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Nitrogen Nutrition in Plants
Mechanism of Biological Nitrogen Fixation
Minerals for Plants
Higher plants generally utilize the oxidized forms such as nitrate (NO) and nitrite (NO) or the reduced form (N) of nitrogen which is made available by a variety of nitrogen fixers.
Natural or Atmospheric nitrogen fixation: By photochemical and electrochemical reactions, oxygen combines with nitrogen to form oxides of nitrogen. Now they get dissolved in water and combine with other salts to produce nitrates.
Physical nitrogen fixation out of total nitrogen fixed by natural agencies approximately 10% of this occurs due to physical processes such as lightning (i.e.. electric discharge), thunder storms and atmospheric pollution.
Due to lightning and thundering of clouds, N2 and O2 of the air react to form nitric oxide (NO). The nitric oxide is further oxidised with the help of O2 to form nitrogen dioxide (NO2).
N2 + O2 2NO
2NO + O2 2NO2
NO2 combines with H2O to form nitrous acid (HNO2) and nitric acid (HNO3). The acid falls along with rain water. Now it acts with alkaline radicals to form water soluble NO (nitrates) and NO (nitrites)."
2NO2 + H2O → HNO2 + HNO3
HNO3 + Ca or K salts → Ca or K Nitrates
The nitrates are soluble in water and are directly absorbed by the plants.
Asymbiotic biological nitrogen fixation: This is done by many aerobic and anaerobic bacteria, cyanobacteria (blue green algae) and. some fungi: e.g.:
Free living bacteria: Free living N2 fixing bacteria add
10-25 kg of nitrogen /ha/ annum.
Aerobic : Azotobacter
Anerobic : Clostridium
Photosynthetic : Chlorobium
Chemosynthetic : Thiobacillis
Cyanobacteria (blue-green algae) e.g., Anabaena, Nostoc, Tolypothrix cylindrospermum, Calotherix and Aulosira etc. They add 20-30 kg of N2-per hactare of soil and water bodies.
Free living fungi e.g., Yeast cells and Pullularia.
Symbiotic biological nitrogen fixation: Symbiotic bacteria are found in the root nodules of the members of family Legurninosae. The best known nitrogen fixing symbiotic bacterium is Rhizobium leguminosarum (Bacillus radicicola).
Several-schemes incorporating such idea have been proposed and Burris (1966) accepts that the total reduction of nitrogen occurs on an enzyme complex (Nitrogenase) without release of intermediates less reduced than ammonia.
The enzyme complex nitrogenase consists of two sub-units
A non-heme iron protein commonly called Fe protein (or dinitrogen reductas, component I).
An iron molybdenum protein called MoFe protein (or dinitrogenase, component II).
According to Burris (1962) hypothesis for nitrogen fixation suggesting the function of ATP and ferredoxin at each step in the reduction of nitrogen, The pretty function of A TP donor is furnished by pyruvate which also acts as electron 'donor for N2 reduction as well.
Pyruvate on one hand acts as ATP donor while on other hand it supplies hydrogen ions and electrons for nitrogen reduction via NADH2 and ferredoxin.
The nitrogenase enzyme require 16 ATP molecules, 8 hydrogen ions and 8 electrons to reduce one molecule of nitrogen to 2NH3 molecules.
N2 + 8e- + 8H+ + 16ATP ® 2NH3 + H2+ 16ADP + 16Pi
Explaining the mechanism of nitrogenase activity, its now believed that electrons are transferred from the reducing agent (Ferredoxin, Flavoprotein or Dithionite) to complex of Mg-ATP and Fe-protein (component II). From here electrons flow to Mo Fe protein (component I) and then to substrate (nitrogen) which is finally reduced (to NH3).
The ammonia formed in biological nitrogen fixation is not liberated. It is highly toxic and is immediately converted into amino acids.
Ammonia + a-ketoglutarate + NADH
Glutamate + NAD+ + H2O.
The amino acids are transported through phloem to other parts of the plant.
