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  • Mineral nutrition in plants

    MINERAL NUTRITION IN PLANTS

    NITROGEN METABOLISM Nitrogen metabolism in brief
    Nitrogen is an inert gas which constitutes 78% of the atmosphere. It is an important mineral present in the bodies olf living organisms. It forms a component of proteins and aminoacids and is also present in nucleic acids, cytochromes, chlorophyll, vitamins, alkaloids and so on.Nitrogen cannot be used directly and is converted to Nitrites, Nitrates and Ammonia by a process called Nitrogen Fixation. There are many free living organisms like bacteria and blue-green algae which are involved in nitrogen fixation. The ammonia and urea present in the soil are directly absorbed by plants.

    Nitrogen Cycle

    The atmosphere is the source of elemental nitrogen which cannot be used directly by plants. The atmospheric nitrogen is converted to ammonia, nitrite, nitrate or organic nitrogen in the soil.The death and decay of organic systems causes cycling of ammonia from amino acids, purnies and pyrimidines. Some of these forms may also be converted to Nitrogen gas and may be cycled back into the atmosphere.The process by which these forms get inter converted to maintain a constant amount of nitrogen in atmosphere, by physical and biological processes is called nitrogen cycle. the cycle includes 5 stages.
    Ammonification
    Nitrification
    Nitrate assimilation
    Denitrification and
    Nitrogen fixation

    Nitrogen cycle
    (i) Ammonification
    This involves conversion of organic nitrogen to ammonium ions by microbes present in the soil. The sources of organic nitrogen in the soil are animal excreta and dead and decaying plant and animal remains which are acted upon by ammonifying saprotrophic bacteria such as Bacillus ramosus, Bacillus vulgaris, certain soil fungi and actinomycetes.
    (ii) Nitrification
    In warm moist soils having a temperature of 30- 35C and neutral pH, ammonia gets oxidized to nitrite (NO2-) and then nitrate (NO3-) by the process of nitrification. Nitrifying bacteria like Nitrosomonas convert ammonia to nitrite and another bacterium called Nitrobacter converts nitrite to nitrate.
    Ammonia -> Nitrobacter -> NO2 Nitrate -> Nitrosomonas -> Nitrite
    (iii) Nitrate Assimilation
    The nitrate present in the soil is absorbed by plants through the root system in the form of NO3-ions. But is cannot be used by plants directly. So it is first reduced to nitrite by the enzyme nitrate reductase. Nitrite is then converted to Ammonia by the enzyme nitrite reductase series of steps requiring a total of eight electrons provided by reduced NAD and Ferredoxin (Fd). This reduction of Nitrate of Ammonia and its incorporation into cellular proteins by aerobic micro organisms and higher plants is called nitrate assimilation.
    (iv) Dentrification
    The process of conversion of nitrate and nitrite into ammonia, nitrogen gas and nitrous oxide (N2O) is called denitrification. This process ends in the release of gaseous nitrogen into the atmosphere and thus completes the nitrogen cycle. A number of bacteria such as Pseudomonas denitrificans, Bacillus subtilis and Thiobacillus dentrificans are involved in this process.
    (v) Nitrogen fixation
    Nitrogen fixation refers to the conversion of elementary dinitrogen (NºN) into organic form to make it available for plants. Nitrogen fixation is essentially of two types-
    (i) Non-Biological or Physical and
    (ii) Biological Nitrogen Fixation
    Non-Biological or Physical Nitrogen Fixation:This involves fixation of nitrogen by chemical process in industry or naturally by electrical discharges such as lightning.
    Biological Nitrogen Fixation:Nitrogen fixation that takes place by living things is called biological nitrogen fixation. These include some bacteria and blue-green algae, which have acquired the capaicity to fix atmospheric nitrogen during the evolutionary process by possessing a set of genes called ‘nif ‘ (Nitrogen fixing) genes. They fix Nitrogen as given in the following reaction.
    These organisms may be freeliving which are otherwise called non-symbiotic nitrogen fixers and may form symbiotic associations in some plants when they are called symbiotic nitrogen fixers.
    Non-Symbiotic Nitrogen Fixation:

    This is carried out by free living organisms in the soil such as Bacteria, blue green algae.Bacteria include aerobic bacteria such as Azotobacter and anaerobic baceria such as Clostridium, Chlorobium and Chromatium.Blue green algae include Chroococcus, Rivularia, Anabaena, Tolypothrix and Nostoc.These organisms contain an enzyme system called Nitrogenase which is a Mo-Fe (Molybdenum-ferredoxin) protein. this progressively reduces the dinitrogen molecules to form ammonia, with the help of Ferredoxin and energy from ATP.
    Symbiotic Nitrogen Fixation:

