Essential Elements Required for Plants

In this article we will discuss about:- 1. Meaning of Essential Elements 2. Sources of Essential Elements for Plants 3. Role 4. Deficiency Symptoms.

Meaning of Essential Elements:

An essential element is known, without which the plant cannot complete its life cycle. It has clear physiological role to play.

The important criteria for an essential element are as follows:

(i) This element is absolutely necessary for supporting normal growth and reproduction of plant.

(ii) This element is always specific, and cannot be replaced by any other element.

(iii) This element is directly involved in the metabolism of the plant.

However, among the mineral elements absorbed by the plants, not all are essential.

Out of 105 or more elements discovered so far, only 20 elements have been found to be essential for growth and development of the plant.

These essential elements are classified into two broad categories called:

(i) Macronutrients, and

(ii) Micronutrients.

The Macronutrients are:

Carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium, magnesium and silicon.

The macronutrients are generally present in plant tissues in concentrations of 1 to 10 mg per gram of dry matter.

The micronutrients or trace elements are: iron, manganese, copper, molybdenum, zinc, boron and chlorine. Recently some other such elements have also been discovered, e.g., cobalt, vanadium and nickel.

The microelements are required in very low amounts, i.e., about 0.1 mg per gram of dry matter.

Sources of Essential Elements for Plants:

All-elements incorporated into plants are ultimately derived from the atmosphere, water and soil.

Carbon (C):

It is derived from atmosphere as CO₂.

Hydrogen (H):

It is mainly obtained from water.

Oxygen (O):

It is either derived from the air or from the water in the form of inorganic ions.

Nitrogen (N):

The plants are unable to use atmospheric nitrogen. It is inert and plants cannot make use of it directly. However, by means of atmospheric activities nitrogen combines with oxygen and is brought down by rain to the soil. Besides, there are certain highly specialised organisms called nitrogen fixers, such as bacteria, and cyanobacteria.

They fix atmospheric nitrogen into the soil in the form of nitrites (NO₂), and nitrates (NO₃). These heterotrophic organisms, occurring in the soil convert nitrogen gas (N₂) to anionic forms such as nitrates (NO₃) or nitrite (NO₂^–), or a reduced cationic form such as NH₄⁺ (ammonium).

These nitrogenous compounds enter plants as nutrients through the roots and are assimilated as organic nitrogen. The plants, in turn, provide organic nitrogen to heterotrophic organisms.

The other elements are absorbed from the soil, such as phosphorus (P) as phosphate (PO₄), and sulphur (S) mainly as sulphate (SO₄₎.

Role of Essential Elements in Plants:

The essential elements perform several important roles.

They may be as follows:

The most important role of the elements is to participate in various metabolic activities, such as regulation of permeability of cell membranes; some elements are required for maintenance of osmotic pressure of cell sap, while others take part in an electron transport system. Some elements are responsible for buffer action, and some for electrical neutrality.

Macronutrients:

Mineral elements include N, P, K, Ca, Mg, S and Fe. They are absorbed from soil.

Nitrogen (N):

About 78% of nitrogen is found in atmospheric air, but this is of no use to plants, in its free state. This enters in the plants through stomata along with other gases, and comes out in the same state, unused.

The plants can take nitrogen, found in the soil in the form of NO₂⁻, NO₃⁻ or NH₄⁻. This is required by all parts of a plant, particularly the meristematic tissues. Nitrogen is found in the constituents of proteins, chlorophyll, protoplasm, nucleic acids, vitamins and hormones. For proper and normal growth of leaves of plants, it is one of the most essential elements.

Phosphorus (P):

It is usually found in nucleoproteins and protoplasm. It also helps in the division of nucleus and the cell. It also helps in the decomposition of carbohydrates during respiration. Phosphorus is a constituent of cell membranes, nucleic acids and nucleotides.

This is needed for all phosphorylation reactions. It plays a very important role in the ripening of the grains and fruits. This also helps a lot in the development of root system. For the development of underground parts of radish, beet root, and potato, etc., this element is required. Phosphorus is absorbed from the soil in the form of phosphate H₂PO⁻ ions.

