In this article we will discuss about:- 1. Classification of Essential Nutrients 2. Criteria of Essentiality 3. Functions.
Classification of Essential Nutrients:
There are 17 essential elements that are required by most plants (Table 6.2). The Big Three: nitrogen, phosphorus and potassium are the primary fertiliser elements used in crop production. Excluding carbon, hydrogen and oxygen, which are obtained from the air, elements provided by soil may be placed in two or three categories according to quantities used by plants as shown in Fig. 6.1.
Sodium, silicon, cobalt and vanadium are beneficial for some plants but have not been established as essential elements for all higher plants. Macronutrients are needed in concentrations of 1000 pg g-1 of dry matter or more, whereas micronutrients (trace elements) are needed in tissue concentrations equal to or less than 100 pg g-1 of dry matter.
The low requirement of plants for trace elements can be accounted for by the participation of these elements in enzymatic reactions and as constituents of growth hormones rather than as components of major plant products such as structural and protoplasmic tissue.
Mengel and Kirkby (1978) classified plant nutrients based on their biochemical behaviour and physiological functions into four groups:
Group 1:
C, H, O, N and S. These are major constituents of organic material involved in enzymatic processes and oxidation-reduction reactions.
Group 2:
P and B. These are involved in energy transfer reactions and esterification with native alcohol groups in plants.
Group 3:
K, Ca, Mg, Mn, CI and Ni. This group plays osmatic and ion balance roles, plus more specific function in enzyme conformation and catalysis.
Group 4:
Fe, Cu, Zn and Mo. Present as structural chelates of metallo-proteins. These elements enable electron transport by valency change.
Nutrients are also classified, based on ionic forms absorbed by plants into cations and anions and metals and nonmetals.
Cations NH4+, Ca2+, Mg2+, K+, Cu2+, Fe2+, Mn2+, Zn2+, Ni2+.
Anions NO3–, HPO42-, H2PO4–SO42- BO33-, MoO42-, CI–.
Metals K, Ca, Mg, Fe, Mn, Zn. Cu.
Nonmetals N, P, S, B, Mo, CI.
Criteria of Essentiality:
Even-though, some of these elements have been known since long, their essentiality has been established in the last century.
Arnon and Stout (1939) and Arnon (1952) proposed the following criteria of essentiality of mineral nutrients:
(1) A deficiency of the element in question results in failure to complete the life cycle.
(2) Deficiency of element in question can be corrected only by supplying that particular element.
(3) The element must extend its effect directly on growth or metabolism and not by indirect effect such as antagonism of another element present at a toxic level.
Arnon criteria of essentiality of elements for plant growth may be more current. However, it appears to be too rigid from practical point of view. Nicholas proposed the term functional or metabolic nutrient to include any mineral element which functions in plant metabolism, whether or not its action is specific.
Elements such as sodium, cobalt, silicon and vanadium are classified as essential when the less restrictive definition of essentiality is used.
Functions of Essential Nutrients:
1. Carbon:
Basic molecular component of carbohydrates, proteins, lipids and nucleic acids.
2. Oxygen:
It is somewhat like carbon in that it occurs in virtually all organic compounds of living organisms.
3. Hydrogen:
Plays a central role in plant metabolism. Important in ionic balance and as main reducing agent plays a key role in energy relations of cells.
4. Nitrogen:
It is a component of many important organic compounds ranging from protein to nucleic acids. It is an integral part of chlorophyll, which is the primary absorber of light energy needed for photosynthesis. It imparts green colour to plants.
5. Phosphorus:
Central role in energy transfer and protein metabolism. It is an important structural component of many bio-chemicals including nucleic acids. DNA and RNA are associated with control of hereditary processes. It is also associated with increased root growth and early maturity of crops.
6. Potassium:
Helps in osmotic and ionic regulation. It functions as cofactor or activator for many enzymes of carbohydrate and protein metabolism. Imparts disease resistance in cereals and drought resistance in many crops.
7. Calcium:
It is involved in cell division and plays a major role in maintenance of membrane integrity.
8. Magnesium:
Component of chlorophyll and a cofactor for many enzymatic reactions. It is a structural component in ribosomes.
9. Sulphur:
Like phosphorus, it is involved in plant cell energetics. It is associated with chlorophyll formation and sulphur containing amino acids.
10. Iron:
An essential component of many hemo and nonhemo Fe enzymes and carriers, including cytochromes (respiratory electron carriers) and the ferredoxins. The latter are involved in key metabolic functions such as N fixation, photosynthesis and electron transfer.
11. Zinc:
It is a constituent of several enzyme systems regulating various metabolic reactions.
12. Manganese:
Involved in oxygen evolving system of photosynthesis. It can substitute for magnesium in many of the phosphorylating and group transfer reactions. It influences auxin levels in plants.
13. Copper:
It acts as electron carrier in enzymes associated with oxidation-reduction reactions. It has indirect effect on nodule formation.
14. Boron:
It is essential for development and growth of new cells in plant meristem. It is necessary for nodule formation in legumes. It is associated with translocation of sugars, starches, nitrogen and phosphorus.
15. Molybdenum:
It is an essential component of enzyme nitrate reductase in plants. It is also a structural component of nitrogenase associated with nitrogen fixation in legumes.
16. Chlorine:
Essential for photosynthesis and as an activator of enzymes involved in splitting water. Associated with osmoregulation of plants growing on saline soils.
17. Nickel:
Essential for regulating N metabolism, grain filling and seed viability.