Physiological Roles and Deficiency Symptoms Of Macroelements
Approximate amount of nitrogen present in the whole plant is 1-3%. It is present in almost all the living matter; chiefly macromolecules like proteins, nucleic acids (RNA and DNA) and thus it is an important element in various metabolic processes. It is also found in amino acids, purines, pyrimidines, porphyrins and coenzymes. It plays an important role in protein synthesis, photosynthesis, respiration, growth and in almost all metabolic reactions.
Deficiency of nitrogen interferes with protein synthesis and hence all forms of growth are affected. An early symptom is yellowing of leaves and development of chlorosis. Lower leaves turn light brown in colour and gradually dry. Stalk are short and slender. Flowering is also delayed or completely suppressed.
Sulphur is essential for the synthesis of some vitamins like biotin, thiamine and coenzyme-A, which are involved in various metabo- lic activities. Sulphur is required for the synthesis of allyl oils of Mustard (viz., sinigrin) which give odours and flavours to Mustard, Onion and Garlic. Sulphur is an important element because
1. It plays a role in the synthesis of chlorophyll.
2. It is a constituent of ferredoxin and some lipids of chloroplasts;
3. It is essential for nodulation in legumes;
4. It determines the structure of protein;
5. It plays a role as a part of active centre of some enzymes and affects various metabolic processes.Deficiency symptoms.
first appear in young leaves. They become reduced, chlorotic followed by anthocyanin pigmentation in certain species. Root system becomes extensive and stems become hard and woody.
A large amount of phosphorus accumulates in storage tissues of fruits and seeds and during active growth in meristematic tissues. It is component of the nucleic acids and proteins (which together form nucleoproteins), phytin, phospholipid, sugar phosphates, ATP, NADP+ and a number of phosphorylated compounds.
Phosphate plays a key role in energy metabolism. Incorporated into ATP, it is part and parcel of the universal “energy currency” of all types of living systems. Phosphorus is actively involved in the protein synthesis, photosynthesis, respiration and other metabolic processes. It also acts as an ac- tivator of some enzymes.
The deficiency of phosphorus is less disastrous than nitrogen. Its deficiency causes: –
(1) Disruption of general metabolism par- particularly at the level of energy generation;
(2) Ab- normalities in the shape and size of chloroplasts;
(3) Anthocyanin pigmentation, yellowing and drying of lower leaves;
(4) Roots and shoots be- come short and slender; and
(5) It promotes leaf fall and delays flowering.
Calcium plays following important roles in plants:
(i) It is chief constituent of cell wall, presentin the form of calcium pectate in middle lamella. boom
(ii) It controls cell permeability.
(iii) It is important in the formation of cell membranes.
(iv) It is essential for continued growth of the apical meristem. Calcium in small amounts is necessary for normal mitosis as it is important in chromatin or mitotic spindle organization.
(v) It is a potent activator of the enzyme-phos- pholipase, arginine kinase, adenosine triphosphatase (ATPase), adenyl kinase and amylase.
(vi) It is responsible to reduce toxicity byforming calcium salts of organic acids.
(vii) It helps in translocation of carbohydrates and amino acids.
(viii) It encourages root development.
(ix) In many plants it is also found as insoluble crystals of calcium oxalate
Calcium deficiency brings about decomposition of root epidermis, roots become mucous, the growing zone and lateral roots rapidly die off. Its deficiency terminates growth of meristematic regions. Margins of younger leaves show chlorosis which become necrotic. Younger leaves become permanently wilted (wither-tip effect) without spotting or marked chlorosis. Twig or stalk just below the tip and seed head often unable to stand erect.
Potassium is rich in actively dividing cells of buds, young leaves and root tips. It is present in fairly less amount is seeds and other mature tissues. It has the following physiological roles.
(i) It is needed for proper growth and development.
(ii) A high amount of potassium is required in the process of protein synthesis.
(iii) It is essential as activator of several enzymes. It is needed in the enzyme system in the change of sugar to starch, amino acids to protein, citric acid synthesis, DNA polymerase and interacts with iron enzymes.
(iv) It is required in maintaining cellular organization, permeability and hydration.
(v) It regulates movement of stomata.
(vi) It is needed in photosynthesis, respiration, translocation, reduction of nitrate and chlorophyll formation.
Deficiency symptoms are first observed on the older, lower leaves. Spots of dead tissue on older leaves are seen which are small, usually at tips, between veins and more marked at margins. The effects are localized. Potas- sium deficiency in cereal crops develop weak stalk. These plants easily bent to the ground by wind or rain.
Magnesium is present in tetrapyrrolic chlorophyll. Thus, it is found in all the green parts of plants and helps in photosynthesis. It also acts as an activator of several enzymes viz., hexokinase, phosphorylase, carboxylase, dehydrogenase and peptidase etc., and hence it plays important role in various metabolic processes including respiration. Magnesium is an important binding agent in microsomal particles where protein synthesis takes place.
Deficiency of magnesium results.
(i) Extensive interveinal chlorosis of theleaves resulting in defoliation;
(ii) Yellowing of the leaves start from basal to younger ones;
(iii) Development of anthocyanin pigment with dead spots;(iv) Tips and margins of leaves turn upward;
(v) Stalk become slender,
Physiological Roles and Deficiency Symptoms of Microelements (or trace elements)
It plays an important role in electron transport systems in photosynthesis and respiration and helps in energy generation. It is essential in chlorophyll synthesis. Iron is a component of flavoproteins active in biological oxidation. It functions in iron-porphyrin proteins which include peroxidases and catalases. It is present in ferredoxin, ferredoxin-reducing substances and cytochromes.
Deficiency of iron causes.
