PLANT NUTRITION

All living organisms require nutrients for their growth and development. Organisms which can make their own food are autotrophs e.g, Plants. Organisms which cannot make their own food and depend on auto – trophs for food are called Heterotrophs. Plants obtain their nutrients (minerals) from soil. This is known as Mineral Nutrition.  2.1 Essential Mineral Elements:– Elements which are required essentially for plants growth and development are called. ‘Essential Mineral Elements’ The criteria for essentiality of an element is: (i) It should be absolutely necessary for supporting normal growth & development of plant (ii) The element required should be specific (iii) The element cannot be replaced by any other element (iv) The element must be directly involved in metabolism of the plant. Total 105 elements are there, out of which 20 are essential for the plant. Essential elements are divided into two categories:– 1- Macro nutrients 2 -  Micro nutrients Macronutrients are the nutrients present in larger quantity in the plants. Their concentration are 1 – 10 mg per gram of dry matter e.g, C, H, O, N, P, S, K, Ca, Mg, Si. Micronutrients are nutrients present in smaller amounts i.e, less than 0.1 mg per gram of dry matter e.g, Fe, Zn, Mn, Mo, Cu, Cl, B. These are also called ‘Trace Elements’. 2.2 Sources of Essential Elements:– Main sources are air and water. Carbon & oxygen are obtained from air. Hydrogen is obtained from water. C, H, O are the building blocks of macromolecules which form the plant body. Nitrogen is present in the air but plants cannot use it as such. So they get nitrogen from the soil as nitrates. P & S are also obtained from soil as Phosphates & Sulphates.  2.3 Role of Macro – and Micro nutrients:– Essential elements perform many roles like:– * Some synthesize components of the cell. * Some take part in synthesis of materials * Some act as components of enzymes * Some help in oxidation – reduction reactions * Some participate in energy transfer, maintenance of osmotic pressure, buffer action etc. HYDROPONICS:– It helps in determining the role of individual elements. It is a technique in which plants are grown with their roots immersed in nutrient solution without soil. In this, plants are raised in small tanks covered with wire net. It is filled with water solution containing all the required elements. Solution is changed from time to time. Oxygen is bubbled in for proper aeration. For studying deficiency symptom, particular ion? is eliminated from the nutrient medium Role of various elements are:– 1 - NITROGEN:– Source:– Soil. It is absorbed as Nitrate, Nitrite or ammonium salts Function:– It is required in greatest amount by plant. It forms proteins, vitamins, nucleic acids & hormones Deficiency Symptoms:– Stunted growth, Yellowing of leaves, late flowering, purple shoot 2 - PHOSPHOROUS:– Source:– Soil, in the form of phosphate ions Function:– It is a constituent of cell membranes, proteins nucleic acids,  nucleotides Deficiency Symptoms:– Poor growth, delay in seed germination purple/red spots on leaves, premature falling of leaves 3 - POTASSIUM:– Source:– Inorganic compounds like Potassium Sulphate (K2SO4), Potassium nitrate (KNO3). Function:– it helps to maintain anion – cation balance in cell, is involved in protein synthesis, opening – closing of stomata, activation of enzymes, maintaining turgidity of cells. Deficiency Symptoms:– Scorched leaf tips, shorter internodes, premature death, bushy growth. 4 - CALCIUM:–  Source:– Soil, It is absorbed in the form of Ca2+ ions from CaCo3 etc. Function:– formation of mitotic spindle during cell division, for normal functioning of cell membranes, activation of enzymes, regulating metabolic activities. Deficiency Symptoms:– Stunted growth, Necrosis, abnormal cell division. 