    This involves nitrogen fixation by micro organisms living in symbiotic association with higher plants which are commonly legumes, but non-legumes may also be involved. A symbiotic association is a mutually beneficial relationship between two living organisms which are called symbionts.
    Nitrogen fixation in non-legumes:An actinomycete like Frankia establishes a symbiotic relationship with roots of higher plants such as casuarina and Alnus. Blue-green algae like Nostoc establish symbiotic relationships in the corolloid roots of Cycas, or thalli of Anthoceros.
    Nitrogen fixation in legumes:
    This is the commonest type of symbiotic nitrogen fixation which has been elaborately studied. A soil bacterium called Rhizobium infects roots of leguminous plants (belonging to Family Leguminosae) and forms the root nodules.These are involved in nitrogen fixation. The bacteria living in the soil enter the root hair and penetrate the root cortex through an infection thread. When the bacteria enter the cortical cells of roots, the latter get stimulated to divide vigorously and form nodules on the root. The bacteria come to occupy the nodules, and at this stage lacl a rigid cell wall being called bacteroids. These make use of the food substances of the root cells and secrete a pinkish pigment called leghemoglobin which is an oxygen carrier like hemoglobin.The Rhizobia in the form of bacteroides contain the enzyme nitrogenase which is responsible for fixation of Nitrogen thus benefitting the host plant. Leghemoglobin is supposed to protect the nitrogenase enzyme as it can function only under anaerobic conditions.

  • #2
    Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. The reaction for BNF is: N2 + 8 H+ + 8 e− → 2 NH3 + H2. This type of reaction results in N2 gaining electrons (see above equation) and is thus termed a reduction reaction. The exact mechanism of catalysis is unknown due to the technical difficulties biochemists have in actually visualizing this reaction in vitro, so the exact sequence of the steps of this reaction are not completely understood. Despite this, a great deal is known of the process. While the equilibrium formation of ammonia from molecular hydrogen and nitrogen has an overall negative enthalpy of reaction (i.e. it gives off energy), the energy barrier to activation is very high without the assistance of catalysis, which is done by nitrogenases. The enzymatic reduction of N2 to ammonia therefore requires an input of chemical energy, released from ATP hydrolysis, to overcome the activation energy barrier.

    Nitrogenase is made up of two soluble proteins: component I and II. Component I known as MoFe protein or nitrogenase contains 2 Mo atoms, 28 to 34 Fe atoms, and 26 to 28 acid-labile sulfides, also known as a iron-molybdenum cofactor (FeMoco). Component I is composed of two copies each of two subunits (α and β); each subunit’s stability depends on the other in vivo. Component II known as Fe protein or nitrogenase reductase is composed of two copies of a single subunit. This protein has four non-heme Fe atoms and four acid-labile sulfides (4Fe-4S). Substrate binding and reduction takes place on component I, which binds to ATP and ferredoxin or flavodoxin proteins (Fdx or Fld).

    The hydrolysis of ATP supplies the energy for the reaction while the Fdx/Fld proteins supply the electrons. Note this is a reduction reaction which means that electrons must be added to the N2 to reduce it to NH4. Thus, the role of component II is to supply electrons, one at a time to component I. ATP is not hydrolyzed to ADP until component II transfers an electron to component I . 21-25 ATPs are required for each N2 fixed. The association of nitrogenase component I and II and later dissociation occurs several times to allow the fixation of one N2 molecule.

    Nitrogenase ultimately bonds each atom of nitrogen to three hydrogen atoms to form ammonia (NH3). The nitrogenase reaction additionally produces molecular hydrogen as a side product, which is of special interest for people trying to produce H2 as an alternative energy source to fossil fuels.

    Protection of nitrogenase from oxygen:

    O2 binds to the iron (Fe) found in nitrogenases and blocks their ability to bind to N2. To protect nitrogenases, there are mechanisms for nitrogen fixers to protect nitrogenase from oxygen in vivo. One known exception is the nitrogenase of Streptomyces thermoautotrophicus, which is unaffected by the presence of oxygen. This is complicated by the fact the bacteria still need the presence of oxygen for proper respiration.

    Some microbes have a proteoglycan rich extra cellular matrix which traps a layer of water, often referred to as a slime layer. This slime layer acts as a barrier for oxygen. The ability of some nitrogen fixers such as azotobacteraceae to employ an oxygen-amendable nitrogenase under aerobic conditions has been attributed to a high metabolic rate, allowing oxygen reduction at the cell membrane; however, the effectiveness of this mechanism is in question.

    Many rhizobia, nitrogen fixing bacteria, live in a symbiotic relationship with plants known as legumes. They have an interesting strategy to deal with O2. In plants infected with Rhizobium, (legumes such as alfalfa or soybeans), the presence of oxygen in the root nodules would reduce the activity of the oxygen-sensitive nitrogenase. In these situations, the roots of such plants produce a protein known as leghemoglobin (also leghaemoglobin or legoglobin). Leghemoglobin buffers the concentration of free oxygen in the cytoplasm of infected plant cells to ensure the proper function of root nodules. Leghemoglobin is a nitrogen or oxygen carrier; naturally occurring oxygen and nitrogen interact similarly with this protein. Leghemoglobin buffers the concentration of free oxygen in the cytoplasm of infected plant cells to ensure the proper function of root nodules. It has close chemical and structural similarities to hemoglobin, and, like hemoglobin, is red in colour. Leghemoglobin has a high affinity for oxygen, about ten times higher than of human hemoglobin. This allows an oxygen concentration that is low enough to allow nitrogenase to function but not so high as to bind all the O2 in the bacteria, providing the bacteria with oxygen for respiration.

    Fate of ammonia:

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