Plants and Mineral Nutrition

Chapter-10

Plants and Mineral Nutrition

Essential mineral, Macro and micronutrients Nitrogen metabolism,

Minerals are very essential even for the normal growth of plants. Their absence or shortage is marked by presence of some deficiency symptoms. Minerals are absorbed in the form of ions. As far as their absorption is concerned it may be of two types that is active absorption and passive absorption.

• Active absorption needs energy while passive absorption does not involve any type of metabolic energy. The minerals which are required in large quantities are called macro-nutrients while those which are absorbed in minute quantities are known as micro-nutrients. Following table indicates the role played by each mineral in plant body.

Mechanism of Absorption: As stated above it may be active or passive absorption.

Passive Absorption:

• (a) Absorption by diffusion: The mineral molecules move by the principle of diffusion.
• i.e. from higher concentration to lower concentration and does not involve any use of energy by root cells.
• (b) Ion exchange mechanism: The process of exchanging negative ions (anions) and positive ions (cations) in the root cells by the ions of equivalent charge from the soil is known as ion-exchange process of passive absorption. The soil particles are negatively charged and to maintain the same these attract positively charged cations from the soil solution and the negatively charged ions are left free.

Active Absorption:

The ions find their entry into the root cells to accumulate in large concentration. These are then moved to the protoxylem by plasmodesma with the help of respiratory energy. The mechanism is explained by the presence of a carrier compound. The ions form ion-carrier complex. This complex gets in through the semipermeable membrane and again breaks into the ion of the carrier. The carrier compound again comes out of the root cells, combines with the new ions and the process goes on. This involves the use of metabolic energy.

Organisms in this universe are essentially composed of carbon, hydrogen, oxygen, nitrogen and many other forms of chemical elements. Nitrogen is one of important constituents of living organisms next to carbon. In the living organisms the nitrogen is present in alkaloids, amides, amino acids, proteins, DNA, RNA, enzymes, vitamins, hormones and many other compounds as one of the essential elements. It is essential cellular requirement of living organism without it, they can not survive. Despite its importance all the organisms can not utilise the nitrogen directly from nature except few organisms like certain bacteria, blue green algae and few fungi, which have the potentiality to utilize molecular nitrogen directly.

                              

• Nitrogen cannot be used directly, except either through nitrate reduction or Nz fixation by bacteria and blue-green algae convert it to Nitrites, Nitrates and Ammonia. The ammonia and urea present in the soil are directly absorbed by plants. This form of nitrogen is biosynthesized to amino acid via the glutamine synthetase (GS) reaction. Thus, the formed amino acids in roots and leaves accumulate in proteins. The main, if not sole function of some proteins is to provide a store of amino acids

Signification of Nitrogen in the Plant

• Nitrogen (N) is an important component of DNA and proteins, the building block of every life. As we are aware, that more than 70% of atmosphere is contributed by elemental nitrogen, but the atmospheric elemental nitrogen due to their strong triple bonds between the nitrogen molecule (N/) became relatively inert and cannot be used directly by plants and animals. The atmospheric nitrogen is converted to organic nitrogen, ammonium (NH* 4), nitrite (NO" 2), nitrate (NO" 3), nitrous Oxide (N2O), nitriC Oxide (NO) or inorganic nitrogen pas (N2) Many of these processes are carried out by microbes, either in their effort to harvest enerpy or to accumulate nitrogen in a form needed for their growth. For example, the used. Organic nitrogen may be in the form of a living organism, humus or in the intermediate products of organic matter decomposition. The processes of the nitrogen cycle is biogeochemical cycle that describes the transformation of nitrogen and nitrogen containing compounds to another form. The diagram shows how these processes fit together to form the nitrogen cycle.

• In the soil, the dead and decaying organic matter is source of nitrogen. In the organic matter, cycling of nitrogen from amino acid, purnies and pyrimidines to ammonia occurs. 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.

Sources of Nitrogen

• Lightening (where nitrogen get oxidized and produces acid rain)

• Inorganic fertilizers,

• Nitrogen fixation

•  Animal residue

•  Crop residue

• Organic fertilizers

Forms of Nitrogen:

Different forms of nitrogen found in the environment are urea, ammonia, ammonium, oxidized nitrogen such as nitrate and nitrite, atmospheric dinitrogen (Nz) is widely distributed.

Role of Nitrogen:

• The plants and bacteria uses nitrogen in the form of ammonium or nitrate.

• It serves as an electron acceptor in anaerobic environment specifically in anaerobic micro-organism.

• It is also most limiting nutrient in soil and water.

• Nitrogen is key element for synthesis of amino acids, nucleic acids, (purines and pyrimedine) and cell wall components for bacteria (N-acetyl muramic acid/ NAM).

• Deficiency of nitrogen causes chlorosis in the leaves of green plants and the cell division as well as cell elongation is also stopped, due to which plant remain dwarf and production of flowers, fruits and seeds are also decreased.

