The following points highlight the seven essential elements of macronutrients. The elements are: 1. Carbon, Hydrogen and Oxygen 2. Nitrogen 3. Phosphorus 4. Potassium 5. Calcium 6. Magnesium 7. Sulphur.
Macronutrients Element: # 1.
Carbon, Hydrogen and Oxygen:
Although these macronutrients elements are not minerals in the true sense, they are still included in the list as they are most essential for plant life. These three elements are also called framework elements. Plants absorb them from air and soil in the form of carbon dioxide and water.
Macronutrients Element: # 2.
Soil is the chief source of nitrogen.
It is absorbed from the soil in two major ionic forms:
Nitrate (NO3–) and ammonium (NH4+). Soils generally remain deficient in nitrogen, and soil fertility always depends on added nitrogen.
Functions of Nitrogen:
(i) The most recognized role of nitrogen in the plant is its presence in the structure of protein molecule.
(ii) It is the constituent of such important biomolecules like purines and pyrimidine’s which are found in DNA and RNA.
(iii) Nitrogen is found in the structure of porphyrin molecules which are the percursors of chlorophyll pigments and cytochromes that are essential in photosynthesis and respiration.
(iv) The coenzymes like NAD+, NADP+, FAD, etc., are essential to the function of many enzymes and nitrogen is a structural component of these coenzymes.
(v) Other compounds in the plant such as some vitamins contain nitrogen.
Deficiency Symptoms of Nitrogen:
(i) A general chlorosis, i.e., the yellowing of leaves, especially in the older leaves, due to a loss in chlorophyll content appears first. In severe cases these leaves become completely yellow and then light tan as they die and frequently fall off the plant.
(ii) This yellowing symptom appears last in the younger leaves, because they receive soluble forms of nitrogen transported from older leaves.
(iii) In some cases production and accumulation of anthocyanin pigments is found. As a result a purplish colouration appears in stems, petioles, and lower leaf surfaces.
(iv) The starch content is increased with the decrease in protein content.
(v) Plant growth remains stunted and lateral buds remain dormant.
(vi) Flowering is suppressed or delayed; in the latter case the fruits and seeds are small and weak.
Macronutrients Element: # 3.
Next to nitrogen, phosphorus is very often the limiting nutrient in soils. It is present in the soil in inorganic and organic forms. It is absorbed primarily as the monovalent inorganic phosphate anions (H2PO4–). The divalent anions (H2PO4)2- are slowly absorbed. The amount of either ion present is dependent on the pH of the soil solution.
In the organic portion of the soil organic forms of phosphorus may be found in nucleic acid, phospholipids and inositol phosphates, which are not the utilizable forms of the element.
These organic compounds are eventually decomposed, and phosphorus is transformed into an inorganic form which is readily absorbed by the root system. After entry into the root or being transported into the shoot, phosphate is converted into organic forms and never undergoes reduction.
Factors controlling the availability of phosphorus are:
(i) pH of the soil solution
(ii) Dissolved aluminum and iron which precipitate out phosphate as un-absorbable aluminum and iron phosphates
(iii) Available calcium which may form salts with all forms of phosphate, that are easily available to the plant due to high solubility in water
(iv) Anion exchange, that takes place between the minerals present in the clay micelles and the phosphate ion under mild acidic conditions
(v) Presence of microorganisms in the soil, which temporarily fix phosphorus in organic structures that is eventually returned to the soil in a bound form for the utilization of plants.
Functions of Phosphorus:
(i) It is a constituent of nucleic acids. Both DNA and RNA have a sugar-phosphate backbone in their structures. Triphosphate forms of nucleotides are precursors of nucleic acids.
(ii) Phosphorus is a constituent of phospholipids or phosphoglycerides or glycerol phosphatides which along with proteins, are characteristic major components of cell membranes; only very small amounts of phosphoglycerides occur elsewhere in cells. The most abundant phosphoglycerides in higher plants are phosphatidyl ethanol amine and phosphatidyl choline (lecithin).
(iii) Phosphorus is a constituent of the coenzymes NAD+ and NADP +, which take part in most of the cellular oxidation-reduction reactions involving hydrogen transfer. Most of the important metabolic processes like photosynthesis, respiration, nitrogen metabolism, carbohydrate metabolism, fatty-acid metabolism, etc., are dependent on the action of these coenzymes.
(iv) Phosphorus is a constituent of ATP and other high energy compounds.
(v) All the intermediate of glycolysis between glucose and pyruvate are phosphorylated compounds.
Their phosphate groups appear to have two functions:
(a) They provide each intermediate with a polar negatively charged group which prevents them from leaking out of cells by simple diffusion,
(b) The phosphate group of the glycolytic intermediates is the conservation of energy, since they ultimately become the terminal phosphate groups of ATP in the course of glycolysis.
