Essential elements perform several functions. They participate in various metabolic processes in
the plant cells such as permeability of cell membrane, maintenance of osmotic concentration of cell
sap, electron transport systems, buffering action, enzymatic activity and act as major constituents of
macromolecules and co-enzymes. Various forms and functions of mineral elements are given below.
- Nitrogen :- This is the mineral element required by plants in the greatest amount. It is absorbed
mainly as NO3– though some are also taken up as NO2– or NH4
+. Nitrogen is required by all parts
of a plant, particularly the meristematic tissues and the metabolically active cells. Nitrogen is
one of the major constituents of proteins, nucleic acids, vitamins and hormones. - Phosphorus :- Phosphorus is absorbed by the plants from soil in the form of phosphate ions (either
as H2PO4– or HPO42–. Phosphorus is a constituent of cell membranes, certain proteins, all
nucleic acids and nucleotides, and is required for all phosphorylation reactions. - Potassium :- It is absorbed as potassium ion (K+). In plants, this is required in more abundant
quantities in the meristematic tissues, buds, leaves and root tips. Potassium helps to maintain
an anion-cation balance in cells and is involved in protein synthesis, opening and closing of
stomata, activation of enzymes and in the maintenance of the turgidity of cells. - Calcium :- Plant absorbs calcium from the soil in the form of calcium ions (Ca2+). Calcium is
required by meristematic and differentiating tissues. During cell division it is used in the
synthesis of cell wall, particularly as calcium pectate in the middle lamella. It is also needed
during the formation of mitotic spindle. It accumulates in older leaves. It is involved in the normal
functioning of the cell membranes. It activates certain enzymes and plays an important role in
regulating metabolic activities. - Magnesium :- It is absorbed by plants in the form of divalent Mg2+. It activates the enzymes of
respiration, photosynthesis and are involved in the synthesis of DNA and RNA. Magnesium is a
constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure. - Sulphur :- Plants obtain sulphur in the form of sulphate (SO42–). Sulphur is present in two amino
acids – cysteine and methionine and is the main constituent of several coenzymes, vitamins
(thiamine, biotin, Coenzyme A) and ferredoxin. - Iron : Plants obtain iron in the form of ferric ions (Fe3+). It is required in larger amounts in
comparison to other micronutrients. It is an important constituent of proteins involved in the
transfer of electrons like ferredoxin and cytochromes. It is reversibly oxidised from Fe2+ to
Fe3+ during electron transfer. It activates catalase enzyme, and is essential for the formation of
chlorophyll. - Manganese : It is absorbed in the form of manganous ions (Mn2+). It activates many enzymes
involved in photosynthesis, respiration and nitrogen metabolism. The best-defined function
of manganese is in the splitting of water to liberate oxygen during photosynthesis. - Zinc : Plants obtain zinc as Zn2+ ions. It activates various enzymes, especially carboxylases. It is
also needed in the synthesis of auxin. - Copper : It is absorbed as cupric ions (Cu2+). It is essential for the overall metabolism in plants. Like iron, it is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu+ to Cu2+.
- Boron : It is absorbed as BO33– or B4O72–. Boron is required for uptake and utilisation of Ca2+, membrane functioning, pollen germination, cell elongation, cell differentiation andcarbohydrate translocation.
- Molybdenum : Plants obtain it in the form of molybdate ions (MoO22+). It is a component of several enzymes, including nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.
- Chlorine :- It is absorbed in the form of chloride anion (Cl–). Along with Na+ and K+, it helps in determining the solute concentration and the anion cation balance in cells. It is essential for the water-splitting reaction in photosynthesis, a reaction that leads to oxygen evolution.
Deficiency Symptoms of Essential Elements
- Whenever the supply of an essential element becomes limited, plant growth is retarded. The
concentration of the essential element below which plant growth is retarded is termed as critical
concentration. The element is said to be deficient when present below the critical concentration. - Since each element has one or more specific structural or functional role in plants, in the absence
of any particular element, plants show certain morphological changes. These morphological
changes are indicative of certain element deficiencies and are called deficiency symptoms. The
deficiency symptoms vary from element to element and they disappear when the deficient mineral
nutrient is provided to the plant. However, if deprivation continues, it may eventually lead to the
death of the plant.
Mobility of Minerals :
- The parts of the plants that show the deficiency symptoms also depend on the mobility of the
element in the plant. For elements that are actively mobilised within the plants and exported
to young developing tissues, the deficiency symptoms tend to appear first in the older
tissues. For example, the deficiency symptoms of nitrogen, potassium, magnesium and
phosphorus are visible first in the senescent leaves. In the older leaves, biomolecules containing
these elements are broken down, making these elements available for mobilising to younger leaves. - The deficiency symptoms tend to appear first in the young tissues whenever the elements
are relatively immobile and are not transported out of the mature organs, for example,
elements like sulphur and calcium are a part of the structural component of the cell and hence are
not easily released. This aspect of mineral nutrition of plants is of a great significance and
importance to agriculture and horticulture. - The kind of deficiency symptoms shown in plants include chlorosis, necrosis, stunted plant growth,
premature fall of leaves and buds, and inhibition of cell division. - Chlorosis is the loss of chlorophyll leading to yellowing in leaves. This symptom is caused by
the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo. - Likewise, necrosis, or death of tissue, particularly leaf tissue, is due to the deficiency of Ca, Mg,
Cu, K. - Lack or low level of N, K, S, Mo causes an inhibition of cell division.
- Some elements like N, S, Mo delay in flowering if their concentration in plants is low.
- You can see from the above that the deficiency of any element can cause multiple symptoms and
that the same symptoms may be caused by the deficiency of one of several different elements.
Hence, to identify the deficient element, one has to study all the symptoms developed in all the
various parts of the plant and compare them with the available standard tables. - We must also be aware that different plants also respond differently to the deficiency of the same
element.
Some physiological deficiency diseases :
Whiptail of cauliflower : caused by deficiency of Mo
Marsh spot of pea : caused by deficiency of Mn
Grey or speck spot of oat : caused by deficiency of Mn
Die back of Citrus : caused by deficiency of Cu
Reclamation disease of cereals and legume crops : caused by deficiency of Cu
Khaira disease of paddy : caused by deficiency of Zn
Brown heart rot of beats : caused by deficiency of B
Stout axis : caused by deficiency of B
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