Plant NutritionDr. Pallavi

J.N.L. College Khagaul

Importance of mineralsThe basic needs of all living organisms are essentially the

same. They require water, macromolecules (carbohydrates, proteins, and fats) and minerals for their growth and development.

Most of the minerals present in soil can enter plantsthrough roots. However, not all the elements areessential for optimal plant growth.

The criteria for essentiality of an element are givenbelow:

(a) The element must be absolutely necessary forsupporting normal growth and reproduction. In theabsence of the element the plants do not complete theirlife cycle or set the seeds.

(b) The requirement of the element must be specific andnot replaceable by another element. In other words,deficiency of any one element cannot be met by supplyingsome other element.

(c) The element must be directly involved in themetabolism of the plant.

The essential elements are further divided into two broad categoriesbased on their quantitative requirements.

(i) Macronutrients

Macronutrients are generally present in plant tissues in large amounts(in excess of 10 mmole Kg –1 of dry matter). The macronutrients includecarbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium,calcium and magnesium. Of these, carbon, hydrogen and oxygen aremainly obtained from CO2 and H2O, while the others are absorbed fromthe soil as mineral nutrition.

(ii) Micronutrients

Micronutrients or trace elements, are needed in very small amounts(less than 10 mmole Kg –1 of dry matter). These include iron,manganese, copper, molybdenum, zinc, boron, chlorine and nickel.

Hydroponics

• Hydroponics is a method of growing plants in a water-nutrient solutioninstead of soil. Since its advent, hydroponics has developed into a growingprocess that researchers often use. This method of growing plants are usedby scientists who are interested in studying plant nutrient deficiencies.

Deficiency Symptoms of Essential ElementsThe 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.

Role of Macro- and Micro-nutrientsNitrogen: This is the essential nutrient element required by plants in thegreatest amount. It is absorbed mainly as NO3

– though some are also takenup as NO2

– or NH4+ . Nitrogen is required by all parts of a plant, particularly

the meristematic tissues and the metabolically active cells. Nitrogen is oneof the major constituents of proteins, nucleic acids, vitamins and hormones.The symptoms of nitrogen deficiency are as follows:(a) Impaired growth(b) Yellowing of leaves due to loss of chlorophyll, i.e., chlorosis.(c) Development of anthocyanin pigmentation in veins, sometimes inpetioles and stems.(d) Delayed or complete suppression of flowering and fruiting..

Phosphorus: Phosphorus is absorbed by the plants from soil in the form of phosphate ions. Phosphorus is a constituent of cellmembranes, certain proteins, all nucleic acids and nucleotides, and is required for all phosphorylation reactions.

Deficiency symptoms

(a) Leaves become dark green or purplish.

(b) Sometimes development of anthocyanin pigmentation occurs in veins which may become necrotic (Necrosis is defined aslocalised death of cells).

(c) Premature fall of leaves

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, openingand closing of stomata, activation of enzymes and in the maintenance of the turgidity of cells.

Deficiency symptoms

(a) Mottled chlorosis followed by the development of necrotic areas at the tips and margins of the leaves.

(b) K+ deficiency inhibits proteins synthesis and photosynthesis. At the same time, it increases the rate of respiration.

(c) The internodes become shorter and root system is adversely affected.

Calcium: Plant absorbs calcium from the soil in the form of calcium ions (Ca2+). Calcium is required by meristematic anddifferentiating 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 thecell membranes. It activates certain enzymes and plays an important role in regulating metabolic activities.

Deficiency symptoms

(a) Ultimate death of meristems which are found in shoot, leaf and root tips.

(b) Chlorosis along the margins of young leaves, later on they become necrotic.

(c) Distortion in leaf shape.

Magnesium: It is absorbed by plants in the form of divalent Mg2+. It activates the enzymes of respiration, photosynthesis and are involvedin the synthesis of DNA and RNA. Magnesium is a constituent of the ring structure of chlorophyll and helps to maintain the ribosomestructure.

Deficiency symptoms

(a) Interveinal chlorosis followed by anthocyanin pigmentation, eventually necrotic spots appears on the leaves. As magnesium is easilytransported within the plant body, the deficiency symptoms first appear in the mature leaves followed by the younger leaves at a laterstage.

(b) Stems become hard and woody, and turn yellowish green.

(c) Depression of internal phloem and extensive development of chlorenchyma.

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.

Deficiency symptoms

(a) Leaf tips and margins roll downwards and inwards e.g., tobacco, tea and tomato.

(b) Premature leaf fall.

(c) Delayed flowering and fruiting.

Iron: Plants obtain iron in the form of ferric ions (Fe3+). It is required in larger amounts in comparison to other micronutrients. It is animportant 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.

Deficiency symptoms

(a) Chlorosis particularly in younger leaves, the mature leaves remain unaffected.

(b) It inhibits chloroplast formation due to inhibition of protein synthesis.

(c) Stalks remain short and slender.

Manganese: It is absorbed in the form of manganous ions (Mn2+). It activates many enzymes involved in photosynthesis, respirationand nitrogen metabolism. The best defined function of manganese is in the splitting of water to liberate oxygen duringphotosynthesis.

Deficiency symptoms:

(a) Chlorosis (interveinal) and necrosis of leaves.

(b) Chloroplasts lose chlorophyll, turn yellow green, vacuolated and finally perish.

(c) Root system is poorly developed.

(d) Formation of grains is badly affected.

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 certainenzymes involved in redox reactions and is reversibly oxidised from Cu+ to Cu2+ .

Deficiency symptoms

(a) Both vegetative and reproductive growth are reduced.

(b) The most common symptoms of copper deficiency include a disease of fruit trees called 'exanthema' in which trees start yieldinggums on bark and 'reclamation of crop plants', found in cereals and legumes.

(c) It also causes necrosis of the tip of the young leaves (e.g., Citrus). The disease is called 'die back'.

Boron : It is absorbed as BO33− or B4 O 7

2− . Boron is required for uptake andutilisation of Ca2+, membrane functioning, pollen germination, cell elongation,cell differentiation and carbohydrate translocation.Molybdenum: Plants obtain it in the form of molybdate ions (MoO2

2+ ). It is acomponent of several enzymes, including nitrogenase and nitrate reductase bothof which participate in nitrogen metabolism.Deficiency symptoms(a) Mottled chlorosis is caused in the older leaves as in nitrogen deficiency, butunlike nitrogen-deficient plants, the cotyledons stay healthy and green.(b) It is also known to inhibit flowering, if they develop, they fall before fruitsetting.(c) It leads to drop in concentration of ascorbic acid.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 incells. It is essential for the water-splitting reaction in photosynthesis, a reactionthat leads to oxygen evolution.

Major Role of Nutrients

Various elements perform the following major role in the plants:

(1) Construction of the plant body: The elements particularly C, H and Oconstruct the plant body by entering into the constitution of cell wall andprotoplasm. They are, therefore, referred to as frame work elements.Besides, these (C, H and O) N, P and S also enter in the constitution ofprotoplasm. They are described as protoplasmic elements.

(2) Maintenance of osmotic pressure: Various minerals present in the cellsap in organic or inorganic form maintain the osmotic pressure of the cell.

(3) Maintenance of permeability of cytomembranes: The minerals,particularly Ca2+, K+ and Na+ maintain the permeability of cytomembranes.

(4) Influence the pH of the cell sap: Different cations and anions influenceon the pH of the cell sap.

(5) Catalysis of biochemical reaction: Several elements particularly Fe, Ca,Mg, Mn, Zn, Cu, Cl act as metallic catalyst in biochemical reactions.