(vi) Phosphorus is also an essential part of all the sugar phosphates in photosynthesis and other metabolic processes.
(vii) High amounts of phosphorus are found in the meristematic regions of actively growing plants where it is involved in the synthesis of nucleoproteins. Another reason is that in this region phosphorus is also involved, through ATP and GTP, in the activation of amino acids and initiation and elongation steps of polypeptide chain synthesis for the protein moiety of the nucleoprotein molecules.
Deficiency Symptoms of Phosphorus:
(i) Phosphorus-deficient plants may develop dead necrotic areas on the leaves, petioles, or fruits.
(ii) The plants show a general overall stunted appearance with often dark green colouration.
(iii) Sometimes phosphorus deficiency may cause leaf-fall and purple or red anthocyanin pigmentation.
(iv) The older leaves are usually affected first and become dark brown because of the mobility of phosphorus to the younger leaves under deficiency conditions.
(v) Sometimes distortion in the shape of the leaves is observed and may be confused with zinc deficiency.
(vi) Large amounts of pith and small amounts of vascular tissues are found in the stems of phosphorus-deficient tomato plants.
(vii) In some cases a deficiency of this element causes an accumulation of carbohydrates.
Macronutrients Element: # 4.
After nitrogen and phosphorus, soils are usually most deficient in potassium. For that reason, commercial mixed fertilizers usually contain these three elements (N, P and K).
Potassium is present in the soil in soluble form, fixed or bound form and in an exchangeable form. Most of the potassium content of the soil is non-exchangeable (fixed) and, therefore, unavailable to the plant. An equilibrium exists in the soil between the three forms of potassium.
When potassium salts are applied to the soil, the bound ions are released and are replaced by the newly added potassium ions. Absorption of soluble K by the plants causes a release of exchangeable K, which, in turn, causes the release of bound K.
Functions of Potassium:
Relatively high amounts of potassium is required by the plants for normal growth, but this situation does not correlate with the observed functions of potassium. It does not enter into the composition of any organic compound in the plant.
This element seems to function mostly as a catalytic agent in several enzymatic reactions. Its probable role is to provide the necessary ionic environment for preserving the proper three-dimensional structure of enzymes for optimal activity.
(a) Physiological Functions:
(i) Potassium has been shown to be linked with carbohydrate metabolism.
(ii) It is essential for translocation of sugar. Spanner, proposed a mechanism of potassium circulation around the sieve plate for increasing translocation of sugar in sieve tubes.
Circulation of potassium establishes a potential difference across the sieve plate which actually favours sugar translocation. So, any factor that increases potassium transport could alter the electro osmotic potential between sieve tubes and influences sugar translocation.
(iii) Stomatal opening in higher plants requires potassium. It has been estimated by the X-ray electron probe micro-analyser, that the guard cells of opened stomata contains a relatively high concentration of potassium as compared to the closed stomata.
There is an influx of potassium ions (K+) into the guard cells during stomatal opening at the expense of ATP. Potassium accumulation in the guard-cell vacuole results in osmotic swelling of guard cell and stomatal opening.
(iv) Potassium has a general role in the regulation of water in plant cells. Under water- stress conditions potassium being absorbed selectively prevents the plant from losing water.
(b) Biochemical Functions:
(i) The reactions, involved in the phosphorylation of carboxyl groups and inter-conversions of enol-keto intermediates are activated by potassium.
(ii) Potassium is required by the enzyme acetic thiokinase from spinach leaves for maximal activity.
(iii) Potassium might act as a regulator of the enzyme pyruvate kinase through repression of synthesis of the enzyme.
(iv) Folic acid metabolism has been shown to require potassium.
(v) γ-glutamylcysteine synthesis specifically requires potassium.
(vi) Potassium is required by the enzyme succinyl-CoA synthetase isolated from tobacco for maximal activity.
(vii) Nitrate reductase formation in rice seedlings specifically requires potassium.
(viii) There is an absolute requirement for potassium by starch synthetase isolated from sweet com.
(ix) Potassium, through its role in ATPase activity, may be involved in ion transport across biological membranes.
Deficiency Symptoms of Potassium:
(i) Due to easy mobility of potassium, deficiency symptoms first appear on older leaves. A mottled chlorosis followed by the development of dark necrotic lesions at the tip and margins of the leaf is generally found. The leaf-tips curve downwards and the margins roll inward towards the upper surface.
(ii) In cereals, cells at the leaf-tip and margin die first, and the necrosis spreads basipetally toward the leaf bases.
