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Monday, 30 January 2017

Mineral Nutrition II

Deficiency Symptoms of Essential elements

  • When supply of essential elements becomes limited, plant growth is retarded. The concentration of essential elements below which plant growth is retarded is called critical concentration.
  • In absence of any particular element shows certain morphological changes. These morphological changes are called deficiency symptoms.
  • The parts of plant that show deficiency symptoms depend upon mobility of elements in the plants. Elements that are actively mobilized (N,Mg,K) shows deficiency in older regions. On the other hand, symptoms appear first in young region if the elements are relatively immobile (Ca) and not transported out of mature tissues.
Kinds of deficiency syndrome are as follows :

Deficiency of any element may cause many symptoms or same symptoms may be caused by different elements. To identify the deficient elements various symptoms are compared with standard chart.

Toxicity of micronutrients :  in higher doses, micronutrients become toxic. Any tissue concentration which reduces dry weight of tissue by 10% is called toxic concentration. Critical toxic concentration is different for different elements.

Mechanism of absorption of elements takes place in two phases. In first phase, rapid intake of ions occurs in free space or outer space of the cells, apoplast. In second phase, ions are taken slowly into inner space, the symplast of the cells.

Passive movement of ions in apoplast occurs through ion channels and trans-membrane protein. On the other hand, movement of ions into symplast occurs by expenditure of energy by active process.

The movement of ion is called flux. The inward movement is called influx and outward movement is called efflux.

Translocation of solutes occur through xylem along with ascending stream of water

Soil as reservoir of essential elements most of the nutrients required for growth and development is obtained from soil by roots. These minerals are formed by weathering or rocks. Soil also harbor nitrogen fixing bacteria and other microbes, holds water and supplies air to roots. Deficiency of essential elements affects the crop yield so, fertilsers are used to supplement these elements.

Metabolism of Nitrogen

Nitrogen is the most prevalent elements in living world along with C, H and O. It is the main constituent of proteins, nucleic acids, fats, hormones, enzymes etc.

The process of conversion of nitrogen to ammonia is called nitrogen fixation. In nature lightening and ultraviolet radiation provide energy to convert atmospheric
nitrogen into nitrogen oxide ( N0, NO2 and N2O).

Industrial combustion, forest fire and automobiles along with thermal power plants produce nitrogen oxides.

The decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.

Ammonia is first oxidized to nitrite by bacteria Nitrosomonas or Nitrococcus. Which is further oxidized to nitrate with help of bacteria Nitrobactor. These process are called nitrification.

Nitrates formed is absorbed by plants and transported to leaves. Nitrates is converted into free nitrogen by the process called denitrificaion by bacteria Pseudomonas and Thiobacilus.

Reduction of nitrogen to ammonia by living organism is called Biological Nitrogen Fixation. The enzyme nitrogenase, present in prokaryotic organism called nitrogen fixer.

Nitrogen fixing microbes may be symbiotic (Rhizobium) or free living (Nostoc, Azotobactor, Anabaena).

Symbiotic biological nitrogen fixation includes legume-bacteria relationship in which rod shaped Rhizobium lives with symbiotic relation with nodules of Leguminous plants.

Central portion of nodule is pink or red due to presence of leguminous haemoglobin or leg-haemoglobin.

Nodule formation involves sequence of interaction between root and Rhizobium as follows: 

Rhizobia increase in number and attach with epidermis of roots. Root hairs curls and bacteria invade it. An infection thread is formed that carrying the bacteria into cortex of root.

Nodule formation starts in cortex of root. Bacteria released from thread to cells which leads to formation of specialized nitrogen fixing cells.

Nodules establish direct vascular connection with host with for exchange of nutrients.

Nodule contain all necessary biochemical components like enzyme nitrogenase and leg-haemoglobin.

Enzyme nitrogense is a Mo-Fe protein and catalyzes the conversion of atmospheric nitrogen into ammonia. The reaction is as follows:

The enzyme nitrogenase is highly sensitive to molecular oxygen and needs anaerobic condition. To protect this enzyme from oxygen, the nodules contain an oxygen scavenger called leg-haemoglobin.

The ammonia synthesized by nitrogenase enzyme require large amount of energy (18ATP) for each NH3 produced

Fat of ammonia- at physiological pH, ammonia is converted into ammonium ions (NH4+).It is toxic for plants in larger concentration and ammonium ion is converted into amino acids by two methods:

a. Reductive animation- in this process ammonia reacts with α-ketoglutaric acid to form glutamic acid

b.  Transamination: it involves the transfer of amino group from amino acids to keto group of keto acid. Glutamic acid is the main amino acid from which transfer of NH3 takes place and another amino acid is formed by transamination.

Two important amides asparagine and glutamine found in plants in proteins. They are formed from aspartic acid and glutamic acid by addition of another amino groups to it.

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