Plant Nutrition

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Plant Nutrition

Green plants, unlike animals, are able to manufacture their major organic constituents entirely from inorganic raw materials that are obtained from soil, water, or atmosphere using energy provided by photosynthesis. Of over fifty elements found in plant tissues, only sixteen are considered essential nutrients for all plants. Of these sixteen, nine are macronutrients, and seven are micronutrients. Macronutrients are required in high amounts and each is present at levels of greater than 0.2 percent of plant dry weight. Most macronutrients are important constituents of organic molecules, and most have more than one role. Micronutrients are required in small amounts often have special purposes. The seven known micronutrients each make up less than 0.1 percent of plant dry weight. A few other elements (nickel, silicon, and sodium) are considered essential only for some plants. Soybeans require nickel; horsetails require silicon; C4 and CAM plants require sodium.

Essential Elements for Proper Plant Nutrition: Roles, Available Forms, and Deficiency Symptoms
Essential Element*	Role	Symbol	Form absorbed	Deficiency symptoms	Leaves affected
Macronutrients
Hydrogen	Component of organic compounds and water; chemiosmotic synthesis of ATP in mitochondria and chloroplasts	H	H2O
Carbon	Component of organic compounds	C	CO2
Oxygen	Component of organic compounds and water; electron acceptor in respiration	O	H2O
Nitrogen	nucleic acids, some hormones, and nucleic acids, some hormones, and chlorophyll	N	NO3–, NH4+	Plants stunted; foliage light green, roots long and slender	Old
Potassium	Enzyme activator, involved in starch formation; regulates osmotic balance and movement of guard cells	K	K+	Stems slender, numerous small necrotic spots form near the margins of leaves	Old
Calcium	Component of middle lamella (Capectate); controls activity of many enzymes; maintains membrane integrity; 2nd messenger	Ca	Ca2+	Plants stunted; terminal bud dies; young leaves hooked; root tips die	Young
Magnesium	Component of chlorophyll; component of middle lamella (Mg-pectate); activates many enzymes	Mg	Mg2+	Leaves with chlorotic spots; tips and margins of leaves turned upward	Old
Phosphorus	Component of nucleic acids, phospholipids, coenzymes; involved in sugar metabolism	P	H2PO4–HPO4 2-	Plants stunted, foliage purple/dark green	Whole plant affected
Sulphur	Components of the amino acids cysteine and methionine; component of coenzyme A	S	SO42–	Young leaves light green, no necrosis	Young
Micronutrients
Chlorine	Involved in water balance; possibly involved in photosynthetic reactions in which O2 is released	Cl	Cl–	Leaves wilted, chlorotic, ultimately necrotic; roots thickened	Whole plant affected
Iron	Component of cytochromes ferredoxin and nitrogenase; cofactor of peroxidase; involved in chlorophyll synthesis	Fe	Fe2+, Fe3+	Stunted growth; interveinal chlorosis of young leaves	Young
Boron	May be involved in sugar transport; regulates enzyme function	B	H2BO4	Terminal bud dies; leaves may be twisted, base of young leaves chlorotic; root tips discolored	Young
Manganese	Activator of enzymes; involved in electron transfer, chlorophyll synthesis, and the photosynthetic evolution of O2	Mn	Mn2+	Interveinal necrosis of young leaves	Young
Zinc	Activates many enzymes; involved in the formation of pollen	Zn	Zn2+	Stems with short internodes; leaves thick; leaf margins distorted	Old
Copper	Component of plastocyanin; present in lignin of xylem elements; activates enzymes	Cu	Cu+, Cu2+	Young leaves permanently wilted; foliage dark green; terminal branches unable to stand erect	Young
Molybdenum	Involved in nitrogen reduction	Mo	MoO42-	Young leaves twisted, chlorotic	Young
* Elements are listed in order of decreasing number of atoms relative to molybdenum.

For an element to be considered an essential nutrient, it must meet the following three criteria: (1) The element must be necessary for normal plant development through a complete life cycle; (2) no other element can substitute for that element; and (3) the element must play a role in metabolism within the plant. Studies to demonstrate whether an element is essential are often very difficult to conduct. Special hydroponic culture in growth chambers that eliminate contamination from the air allows scientists to eliminate a particular element and determine plant response to the deficiency.

Roles of Nutrients

Structural and Metabolic Components. Carbon, hydrogen, and oxygen comprise a major portion of organic compounds that make up plant cells. Nitrogen and phosphorus are found in phospholipids and nucleic acids. Copper and iron are components of electron transport systems in mitochondria and chloroplasts. The middle lamella that cements adjacent plant cells together is rich in calcium and magnesium pectate. Magnesium is also a component of chlorophyll.

Enzymatic Role. Many minerals serve as enzyme activators. Potassium, for example, is involved in the activation of many enzymes. Calcium binding protein (calmodulin) regulates many cellular activities. Manganese is essential in the photosynthetic release of O₂ in photosystem II.

Osmotic Role. Potassium plays a major role in opening and closing movements of guard cells of the stomatal apparatus. Hydrogen ion gradients are important in the generation of adenosine triphosphate (ATP) in mitochondria and chloroplasts.

Deficiency Symptoms

Chlorosis, a yellowing of leaf and stem tissue, is a common symptom of mineral deficiencies. In nitrogen deficiency a general chlorosis is exhibited, but in iron-deficient plants, chlorosis is confined to areas between leaf veins. Occasionally, plants will develop a purple coloration due to the production of large amounts of anthocyanins , when certain elements, such as phosphorus, are deficient. Necrosis (death of tissue) may follow chlorosis as deficiencies become more acute. In potassium-deficient plants necrosis occurs along leaf margins, but in manganese-deficient plants necrosis occurs between veins.

For several essential nutrients, young leaves show symptoms first, which means that the element is not easily translocated from old to young leaves, as is the case with iron deficiency. Nitrogen, potassium, and magnesium are easily loaded into sieve tube members of the phloem and translocated from old leaves to younger developing leaves. In those cases the older leaves exhibit the symptoms.

see also C4 and CAM Plants; Chloroplast; Photosynthesis; Translocation

George H. Wittler