5 - MAGNESIUM:– Source:– Soil, absorbed as Mg2+ ion Function:– Activation of enzyme in respiration & photosynthesis, synthesis of DNA & RNA, maintains ribosome structure Deficiency Symptoms:– Chlorosis, Necrosis, Premature leaf abcission 6 - SULPHUR:–  Source:– Soil as sulphates (So42–) Function:– Forms aminoacids cysteine & methionine, constituent of coenzymes, vitamins Deficiency Symptoms:– Chlorosis, stunted growth, anthocyanin accumulation, change in pigmientation 7 - IRON:–  Source:– Soil, It is obtained as Ferric ion (Fe3+) Function:– It is important constituent of proteins like Ferredoxin. Takes part in electron transport, photosynthesis development of chloroplast Deficiency Symptoms:– Chlorosis of leaves, reduced growth 8 - MANGANESE:–  Source:– from soil as Mn2+ Function:– Activation of enzymes involved in photosynthesis respiration and nitrogen metabolism Deficiency Symptoms:– Chlorosis, grey spots on leaves, sterile flowers 9 - ZINC:–  Source:– obtained as Zn2+ ions from minerals containing zinc Function:– Activation of enzymes, synthesis of Auxins Deficiency Symptoms:– Distortion of leaves, reduction in fruits and leaves, smaller leaves 10 - COPPER:–  Source:– From soil as Cupric ion (Cu2+) Function:– Overall metabolism of plant, chlorophyll synthesis Deficiency symptoms:– Necrosis of leaf, dieback of shoot, rough bark 11 - BORON:–  Source:– From soil as BO33– or B4O72– Function:– Helps in uptake & utilization of Ca2+, membrane function pollen germination, cell elongation, cell differentiation. Deficiency Symptoms:– Death of root/shoot tips, loss of apical dominance, small fruit size, stunted growth. 12 - MOLYBDENUM:–  Source:– Absorbed as molybdate ion (MoO22+) Function:– Component of enzymes participating in Nitrogen metabolism Deficiency Symptoms:– ‘Whiptail’ disease in cabbage where young leaves get distorted causes nitrogen deficiency 13 - CHLORINE:–  Source:– As chlorine gas or chlorides in soil Function:– Cell division, in photosynthesis Deficiency Symptoms:– Wilted leaves, stunted growth, reduced fruiting 2.4 Mechanism of Absorption of Elements:– Plants absorb mineral ions from the soil through roots and then translocate these to other parts of the plant. In this process, two main phases are seen:– 1 - In first phase, ions are taken into ‘free space’ / ‘outer space’ of the cells. It includes intercellular space & cell wall. Here ions can be exchanged freely. Energy is not required in this process, so this is passive 2 - In second phase, ions are taken into ‘inner space’ which means cytoplasm & vacuoles. Here ions are not exchangeable. This process requires metabolic energy and so it is an active process. Flux:– The movement of ions is called ‘Flux’ Influx:– Movement of ions into the cell is ‘Influx’ Efflux:– Outward movement of ions is ‘Efflux’. Mineral absorption occurs by 2 methods – Passive and Active (A) Passive Absorption:– Energy is not required in this type of absorption. It occurs along concentration gradient for unchanged minerals (i.e, from higher concentration to lower concentration) and along electrochemical gradient for ions (as ions have charge also). Passageway is provided by membrane channels Various theories of passive absorption are:–  (i) Ion Exchange:– Exchange of ions takes place between roots and soil. Cations are exchanged with H+ ions Anions are exchanged with OH– or HCO3– ions. It can occur either bycarbonic acid exchange or by contact exchange. In carbonic acid exchange, CO2 dissolves in H2O to form H2CO3 which breakes up into H+ & HCO3– . These can then be exchanged. In contact exchange, ions present on the surface of roots come in contact with ions in the soil & get exchanged. (ii) Donnan Equilibrium:– All cells have some fixed ions present on inner side which cannot diffuse out So, for reaching the equilibrium, additional ions (opposite in change to the ones fixed inside) are required. This continues till Donnan Equilibrium is reached when product of anions & cations in internal solution becomes equal to the product of anion & cations in external solution.          [ Ci+] [Ai–] = [Co+] [Ao–] where            Ci+ = cations inside      Co+ = cations outside A i – = Anions inside    Ao– = Anions outside (iii) Mass Flow Hypothesis:– According to this theory, ions are absorbed by the root along with mass flow of water under the influence of transpiration (B) Active Absorption:– It is the absorption of mineral ions which involves the use of energy. The movement of ions is against concentration gradient. It occurs with the help of certain carriers present in the plasma membrane. Activated ions combine with these carrier proteins to form ion – carrier complex. This complex moves across the membrane & reaches inside where it breaks to release the ions into the cytoplasm. Ion Traffic Into the Root:– Mineral ions are absorbed by roots and then carried to xylem. It occurs by two pathways:– * Apoplastic:– Water & ions move through the cell walls without crossing any membrane and reach xylem. * Symplastic:– Water & ions move across the cytoplasms of cell through plasmodesmata & reach xylem. So in this, they cross membranes & reach cytoplasm 2.5 Translocation of Solutes:–  Solutes move up in the plant along with the upward moving water. Water is moving up due to transpirational pull. So, the rate at which solutes are translocated corresponds to the rate of translocation of water. 