Potassium (K):

This element is usually found in the growing regions of the plant. It is one of the constituents of the protoplasm. It is related to metabolic activities. This helps in the synthesis of carbohydrates and proteins. The plant grows normally in its presence, and the fruits and seeds remain quite healthy.

On the scarcity of potassium, the normal growth of the leaves become yellowish, and the stem becomes somewhat thin. The plants take potassium from the soil in the form of potassium nitrate and potassium chloride, etc.

Potassium helps to determine anion-cation balance in cells, and is involved in protein synthesis, opening and closing of stomata, activation of enzymes, and maintenance of turgidity’ of cells. It is absorbed as K⁺ ion by plants from the soil.

Calcium (Ca):

This element is always found in green plants. The middle lamella of the cell wall consists of calcuim pectate. Only because of this element, the permeability of the protoplasm is maintained. Usually for the growth and development of the fruits, the calcuim is very essential.

In its deficiency the cells of the plant become disorganized, and the growth of the plant is also checked. But, usually the leaves become chlorotic, when its overdose is present in the soil. Calcium is required by meristematic and differentiating tissues.

It is also used in the mitotic spindle in cell division. It activates certain enzymes and plays an important role in regulating metabolic activities. Calcium accumulates in older leaves. Calcium is absorbed by the plants from the soil in the form of Ca⁺ ions.

Magnesium (Mg):

It is found in the chlorophyll. The chlorophyll does not form in its absence, the leaves become chlorotic, and the growth of the plant is retarded. In the seeds of pulses and cereals, this element is found in sufficient quantity.

Magnesium activates enzymes in respiration and photosynthesis. It helps in synthesis of DNA and RNA. This also maintains ribosome structure. Magnesium is absorbed by the plants from the soil in the form of divalent M²⁺.

Sulphur (S):

It is usually found in the complex proteins of the plants. It is found in sufficient quantity in the mustard oil. It is also found in the protoplasm. In its deficiency, the leaves become chlorotic, and the stem becomes thin. The plants take this element from the soil in the form of calcium and potassium sulphate (SO₄^2-).

Sulphur is present in two amino acids, i.e., cysteine and methionine. It is main constituent of several coenzymes, vitamins {i.e., thiamine, biotin, CoA) and ferredoxin.

Iron (Fe):

This also plays an important role in the formation of chlorophyll, though it is not found in the constitution of the chlorophyll. This element is always found to be present in the chromatin material of nucleus and protoplasm.

It is an important constituent of proteins like ferredoxin and cytochromes which are involved in transfer of electrons. It is reversibly oxidised from Fe²⁺ to Fe³⁺ during electron transfer. It activates catalase. Iron is absorbed by the plants from the soil in the form of ferric (Fe³⁺) ions.

Non-mineral elements include C, H and O:

Carbon (C):

Carbon is the main constituent of the plant material. The main part of the plant consists of it. It is usually half of plant’s dry weight. Carbon is found in all organic compounds present in the plant.

The plants get carbon from the atmosphere in the form of carbon dioxide. Usually, .03% carbon dioxide is found in the atmosphere, but this percentage goes down even to .01% in day time, because of its use in the photosynthesis.

Hydrogen (H):

Free hydrogen does not play any role in the plant life. When it combines with oxygen, water is formed, which is absorbed by the plants. The hydrogen is found in so many organic and inorganic compounds of the plant.

Oxygen (O):

Free oxygen is taken from the atmosphere by the plants, which helps in the respiration of the living cells. Besides this, the oxygen is found in many organic compounds of the plant.

Micronutrients:

Boron (B):

This is also an essential element for the growth of nearly all the plants. The plants of tomato, tobacco, lemon, root, mustard, cotton and others are sufficiently benefitted by these elements. Boron also helps in the formation of the nodules of the leguminous plants. It also adds to the quantity of the sugar of beet root.

The beet root becomes susceptible to various diseases, because of its deficiency. In its deficiency, the normal growth of the plant is obstructed and various leaf spots are developed. In its absence, the apices of roots and stems become brittle, and gradually they die.

Boron is also required for uptake and utilisation of Ca²⁺, membrane function, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.

Manganese (Mn):

Manganese activates many enzymes which are involved in photosynthesis, respiration and nitrogen metabolism. Manganese helps in splitting of water to liberate oxygen during photosynthesis. Various vitamins are formed in the fruits, only because of its presence. The cabbage plants, pine trees and the leguminous plants require its optimum doses. Manganese is absorbed by plants in the form of manganous cation (Mn²⁺).