Nitrogen cycle

The nitrogen cycle includes five stages:

• Nitrogen fixation

• Ammonification

• Nitrification

• Nitrate assimilation and Denitrification

1. Ammonification:

• In this stage, the nitrogen present in organic form converts to ammonium ion with the help of microbes present in the soil. The animal excreta, dead and decaying plants and animals are the sources of organic nitrogen in the soil. The ammonifying saprotrophic bacteria such as Bacillus ramosus, Bacillus vulgaris, certain soil fungi and actinomycetes are responsible for ammonification.

Nitrification

In the warm moist neutral soil, 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. The bacteria involved in this process are known as chemoautotrophs.

Nitrate Assimilation:

The plants through roots absorb NO3 ions, but plants cannot utilize these nitrite. So, through series of reaction and presence of nitrate reductase convert the nitrate to ammonia. This reduction of Nitrate of Ammonia and its incorporation into cellular proteins by aerobic micro-organisms and higher plants is called nitrate assimilation.

Denitrification:

The process of conversion of nitrate and nitrite into ammonia, nitrogen gas and nitrous oxide (NGO) is called denitrification. This process ends with the release of gaseous nitrogen into the atmosphere and thus completes the nitrogen cycle. A number of bacteria such as Pseudomonas dentifrices, Bacillus subtilis and Thiobacillus dentifrices are involved in this process.

Nitrogen Fixation:

The nitrogen which makes up 75% of air in environment, can not be used directly by plants to synthesize essential biological nitrogen containing compounds, such as amino acids and nucleic acid. The process of conversion of molecular nitrogen to plant usable nitrogen i.e. ammonia, nitrate and nitrite is known as nitrogen fixation. There are three nitrogen fixation processes that are:

• (a) Biological nitrogen fixation

• (b) Non-biological nitrogen fixation

• (i) Atmospheric nitrogen fixation

• (ii) Industrial nitrogen fixation

Biological nitrogen fixation

In this type of fixation, the molecular nitrogen is converted by microbes to usable nitrogen form. These microbes either fix the nitrogen non-symbiotically by living freely in the soil or fix the nitrogen symbiotically by living in the cells of higher plants, which later are absorbed by the plant.

• (i) Non-symbiotic Nitrogen Fixation: Free living/ non-symbiotic nitrogen fixation is carried out by free living nitrogen fixing bacteria, yeast and algae which live in soil are Bacteria such as Azotobacter, Closterium, Aerobacter, Rhodospirillum, Rhodopseudomonas, Chloronium, Bacillus polymyxin; Yeast such as soil yeast; Algae such as nostoc, anabaena, Calothrix, cylindriospermum, and polypterid

Some of the bacteria present in the soil have ability of fixing the nitrogen from the air and soil and convert them to nitrates and then amino acid, protein for them. After the death of such bacteria, the other nitrifying bacteria again transform them to usable form of nitrogen i.e. nitrate and absorbed by green plant. The capacity of nitrifying bacteria mentioned above have less efficiency of nitrogen as fixation compared with symbiotic bacteria.

• (i) Symbiotic Nitrogen Fixation: Since long time it is known that Leguminosae plants have ability to utilize free nitrogen (Nz) of the soil and air for their growth. Later, in 1887, it was found that root of these plant possess swelling called nodules or tubercles, which is infection of specific bacteria i.e. Rhizobium legumin Sarum species which is responsible for the nitrogen fixation.

Rhizobium leguminosarum species is symbiotic aerobic bacteria and helps in nitrogen fixation in anaerobic condition. As it secretes nitrogenase enzyme which is molybdenum iron protein which only shows enzymatic potential in anaerobic condition

Nodule formation: Infection of bacteria takes place through the root hair. Infection takes place through thread of bacteria and enters in to the cortical cells of roots, this results into the curling of the root hair. Some of the bacteria as bacteroids initiate the cell division in cortex, results into generation of compact mass of cells which leads to formation of root nodules; while some of bacterial cells act as nitrogen fixing bacteria which secrete nitrogenase enzyme in anaerobic conditions of root nodules.

• Non-Biological nitrogen fixation

(i) Atmospheric Nitrogen fixation: Due to thunderstorm the atmospheric nitrogen combines with oxygen to form nitric oxide (NO), which finally converts to nitrogen oxide (NOz). This react with water of the rain in the presence of oxygen to form nitric acid. The nitric acid comes to the earth, here it reacts with basic substances present in the soil and form nitrate which is the life source of the plant in the soil. Thus, the molecular nitrogen converts to usable nitrogen form.

(ii) Industrial nitrogen fixation: In this fixation, the molecular nitrogen is converted to usable form of nitrogen by means of reaction in the industry. Here, the hydrogen and nitrogen are made to react and form ammonia, later this ammonia is reacts with carbon dioxide to form urea. This urea is used as fertilizers for plants. Urea in the soil dissociates to form ammonium ions.