(i) Marked chlorosis particularly in younger leaves, the mature leaves remain uneffected ;
(ii) Characteristic interveinal chlorotic spots develop and the principal veins remain typically green showing fine network of reticulate venation:
(iii) Inhibits chloroplast formation;
(iv) Stalks become short and slender,
Boron plays following physiological roles:
(1) It regulates carbohydrate metabolism spe- cially pentose phosphate shunt.
(2) It regulates many growth phenomenon such as regeneration, fruiting and cell division
(3) It facilitates the translocation of sugars inplants.
(4) It forms complex at cell membrane which facilitate its passage across the membrane.
5) It regulates cellular differentiation and development.
(6) Boron also regulates water relations, active salt absorption, nodulation in legumes, fat metabolism, hormone metabolism, fertilization, phosphorus metabolism and photosynthesis.
Deformation, dis-colouration and disorganisation of meristematic tissue and finally death of growing point in severe boron deficiency have been noted. It causes abscission of flowers, increased fruit drop, inhibition of flower formation and retards root growth.
Its deficiency causes curling and twisting of leaves which become quite brittle. Its deficiency causes an abrupt growth of lateral shoots but tips of such shoots soon die. Boron deficiency causes heart rot of sugar beet, internal corn formation in apples and the development of watercore is inhibited in the turnip.
Manganese primarily functions as activator of several enzymes such as
i) malic dehydrogenase and oxalosuccinic dehydrogenase-the enzymes of Kreb’s cycle;
(ii) nitrate reductase and hydroxylamine reductase-thus play important role in nitrogen metabolism;
(iii) arginase and
(iv) carboxylase etc
Manganese plays a role in the photo-oxidation of H2O and release of molecular O₂ in photosynthesis. It is also involved in the synthesis of chlorophyll and oxidation of auxin (indole-3-acetic acid).
Deficiency of manganese causes chlorosis and nacrosis in the inter-veinal areas of leaves. However, the veins remain green, producing a checkered or reticulating effect. The chloroplasts are devoid of chlorophyll and starch grains, become yellow-green in colour, vacuolated and finally disintegrate.
Copper is a component of plastocyanin and therefore, acts as a key role in the electron transport chain in photosynthesis. It acts as an activator of several enzymes such as polyphenol oxidase, lactase and oxidase. It is a component of enzyme involved in the synthesis of ascorbic acid (vitamin-C).
Deficiency of copper causes distortion and chlorosis in leaves followed by necrosis of the tips of young leaves that proceeds along the margin of the leaf. Under severe conditions the whole plant may appear wilted and the leaves may be lost.
Zinc plays an important role in the synthesis of tryptophan which is a precursor of a plant growth hormone-auxin (Indol-3-acetic acid). It acts in the metabolism as an activator of several enzymes, such as
(i) carbonic anhydrase- catalyze the decomposition of carbonic acid to carbon dioxide;
(ii) alcohol dehydrogenase;
(iii) pyridine nucleotide dehydrogenase;
(iv) hexose kinase and
(v) phosphorylation enzymes.
Deficiency of zinc causes
(i) shortening of internodes with the result plants become stunted;
(ii) reduction in the size of leaves so that the leaves become very small;
(iii) interveinal chlorosis of older leaves starting at tip and extending to the margins;
(iv) interveinal necrosis, the spots of dead tissues enlarge in the areas between veins,
(v) suppression of seeds formation
Molybdenum is required. for nodulation in legumes, synthesis of tannins and reduction of nitrates to nitrites. It regulates the amino acid concentration in plant systems. It is also required in ascorbic acid metabolism. It has a protective role in the chloroplasts.
Deficiency of molybdenum develops chlorosis coupled with poor leaf growth. Nitrogen fixation is reduced in symbiotic plants. Its deficiency causes shoot growth die back. Flower formation is also inhibited.
Chlorine plays an important role in photosystem II in photosynthesis. It is essential in the transfer of electrons from water to photoxidized chlorophyll in photosystem II. It also plays a role in transpiration.
Its deficiency in nature cannot be encountered because of the prevalance of the cyclic salt of chloride. The deficiency symptoms consists of wilted leaves, which then become chlorotic and necrotic. Roots become short, thick and club-shaped near the tip.
Nickel is an essential micronutrient (Dalton et al., 1988). It is an essential part of enzyme urease which catalyzes hydrolysis of urea to CO2 and NH4. Urea is formed in most or all plants and needs the enzyme urease for its hydrolysis.
Deficiency of nickel causes accumulation of so much urea in leaves that they develop necrotic spots in their tips. Nickel deficiency in barley plants results in the production of nonviable seeds which show various anatomical abnormalities.
The plants require various mineral elements for their normal growth development and metabolism. They fulfil their mineral requirement usually from the soil. That means all the essential elements must be present in the soil where a plant grows. Now suppose a particular element or a group of elements are missing in the soil, the element(s) will, therefore, be deficient. If the plant also fails to get the particular element from the external source, it will show deficiency symptoms.
Usually, all the essential elements play specific vital roles in the plant metabolism. For example, magnesium is an essential element required as a constituent of chlorophyll molecule. It is also involved in various enzymatic reactions associated with phosphorus metabolism. If magnesium is deficient the chlorophyll will not be synthesized and the plants turn chlorotic. The plants will ultimately be sickly in appearance and termed as diseased. Such diseases caused by the deficiency of one or many elements are called deficiency dis- eases.
Such diseases are not infectious and can not be transmitted from one plant to another.Examples of deficiency diseases are-die- back disease of citrus and redamation disease of cereals caused by deficiency of copper; little leaf disease and mottle leaf disease of apples caused by deficiency of zinc; whip tail disease of cauliflower caused by deficiency of molebdenum ; heart rot of sugar beet and water core in turnip caused by deficiency of boron, etc. readmore