(iii) Potassium deficient cereal grains develop weak stalks, and their roots become susceptible to root rotting organisms. As a result, the plants easily get lodged by wind or rain.
(iv) Generally, a potassium deficient plant exhibits stunted growth with shortened internodes.
(v) Anatomically, potassium deficiency causes disintegration of pith cells and formation of secondary pholem in tomato plants.
Macronutrients Element: # 5.
Calcium is present adequately in most soils for favourable plant growth. It is absorbed as divalent Ca2+. The non-exchangeable form of calcium present in the soil is CaAl2SiO8 (anorthite).
Calcium is made available from anorthite through weathering. Calcium carbonate and insoluble calcium phosphate salts are also found in the soils of arid and semi-arid regions and in alkaline soils respectively. This calcium to some extent is available to the plant depending on the solubility of the salt and the degree of alkalinity.
Functions of Calcium:
Ca2+ functions both as a structural component and as a cofactor for certain enzymes.
(a) Structural Functions:
Calcium has been associated with the cell wall structure. It is required to bind pectate polysaccharides that form a new middle lamella in the cell plate that arises between daughter cells.
Ethylenediamineteracetic acid (EDTA), a chelating agent, stimulates growth of the Avena coleoptile through increased plasticity caused by the partial removal of pectate-bound calcium. Protein as well as calcium are involved in binding cell walls together. Calcium in the cell wall remains bonded with ionized carboxyl groups of cellulosic fraction. Low pH has been found to mimic IAA induced cell elongation.
The possible explanation might be that an increase in hydrogen ions within cell walls would decrease ionization of carboxyl grouping of cellulose, which in turn decreases the amount of bonding of calcium between associated carboxyl’s.
Cell wall extension induced by CO2 and low pH is reduced when calcium is added in the experimental solution, supporting the hypothesis that calcium influences cell wall rigidity through its effect on ionic bridging between cellular molecules.
(b) Membrane and Ion Regulation:
(i) Calcium is involved in spatially three dimensional arrangement of the membranes. Calcium salt of lecithin, a lipid compound, is involved in the formation or organization of cell membranes. EDTA stimulated Avena coleoptile growth may also be caused by an increase in cell permeability through the removal of calcium.
(ii) All organisms maintain low concentrations of free Ca2+ in their cytosol. This is also true even though calcium is abundant in many plants. Mostly calcium remains in vacuoles and bound in cell walls to pectate polysaccharides.
Calcium is frequently stored in vacuoles as insoluble crystals of oxalates, carbonates, phosphates or sulfates. Low concentrations of calcium in cytosol must be maintained partly to prevent formation of insoluble calcium salts of ATP and other organic phosphates, and calcium inhibition of the action of many enzymes present in the cytosol.
(iii) Calcium ions can serve a protective function. It protects plants from the injurious effects of hydrogen ions, high salt concentration in the environment, and toxic effects of other ions in the environment.
(c) Physiological and Biochemical Functions:
(i) Relatively high concentration of calcium is required for nodulation and successful symbiotic nitrogen fixation.
(ii) Much calcium within the cytosol becomes reversibly bound to a small, soluble protein called Calmodulin. Calmodulin is activated by calcium binding in such a way that it then activates several enzymes. So an enzyme activating role of Ca2+ exists mainly when the ion is bound to calmodulin or closely related proteins.
(iii) Ca2+ is required for the barley a-amylase activity.
(iv) Adenine triphosphatases (ATPases) found in different plant tissues show a varied response to calcium.
(v) Phospholipase D from cabbage and carrot is activated by Ca2 +.
(vi) Calcium may also be an activator for the enzymes arginine kinase, adenylkinase and potato apyrase.
(vii) The mitochondrial number in wheat roots is reduced under calcium-deficiency condition.
(viii) Increased levels of carbohydrates in the leaves and decreased levels in the stems and roots of cotton plants result from calcium deficiency. This is interpreted as decrease in carbohydrate translocation owing to calcium deficiency, an effect similar to that found in boron deficiency.
(d) Cytological Functions:
(i) Calcium in small amounts is necessary for normal mitosis, because Ca2+ is needed to form microtubules of the spindle apparatus.
(ii) Calcium is also involved in chromatin organization, it is suggested that nucleoprotein particles are held together by Ca2+. So calcium deficiency results in abnormal mitosis and thus chromosomal aberration.
Deficiency Symptoms of Calcium:
(i) A plant found deficient in calcium shows stem collapse and subsequent termination of growth in the apical meristematic regions at stem, leaf, and root tips.
(ii) Chlorotic patches appear near the margin of younger leaves.
(iii) Roots may become short, stubby, and brown.
(iv) Distortion or disfiguration of the growing shoot tips is characteristic of calcium-deficient plants.