Application of fertilizers: Most of the soil usually contain sufficient amounts of essential mineral elements for the better crop production. Some of them are, however, deficient in certain elements. These elements are required to be supplemented externally by adding the appropriate fertilizers. Moreover, constant agricultural cultivation in field may also cause depletion of certain elements which must be replenished in order to improve the fertility of soil. The important elements need to be replenished in crop fields are nitrogen, phosphorus and potassium. These are grouped as nitrogenous ferttlizers, phosphate fertilizers and potash fertilizers. These are abbreviated as NPK. Common sources of NPK are ammonium chloride, ammonium sulphate, ammonium nitrate, bone meal, calcium magnesium phosphate and nitrate of soda.
Nitrogen (Imp. In growth, metabolism, Heredity, Reproduction) NO3- from (Nitrate)
Sulphur SO42- (Sulphate form)
Phosphorus H2PO4- & Anthocyanin HP4- (Orthophosphate anion form)
Calcium Ca++ form
Molybdenum MoO42- Form (Molybdateion)
Potassium K+ is only monovalent cation in free form
Magnesium Mg++ form
Iron (Fe) Absorption in Fe++ (us) form, which is active form
Manganese Mn++ forms
Boron H3BO3- or B(OH)3 or BO3-3 (Borate)
Copper Cu++ form toxic in high conc.
Zinc Zn++ form
(i) Imp. constituent of proteins (AA), RNA, DNA.
(ii) Present in porphyrins of chlorophylls & cytochromes, thus active role in photosynthesis & respiration. (ETS)
(iii) Parts of vitamins, Co-enzymes (NAD, NADP) & alkaloids
(iv) Constituent of plant hormones IAA, ATPs.
(v) Absorbed from soil as NO3-, NO2,
NH4+ some plants from air by nitrogen fixeres (Rhizobia, Azolla, fungi)
(i) Parts of cystine, cystein, & methionine amino acids.
(ii) Vito Biotene, thiamine, Co-A in respiration.
(iii) Disulphide linkage (-S-S) for protein orientation
(iv) Sulph hydril (-S-H) for active site of enzyme.
(v) Role in oil synth, chlorophyll synthesis & part of ferredoxin.
(vi) Root nodule formation.
(i) Very imp. to RNA, DNA (Heredity) Phospholipid (cell membrane), NADP, (Co-enzyme), A TP (energy reactions)
(ii) Imp. in Photosynthesis (NADP), Protein synth. (DNA, RNA, A TP, AA)
(iii) In oxidation-reduction reactions, fat metabolism.
(iv) In growth of roots, dev. of leaf form of seeds and crop yield.
(v) Important for endergonic & exergonic reactions.
(i) Imp. for mechanical strength because Ca is constituent of middle lamella (Ca-pectate in cell wall)
(ii) Permeability of biomembrane maintained by calcium.
(iii) Stability of chromosome stnicture & in spindle formation. (Hewitt 1963)
(iv) Detoxification (Oxalic acid ® Co-oxalate), Na+, K+.
(v) Activator of enzymes-Phospholipase, arginin, kinase, ATPase, Amylase.
(vi) Essential for growth of apical meristems.
(i) Role as prosthetic group of nitrate reductase and nitrogeneous in nitrogen metabolism.
(ii) Tanin synthesis process.
(i) Not a essential contituent of organic matter but imp. for respiration, photosynthesis, protein synth and DNA synthesis as activator.
(ii) Key role in stomatal movement and transpiration.
(iii) In starch synthesis & distribution, reagulation of permeability and charge of cells (cation-anion balance)
(i) Constituent of chlorophyll and in ribosomal units binding.
(ii) Essential for phosphate transfer reactions (P-metabolism)
(iii) Activator of many enzymes in carbohydrates metabolism. Eg. Hexokinase
(iv) In cell wall formation
(i) Absorption in acidic soil, because present in soluble form.
(ii) Iron-Porphyrin protein for cytochromes, Peroxidase, Catalases (Photorespiration)
(iii) Fe imp. to Ferrodoxin ® Biological N2 fixation and ETS.
(iv) Essential role in chlorophyll synthesis.
(v) In Aconitase enzyme of Krebs cycle.
