Let us make an in-depth study of the sources, functions and deficiency symptoms of some essential mineral elements in plants.
The seven major essential mineral elements in plants are: (1) Carbon, Hydrogen and Oxygen (2) Nitrogen (3) Phosphorus (4) Potassium (5) Calcium (6) Magnesium and (7) Sulphur.
Element # 1. Carbon, Hydrogen and Oxygen (C, H and O):
These are the non-mineral essential elements commonly enter a plant body as CO2, H2O. These elements are the building blocks of macromolecules and constitute over 90% of the total dry matter of the plant. Hence these are commonly known as framework elements.
Element # 2. Nitrogen (N):
Source:
Plants absorb nitrogen from the soil in three forms: nitrate (NO3- ), Nitrite (NO2-) or Ammonium (NH4+).
Regions of requirement:
Nitrogen is required in all plant parts particularly in meristematic tissues.
Functions:
(i) It is the major constituent of proteins, purines, pyrimidines, vitamins, cholorophyll and hormones,
(ii) It is also present as a component of coenzymes like FAD, NAD, NADP etc.
(iii) Older leaves when turn yellow, their nitrogen passes to younger parts in the form of amines and amides.
Deficiency Symptoms:
(i) Stunted growth due to reduced cell division and dormant lateral buds.
(ii) Chlorosis (yellowing of leaves),
(iii) Suppressed or late flowering,
(iv) Increase in starch content but decrease in protein content,
(v) Wrinkling of cereal grains,
(vi) Purple colouration appears in shoot axis.
Element # 3. Phosphorus (P):
Source:
Soil contains phosphorus in organic and inorganic forms. Plant absorbs only inorganic forms: monovalent phosphate anions (H2PO4) and divalent phosphate anions (H2PO4)2-. The organic forms will be available only after transformation into inorganic forms.
Regions of requirements:
Used mostly in younger tissues.
Functions:
It is a constituent of phospholipids (membrane lipids), nucleic acids, nucleotides, coenzymes, ATP, metabolic intermediates, sugar phosphates in photosynthesis etc.
Deficiency Symptoms:
(i) Purple or red pigmentation on leaves
(ii) Premature fall of leaves and floral buds
(iii) Delay in seed germination
(iv) Older leaves affected first and become dark brown,
(v) Stunted and slender stem in young plants,
(vi) Accumulation of carbohydrates in Glycine max (Soybean),
(vii) Vascular tissues reduce in tomato plants.
Element # 4. Potassium (K):
Source:
It is absorbed as potassium ion (K+).
Regions of requirements:
Required in meristematic tissues, leaves and root tips. It accumulates in older leaves.
Functions:
(i) Essential for stomatal opening, translocation of sugar, protein synthesis, activation of enzymes and maintenance of cell turgor.
(ii) Determine anion-cation balance in cell.
Deficiency Symptoms:
(i) Mottled or marginal chlorosis followed by necrosis of leaf tips, margins and between veins
(ii) Loss of apical dominance, leads to rosette or bushy habit
(iii) Loss of cambial activity
(iv) Disintegration of plastids
(v) Rate of respiration increases
(vi) Dieback of shoots i.e. progressive death from shoot tip towards base
(vii) Increased tendency to lodging (bent to the ground) in corn.
Element # 5. Calcium (Ca):
Source:
Plant absorbs calcium from the soil in the form of Ca2+ ions. Deficient in sandy soils.
Regions of requirements:
Meristematic and differentiated tissues; accumulated in older leaves.
Functions:
(i) Used in synthesis of calcium pactate in middle lamella of cell wall
(ii) Involved in normal functioning of cell membrane
(iii) Used in formation of mitotic spindle
(iv) Serve as a second messenger in action of phytohormones.
(v) Activates certain enzymes like ATPase, kinases and succinate dehydrogenase.
Deficiency Symptoms:
(i) Stunted growth
(ii) Chlorosis, downward hooking and deformation in young leaves,
(iii) Necrosis of young meristematic regions such as root tips or young leaves.
Element # 6. Magnesium (Mg):
Source:
Like calcium, magnesium is also available in the soil in form of exchangeable cation. It is absorbed as divalent Mg2+.
Regions of requirements:
It is required in leaves. It is withdrawn from ageing leaves and exported to developing seeds.
Functions:
(i) Important constituent of chlorophyll
(ii) Maintains ribosome structure and chromatin fibre
(iii) Activates enzymes in respiration, photosynthesis and synthesis of DNA and RNA.
Deficiency Symptoms:
(i) Chlorosis between the leaf veins
(ii) Necrotic or purple spots on older leaves.
(iii) Premature leaf abscission
(iv) Extensive development of chlorenchyma and scanty pith formation.
Element # 7. Sulphur (S):
Source:
Plants obtain sulphur from soil as divalent sulphate anions (SO42-). Atmospheric SO2 and SO3 are also absorbed directly.
Regions of requirements:
In plants sulphur is required in stem, root tips and young leaves.
Functions:
(i) It is a constituent of amino acids like cysteine, cystine and methionine,
(ii) It is also the main constituents of Coenzyme A, Vitamins (thiamine and biotin), ferridoxin etc.
(iii) It is essential for stabilizing the structure of protein by formation of disulfide bond (S-S) between two cysteine residues to form a cystine,
(iv) Characteristic pungent odour of mustard, onion, garlic etc. is due to presence of sulphur containing volatile oils.
Deficiency Symptoms:
The sulphur deficiency symptoms are similar to those of nitrogen deficiency because sulphur and nitrogen are constituents of proteins.
Sulphur deficiency causes:
(i) Chlorosis of younger leaves
(ii) Stunted growth
(iii) Accumulation of anthocyanin
(iv) Terminal bud growth in inhibited
(v) Lateral buds develop prematurely.