(v) Cell walls become rigid and brittle in calcium deficient plants. Calcium deficiency results in cell enlargement, vacuolation and differentiation in the shoot apex of many plants.
Macronutrients Element # 6.
Like calcium, magnesium is also an exchangeable cation. It is present in the soil in water soluble, exchangeable, and fixed form. Magnesium is found to be present in soil abundantly as magnesium silicate, an unavailable form which becomes available to plants after weathering. Magnesium is absorbed as divalent Mg2 +.
It may be available to plants from some fixed forms of minerals like magnesite (MgCO3), livine [(MgFe)2 SiO4], and dolomite (MgCO3.CaCO3). Of them dolomite is the most popular and economical source of magnesium fertilizer.
Functions of Magnesium:
Magnesium, like calcium, also serves as a structural component and is involved as a cofactor in many enzymatic reactions.
(a) Structural Functions:
(i) Magnesium is a component of the chlorophyll structure.
(ii) Magnesium is required to maintain ribosome integrity.
(iii) Magnesium is necessary to maintain the structural integrity of chromatin fibre. It is involved in coiling of 110Å thick DNA histone protein fibre to form a 300Å thick chromatin fibre.
(b) Physiological and Biochemical Functions:
(i) Magnesium plays two very important roles in plant in photosynthesis and carbohydrate metabolism. Almost all the phosphorylating enzymes involved in carbohydrate metabolism require Mg2+ as an activator for maximal activity. Generally ATP is involved in these reactions. It has been suggested that Mg2 + forms a chelated complex with the phosphate groups, establishing the configuration that allows maximal activity.
(ii) The release of energy in the hydrolysis of high energy compounds like ATP is greatly influenced by Mg2+. It complexes with ATP, ADP and AMP with differing affinities, resulting in hydrolysis of these compounds.
Magnesium through complex formation with phosphate groups, exerts a controlling power on the steady state concentrations of high energy phosphates and influences the rate and extent of the phosphate-transfer reactions.
(iii) Mg2+ has also a direct role on potassium-sodium stimulated ATPase activity.
(iv) Mg2+ is necessary for full activity of the two principal CO2 fixing enzymes, PEP carboxylase and RuBisCO.
(v) Mg2 + is also an activator for DNA and RNA polymerases involved in DNA and RNA synthesis from nucleotide triphosphates. Thus Mg2 + helps in protein synthesis by activating enzymes of nucleic acid synthesis and forming imitation complexes with mRNA, ribosome and fMet initiator tRNA.
Deficiency Symptoms of Magnesium:
(i) Extensive interveinal chlorosis of the older leaves is the first symptom, and as the deficiency becomes more acute, eventually reaches the younger leaves. This is because magnesium is a mobile element.
(ii) Chlorosis is followed by anthocyanin pigmentation and then necrotic spotting.
(iii) Anatomically magnesium deficiency causes extensive chlorenchyma development and scanty pith formation.
Macronutrients Element: # 7.
Sulphur is taken up by the plants from the soil as divalent sulphate anions (SO42-), which is enzymatically converted into an activated form before it can be incorporated into organic compounds. The activated sulphate is converted into reduced sulfur compounds, a wide variety of sulphate esters, and sulpholipids in plants.
Functions of Sulphur:
(i) Sulphur is a constituent of certain amino acids like cystine, cysteine and methionine. Thus sulphur participates in protein structure. The absorbed SO4-2 is converted subsequently into an activated form, which is then reduced and incorporated into cystine, cysteine, and methionine, and finally into the protein structure. Sulphur forms cross-links in the protein molecule and thus stabilizes the tertiary structure of protein.
(ii) Sulphur is a constituent of biotin, thiamine, coenzyme A and lipoic acid which are involved in cellular metabolism.
(iii) Sulphur along with iron forms Fe-S centres of many enzymes, which are important electron carriers in photosynthesis, respiration, nitrogen metabolism, etc.
(iv) Sulphur is a component of S-adenosyl-methionine, which is involved in lignin and sterol biosynthesis.
(v) It is a constituent of volatile oils which give characteristic pungent odours to cruciferous plants, onion, garlic, etc.
Deficiency Symptoms of Sulphur:
As most soils contain enough sulphate, sulphur-deficient plants are fairly uncommon.
(i) A general chlorosis throughout the entire leaf, including veins appears first in younger leaves.
(ii) Rapid leaf fall takes place.
(iii) Leaf tips and margins are rolled inwards.
(iv) Terminal bud growth is inhibited and lateral buds develop prematurely.
(v) Sulphur deficiency results in marked decrease of stroma lamellae and an increase in grana stacking.