Over 95 percent of the dry weight of a flowering plant is made up of three elements—carbon, hydrogen, and oxygen—taken from the air and water. The remaining 5 percent of the dry weight comes from chemicals absorbed from the soil. Roots absorb the chemicals present in their surroundings, but only 14 of the elements absorbed are necessary for plant growth. These 14 elements, along with carbon, hydrogen, and oxygen, are called the
17 essential inorganic nutrients, or
elements. Some of the essentials are needed in larger amounts than others and are called the
macronutrients; those needed in lesser amounts are the
micronutrients. All elements are needed in specific amounts. Note that there is a dispute among plant physiologists concerning the role of nickel in plant nutrition. Since many physiologists exclude it as essential, in some textbooks, lists like the following consist of only 16 essential inorganic nutrients. The 17 are:
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Macronutrients absorbed from the air: oxygen, carbon, and hydrogen.
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Macronutrients absorbed from the soil: nitrogen, potassium, magnesium, phosphorus, calcium, and sulfur.
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Micronutrients from the soil: iron, boron, chlorine, manganese, zinc, copper, molybdenum, and nickel.
An element is essential if it: 1.) is required for normal growth and reproduction; 2.) can not be replaced by another element; 3.) can be shown to be part of a molecule clearly essential to the plant structure or metabolism.
Plants use elements in differing amounts and forms, some as cations, others as anions. Almost all elements are used in a variety of ways, such as as catalysts for enzymatic reactions (either as part of the enzyme structure or as regulators or activators), as regulators of the movement of water in or out of the cell and maintenance of turgor pressure, as regulators of membrane permeability, as structural components of the cell or of electron receptors in the electron transport system, or as buffers (which maintain the pH within cells).
Two-thirds of all the naturally occurring chemical elements have been found in plants. Some odd kinds are known to be used metabolically by particular species, but others with no known function are accumulated apparently because they are present in the soil from which the plant is extracting water and ions. These non-useful chemicals are sequestered in cell vacuoles, as crystals, or as non-soluble compounds and remain in the plant throughout its life. Plants, therefore, can be useful in locating deposits of minerals, e.g. gold or uranium, and have been used by modern prospectors who collect the vegetation from a site and run spectroscopic analyses on the tissues. Some plants grow only in soils in which a particular element is present and are said to be
indicator plants of that element.
Table
1 highlights the roles of the essential elements in plant nutrition.
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TABLE 1
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The Role of Inorganic Elements in Plant Nutrition and Their Deficiency Symptoms
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Element
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Form in which Absorbed
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Important Roles/Functions
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Deficiency Symptoms
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Macronutrients
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carbon
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CO2
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major component of organic compounds; presence defines “organic”
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rarely limiting as a nutrient; no specific symptoms
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hydrogen
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H2O
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major component of organic compounds
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rarely limiting as a nutrient; no specific symptom
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oxygen
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H2O, O2
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major component of organic compounds
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rarely limiting as a nutrient; no specific symptoms
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nitrogen
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NO3−, NH4−
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part of amino acids, proteins, nucleotides, nucleic acids, chlorophylls, coenzymes
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chlorosis; severe cases: turn yellow, die; some plants turn purple as anthocyanins accumulate in vacuoles; nutrient most likely to be deficient in soil
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potassium
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K+
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involved in osmosis, ionic balance, opening and closing of stomata; activator of enzymes; necessary for starch formation
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weak, spindly stems and roots; older leaves especially affected—mottled with dead spots along margins and dead tips; roots more susceptible to disease
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calcium
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Ca2+
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component of middle lamella of cell walls; enzyme cofactor; involved in membrane permeability; component of calmodulin (regulator of membrane and enzyme activities)
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root and shoot tips die; young leaves and shoots most affected, die back at tips and margins first
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phosphorus
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component of ATP and ADP (essential energy-carrying compounds), nucleic acids, several essential coenzymes, phospholipids of membranes
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stunted growth of whole plant; dark green color; antho cyanins accumulate in vacuoles giving purple tinge to leaves; second most-likely nutrient to be deficient in soil
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magnesium
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Mg2+
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center of chlorophyll molecule; activator of many enzymes
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leaf tips and margins turn upward on mostly older leaves; chlorosis, mottling, some dead spots and reddish color of leaves
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sulfur
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SO42−
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component of some amino acids, proteins, and coenzyme A; can be absorbed through stomata as gaseous SO2
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young leaves with chlorosis between the veins: sulfur is rarely limiting
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Micronutrients
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iron
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Fe2+ or Fe3+
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required for chlorophyll synthesis; component of cytochromes and nitrogenase (important in respiration and photosynthesis)
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short, slender roots; chlorosis between the veins in leaves
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zinc
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Zn2+
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activator or component of several enzymes; involved in auxin synthesis, maintenance of ribosome structure
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leaf size and internodal length much reduced; leaf margins deformed; chlorosis between veins, especially in older leaves
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molybdenum
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MoO42+
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required for nitrogen fixation and nitrate reduction (nitrate reductase)
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chlorosis starting in older leaves and progressing to younger; death of interveinal areas and then of whole leaf
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boron
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influences Ca2+ utilization, formation of nucleic acids, maintenance of membranes; essential for growth of pollen tubes
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young tissues most affected; apical meristems die; root tips swollen and discolored; young leaves yellow at base, twisted
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copper
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Cu2 or Cu2+
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activator of enzymes, present in some; involved in oxidation-reduction
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wilting and twisting of dark green young leaves; often with numerous dead spots on blades; copper is rarely deficient
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manganese
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Mn2+
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activator of enzymes, required for O2 release in photosynthesis, integrity of the chloroplast membrane; electron transfers
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interveinal chlorosis and dead spots; thylakoid membranes disintegrate
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chlorine
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Cl−
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involved in water balance (osmosis), ionic balance; probably essential in photosynthetic O2−-releasing reactions
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leaves wilt; turn reddish bronze in color; chlorosis, dead spots; stunted roots with abnormal thickening near tips
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nickel
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Ni
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essential part of enzyme in nitrogen metabolism
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leaf tips with dead spots
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Themes of Plant Biology
Mineral Nutrition
