounds have been found in plants, few functional roles forthese compounds are known. Chloride is required for the

water-splitting protein complex of photosystem II, itstimulates the activity of the vacuolar proton-pumpingATPase, and it can function in osmoregulation, especiallyin stomatal guard cells.

Molybdenum and nickelThe requirements of plants for molybdenum and nickelarethe least of all the mineral nutrients. These minerals aretransition elements capable of existing in multiple oxida-tion states, and thereby function in redox reactions.Only afew enzymes have been conrmed that require theseminerals: these are nitrate reductase, nitrogenase and

xanthine oxidase/dehydrogenase for molybdenum, andurease for nickel. Thus, both of these minerals serveimportant roles in nitrogen metabolism.

Nutrient DeficienciesA plants ability to obtain adequate levels of essentialminerals depends in part on the availability of theseminerals in the soil environment, as well as the presence of appropriate transport proteins and related ion acquisitionmechanisms in the plasma membranes of root cells(Kochian, 1991). Availability is not solely a function of how much of a mineral is in the soil matrix, but moreimportantly depends on the molar fraction present in soilsolution, and on the speciation of that minerals ions(Lindsay, 1991). Abiotic factors such as pH, redox state,and temperature can inuence mineral speciation andsolubility, as can biotic factors such as microbial release of organic acids and phenolic compounds, either generatedmetabolically or through degradation of soil organicmatter. Plant roots also can modify the rhizosphere toaffect nutrient availability, through the release of protons,chelators and/or chemical reductants. When challengedwith a specic nutrient deciency, i.e. low availability,plants can induce high-affinity transporters and othermechanisms in their roots, to assist in meeting their mineralnutrient requirements.

Mineral deciencies affect plant growth by limiting thebiosynthesis or expression of key components of energycapture and/or metabolism. For example, deciencies of N, Fe or Mg reduce chlorophyll synthesis and thusphotosynthetic capacity, and result in chlorosis, or yellow-ing, of leaves. Deciencies of P, K or S impact metabolitesor enzymesinvolvedin photosynthesis andrespiration; this

leads to inadequacies in the transfer of light energy tochemical bonds, or in the export of sugars from chlor-oplasts, and can result in the development of necroticlesions on leaves. Similarly, deciencies of Zn, Cu or Mncan impair enzyme activity or the function of structuralproteins, such that overall metabolism is reduced; thesedeciencies also lead to chlorotic or necrotic symptoms.Interestingly, the occurrence of deciency symptomsthroughout the plant can differ from older to youngerleaves,depending on whether the mineral can be mobilizedin the phloem pathway from older senescing tissues toyoung growing regions of the plant. For instance, Ca haspoor phloem mobility, and Ca deciency leads to necrosisof new leaves and apical meristems, whereas a deciency of K, which is highly phloem mobile, appears initially onolder, more mature leaves at the base of the plant.

Because mineral deciencies impair plant growth andmetabolism, their most signicant outcome in the case of agronomically important crop plants is a reduction inharvest yields, or in some cases, total loss of the crop.However, even moderate nutrient deciencies can reducethe general health of the plant, inhibiting its ability towithstand environmental or biotic stresses. Additionally,as all the essential minerals required by plants are essentialfor the health of humans and other animals, plant mineraldeciencies can reduce the nutritional content and qualityof our harvested food supply (Grusak and DellaPenna,1999).

ReferencesArnon DI and Stout PR (1939) The essentiality of certain elements in

minute quantity for plants with special reference to copper. PlanPhysiology 14 : 371375.

Blevins DG and Lukaszewski KM (1998) Boron in plant structure andfunction. Annual Review of Plant Physiology and Plant MolecularBiology 49 : 481500.

Brown PH, Welch RM and Cary EE (1987) Nickel: a micronutrientessential for higher plants. Plant Physiology 85 : 801803.

Bush DS (1995)Calciumregulation in plant cells andits role insignaling.Annual Review of Plant Physiology and Plant Molecular Biology 4695122.

Epstein E (1972) Mineral Nutritionof Plants: Principlesand PerspectivesNew York: Wiley.

Grusak MA and DellaPenna D (1999) Improving the nutrientcomposition of plants to enhance human nutrition and health. AnnuaReview of Plant Physiology and Plant Molecular Biology 50 : 131161

Kochian LV (1991) Mechanisms of micronutrient uptake and transloca-tion in plants. In: Mortvedt JJ, Cox FR, Shuman LM and Welch RM(eds) Micronutrients in Agriculture , pp. 229296. Madison, WI: SoilScience Society of America.

Lindsay WL(1991) Inorganicequilibriaaffectingmicronutrients in soils.In: Mortvedt JJ, Cox FR, Shuman LM and Welch RM (eds)Micronutrients in Agriculture , pp. 89112. Madison, WI: Soil ScienceSociety of America.

Sandmann G andBo gerP (1983)The enzymatologicalfunctionof heavymetals and their role in electron transfer processes of plants. In:La uchli A and Bieleski RL (eds) Encyclopedia of Plant Physiology,New Series , vol. 15A, pp. 563596. Berlin: Springer-Verlag.


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Vallee BL and Auld DS (1990) Zinc coordination, function, andstructure of zinc enzymes and other proteins. Biochemistry 29 : 5647 5659.

Further Readingda Silva JJRF and Williams RJP (1991) The Biological Chemistry of the

Elements:The InorganicChemistryof Life . Oxford: Oxford UniversityPress.

Fox TC and Guerinot ML (1998) Molecular biology of cation transportin plants. Annual Review of Plant Physiology and Plant MolecularBiology 49 : 669696.

Grusak MA, Pearson JN and Marentes E (1999) The physiology of micronutrient homeostasis in eld crops. Field Crops Research 604156.

Loneragan JF (1997) Plant nutrition in the 20th and perspectives for the21st century. Plant and Soil 196 : 163174.

MarschnerH (1995) Mineral Nutritionof Higher Plants . San Diego, CA:Academic Press.



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Plant Macro-And Micronutrient Minerals