(i) Mn++ is activator of many enzymes Nitrite reductase, hydroxyl amine reductase decarboxylase, dehydrogenase
(ii) Essential for O2 evolution and photolysis of water in light reaction.
(iii) Chlorophyll & IAA formation;
(iv) Respiratory metabolism
(i) B is only micronutrient which is not . associate with enzymes.
(ii) Key role in sugar translocation (Phloem conduction)
(iii) Must for cell division, flowering, fruiting, active salt absorption, nodule formation in legumes.
(iv) B is essential in pollen tube formation.
(v) Lethal effect at carbohydrate metabolic site
(i) Oxidation-reduction process, as parts of enzymes, cytochromes (PC & a)
(ii) Vit.-C (ascorbic acid) formation
(i) Specific role in Auxin (IAA) Hormone synthesis in cell.
(ii) Activator of Carbonic anhydrase, alcohol dehydrogenase, - Peptidase
(iii) In seed formation
(i) Chlorosis (yellowing) in older leaves (Highly mobile).
(ii) Anthocyanin formed in stem, petioles & leaf. (Tomato etc.)
(iii) Plant growth shunted (cell div. & respiration reduced).
(iv) Protein synthesis, cell enlargement, chi-synthesis decreased.
(v) Late flowering & plant become more suceptible to fungal disease due to excessive nitrogen.
(vi) Seed dormancy increased.
(i) Chlorosis in yellowing in younger leaves with anthocyanin.
(ii) Stem & roots become woody (hard) because sclerenchyma development.
(iii) Tip & margins of leafs curved inwardly "Tea yellow disease".
(iv) Cell division reduced & checked fruiting.
(i) Premature leaf fall, Necrosis, formation.
(ii) Protein synthesis decrease.
(iii) Growth of roots, shoots checked delay in flowering.
(iv) Xylem & Phloem differentiation reduced.
(v) Inhibit seed germination.
(i) Disintegration of growing apices (Root, shoot, leaf apex)
(ii) Irregular cell division (Mitosis) and death of meristem.
(iii) Chlorosis on margins of younger leaves malformation.
(iv) Flower falling Necrosis.
(v) Abnormalities in chromosomes.
(i) Interveinal chlorosis (Lemon).
(ii) Whip tail of cauliflower.
(iii) Inhibition of flowering.
(i) Mottled (Interveinal chlorosis & shorter the internodes. (Bushy habit)
(ii) "Die-back" disease.
(iii) Necrosis & Blight effect on leaf tips, margin curved downwards.
(iv) Stop the carbohydrate metabolism, storage of carbohydrate in potato, beet, inhibited.
(v) Decrease the apical dominance, seeds less developed.
(i) Interveinal chlorosis on large scale and form of anthocyanin in older leaves.
(ii) Necrotic spots
(iii) Inhibition of Glycolysis, Krebs cycle (Carbohydrate metabolism).
(i) Rapid interveinal chlorosis (New leaves )
(ii) Inhibition of respiration.
(iii) Disintegration of chloroplast.
(i) Deficiency cause chlorotic & necrotic spots on leaves. (Mosaic pattern)
(ii) Chlorophyll & starch disappears from plastids.
(iii) Marsh spot of pea, and grey speak of oat.
(iv) Chlorosis in young & older leaves.
(i) Stem and root tips (Apex) dies. Root growthshuted.
(ii) Flower formation suppressed.
(iii) EMP pathway change in HMP (PPP) pathway.
(iv) Physiological diseases - top rotten in tobacoo water core in turnip, brown heart rot of beets, Brittleness of Celeary stem, Heart rot in carrot & marigold, fibers in applied fruit.
(i) Necrosis of tip in young leaves (wither tip)
(ii) "Die-back of citrus" and other fruit trees Exanthema in trees.
(iii) Reclamation disease of cereals and legume crops.
(i) Checked veg. growth and shorter the internodes leaf deformation.
(ii) Mottle leaf disease in fruit trees "little leaf disease”.
(iii) Khaira disease of paddy' Rosset disease in walnut.
(iv) Inhibit seed formations, white but disease.
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Mineral Absorption Table of Content Passive...