2.6 Metabolism of Nitrogen:– Nitrogen is absorbed in the form of nitrate (NO3–) and ammonium (NH4+) from the soil. It is essential for living organisms. Nitrogen Cycle:– Atmospheric nitrogen is present in fixed forms as aminoacids, proteins, nucleic acids etc. in living organisms. When they die and decay, inorganic nitrogen is released. Dead remains decompose to produce ammonia. Ammonia converts into nitrites & then nitrates by process of Nitrification. Bacteria Nitrosomonas converts ammonia to nitrate. Bacteria Nitrobacter converts nitrite to nitrate. Nitrate is then available to plant or gets converted into Nitrogen gas by process of Denitrification. Bacteria like Pseudomonas helps in this process. Nitrogen gas may then be fixed again in the form of NH4 by biological fixation Biological Nitrogen Fixation:– Nitrogen fixation done by microorganisms is called biological Nitrogen fixation. It is done by two ways:– * By free living bacteria:– Free living N2 – fixers include cyanobacteria, Azotobacter, Clostridium * By Symbiotic bacteria:– Most common symbiotic bacteria is Rhizobium which lives in root nodules of leguminous plants. In symbiotic association, both the partners are mutually benefited. Nodules are the outgrowths of roots and act as site for N2 – fixation. These contains enzyme – Nitrogenase (a Mo – Fe protein), pink colour pigment – Leghaemoglobin (which protects nitrogenase from oxygen) Formation of Root Nodules:– Root hair of a plant curls when it comes in contact with Rhizobium due to chemicals secreted by it. At the place of curling, bacteria invades root tissues and sometimes forms large structures called – Bacteroids. Cell walls of root hairs degrade and membrane forms infection thread by growing inwards Cell division in infected tissues continues and nodules are formed. Auxins (produced by plant cells) and cytokinins (produced by invading bacteria), together, promote cell division and formation of nodule. N2 – fixation is controlled by genes:– plant ‘nod’ genes, bacterial ‘nod’ genes, ‘nif’ & ‘fix’ gene clusters Mechanism of N2 fixation:– In N2 fixation, atmospheric nitrogen gets reduced by addition of H – atoms. The three bonds (N º N) between two nitrogen atoms are broken. NH3is formed which is used for the synthesis of amino acid. N2 fixation requires 3 components:– (i) A strong reducing agent (ii) ATP (iii) Enzyme systems Synthesis of Aminoacids:– Aminoacids are initial products of nitrogen assimilation Each amino acid has an –NH2 (amino) group and –COOH (carboxyl) group. Aminoacid synthesis occurs in following step: (i) Reductive amination:– Amino acid reacts with  – ketoglutaric acid (comes from kreb’s cycle) and forms Glutamic acid. (ii) Transamination:– In this, amino group from one amino acid is transferred to the keto group of keto acid 17 amino acids can be made from Glutamic acid by transamination. Enzyme acting is Transaminase Amides:– eg, Asparagine, Glutamine. These are formed from aspartic acid and Glutamic acid when – OH part of acid is replaced by –NH2. Enzymes Glutamine synthetase or Asparagine synthetase help in the reaction Amides form proteins. Protein Synthesis:– Many amino acids join to form a protein (or polypeptide). Peptide bonds join the amino acids or amides to form proteins. 2.7 Heterotrophic Nutrition:–  Organisms not capable of making their own food are called Heterotrophs. e.g, Phanerogams, fungi, bacteria etc. Heterotrophs can be:– (a) Parasitic:– These obtain food from tissues of other plants. They have haustoria which penetrate into host plants vascular bundle and draws nutrition (b) Saprophytic:– These grow on decaying animal or vegetable matter and absorb organic food from it eg Monotropa. (c) Symbionts:– In this, two organisms live together and benefit each other mutually e.g, Lichen, mycorrhiza (d) Insectivorous:– These plants obtain their food by catching insects and digesting them. Trapped insects are killed & their protein is digested byu proteolytic enzymes. Amino acids formed are absorbed by plant e.g, Drosera, Utricularia, Nepenthes.