Zinc (Zn):

Zinc helps in the formation of chloroplasts. It activates various enzymes, especially carboxylases. Zinc is required in synthesis of auxin. It is absorbed by plants from soil’ in the form of (Zn²⁺) ions,

Copper (Cu):

Copper helps in formation of starch. It is required for the overall metabolism in plants. It is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu⁺ to Cu²⁺. Copper is absorbed by plants as cupric ion(Cu ²⁺).

Molybdenum (Mo):

This helps in formation of proteins. It also helps in fixation of nitrogen in soil by Azotobacter and Rhizobium. This is a constituent of several enzymes, including nitrogenase and nitrate reductase which take part in nitrogen metabolism. This is absorbed by plants from soil in the form of molybdate ion (MoO₂²⁺).

Chlorine (CI):

With Na⁺ and K⁺, chlorine helps in determining solute concentration and anion-cation balance in cells. It is required for cell division in roots and leaves. It is needed to perform water-splitting reaction in photosynthesis, which evolves oxygen.

Deficiency Symptoms of Essential Elements:

Deficiency Symptoms (Macronutrients):

Nitrogen (N):

Nitrogen deficiency causes yellowing of older leaves (chlorosis). The plant growth is stunted as protein content; cell division and cell enlargement are decreased. It also causes dormancy of lateral buds, late flowering, purple colouration and shoot axis surface and wrinkling of cereal grains.

Phosphorus (P):

Phosphorus deficiency causes premature leaf fall. Dead necrotic areas develop on leaves or fruits, and leaves turn dark to blue-green in colour. It also causes delay in seed germination.

Potassium (K):

Mottled chlorosis of leaves occurs. Necrotic areas are developed at the tips and margins of leaves, and they curve downward. Intemodes become short, and plants adopt bushy habit. It also causes loss of cambial activity, disintegration of plastids and increase in rate of respiration.

Calcium (Ca):

Calcium deficiency causes disintegration of growing meristematic regions of the root, stem and leaves. Chlorosis occurs along the margins of the younger leaves, and malformation occurs. This also leads to stunted growth of the plant.

Magnesium (Mg):

Magnesium deficiency causes interveinal chlorosis of the leaves. The older leaves are affected first, and dead necrotic patches appear on the leaves. This also causes premature leaf abscission.

Sulphur (S):

Sulphur deficiency causes yellowing (i.e., chlorosis) of leaves, younger leaves are affected first, tips and margins of leaves roll inward, and stem becomes hard due to development of sclerenchyma. These symptoms are similar to those of nitrogen deficiency symptoms, as sulphur and nitrogen are constituents of proteins.

Iron (Fe):

Iron deficiency causes rapid chlorosis of the leaves which is generally interveinal.

Deficiency Symptoms (Micronutrients):

Manganese (Mn):

Manganese deficiency causes chlorotic and necrotic spots in the interveinal areas of leaves.

Zinc (Zn):

Zinc deficiency causes chlorosis of older leaves which starts from tips and margins. Malformation of leaves takes place, and stunted growth of plant occurs. Its deficiency causes mottle leaf disease in apple, citrus, walnut, etc., and khaira disease in rice.

Copper (Cu):

Copper deficiency causes necrosis of the tips of young leaves. It causes die-back of citrus and other fruit trees where leaves wither and fall, bark becomes rough and splits exuding gummy substances. Reclamation disease of cereals and leguminous plants is also caused due to its deficiency.

Boron (B):

Boron deficiency causes death of the shoot tip. Flower formation is suppressed, root growth is stunted and shoot apices die. Fruits become of small size and root nodules in leguminous plants are not formed, and leaves become coppery in texture.

Molybdenum (Mo):

Molybdenum deficiency causes chlorotic interveinal mottling of the older leaves. This may cause nitrogen deficiency, as it is component of enzymes involved in nitrogen metabolism. Flower formation is inhibited, and also causes whip tail disease in cauliflower plants.

Chlorine (CI):

The deficiency of chlorine in plants causes wilting of leaves, stunted root growth and reduced fruiting.