MINERAL NUTRITION

SubtopicsMinerals and its importance

Essential Minerals- Classification

Minerals- Roles and properties

Absorption and Translocation

Metabolism of Nitrogen

Introduction• Living organism- Macromolecules (Carbohydrates,

proteins & fat), water and minerals for growth and

development.

• Def.- A mineral is a chemical element which naturally

occurs as inorganic nutrients in the food and soil, and

are essential for the proper functioning of the plant and

animal body.

• Other than carbon, hydrogen, oxygen & sulphur- organic

molecules

Methods to study Mineral requirements of Plants• Julius von Sachs, German botanist in

1860- that plants could be grown to

maturity in a defined nutrient solution in

complete absence of soil- hydroponics.

• Method involves growing plants in purified

water & specific mineral nutrient salts-

nutrient solution is aerated for optimum

growth

• Concentration required is determined-

adding/removing mineral solution- identify

essential elements & deficiency

• Hydroponics- commercial production of vegetables such as tomato, seedless cucumber and lettuce

Criteria for Essentiality of minerals• Minerals present in soil enters plant- roots

• Criteria for essentiality of mineral elements:

a) The element must be absolutely necessary for supporting normal

growth and reproduction. In the absence of the element the plants do

not complete their life cycle or set the seeds.

b) The requirement of the element must be specific and not replaceable

by another element. In other words, deficiency of any one element

cannot be met by supplying some other element.

c) The element must be directly involved in the metabolism of the plant.

• Based on quantitative requirement by plants:1) Macronutrients2) Micronutrients

MACRONUTIENTS• Large amounts in plant tissues (in excess of 10 mmole Kg-1 of dry

matter)• Carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium,

calcium and magnesium- CO2, H2O & soil

MICRONUTRIENTS• Trace elements, less than 10 mmole Kg-1 of dry matter• iron, manganese, copper, molybdenum, zinc, boron, chlorine and nickel

• Higher plants- sodium, silicon, cobalt and selenium

Classification of Elements based on function

• Based on diverse function:1. Structural Elements: Components of biomolecules

(Carbohydrates, proteins, lipids & nucleic acid). Eg. C, H, O & N2. Energy related chemical compounds: Provide energy to plants.

Eg. Mg in chlorophyll & Phosphorus in ATP3. Elements activate & inhibit enzymes: Activates or inhibits

enzymes during metabolism. Eg. Mg2+ - activator of ribulose bisphosphate carboxylaseoxygenase & phosphoenol pyruvate carboxylase-photosynthetic carbon fixation; Zn2+ - an activator of alcohol dehydrogenase & Mo of nitrogenase- nitrogen metabolism.

4. Elements altering water potential: Alters osmotic potential of cell. Eg. K- opening and closing of Stomata; regulates water potential of cells

Toxicity of Micronutrients• Micronutrients- low

amounts• Decrease- deficiency,

moderate increase- toxicity• Any mineral ion

concentration in tissues that reduces the dry weight of tissues by about 10 per cent is considered toxic.

• Critical concentration- vary & toxicity level vary with different plants

• Excess micronutrients- Toxicity Eg. Manganese toxicity

Manganese Toxicity• Brown spots around chlorotic

veinsMODE OF TOXICITY:a) Manganese competes with

iron and magnesium for uptake

b) Magnesium for binding with enzymes

c) Inhibit calcium translocation in shoot apex.

RESULT:• Excess of manganese- Induce

deficiencies of iron, magnesium and calcium.

Absorption of Elements• Studies carried on cells/tissues/organs- occurs in 2 phase:1. First phase- Rapid uptake of ions- ‘free space/ outer space’-

the apoplast; passive; occurs through ion-channels, the trans-membrane proteins which functions as selective pores.

2. Second phase- Ions are taken slowly- inner space- the symplast; entry and exit of ions require metabolic energy

• Inward movement- Influx & Outer movement- Efflux

Translocation of Solutes• Occurs through- xylem along with the ascending stream of water,

which is pulled up through the plant by transpirational pull.• Xylem sap- mineral salts

Soil as reservoir of Essential elements

• Nutrients essential for growth & development- weathering & breakdown of rocks

• Enrich soil with dissolved ions & inorganic salts• Since nutrients derived from rock minerals so their role-

mineral nutrition• Other function of soil- harbour nitrogen- fixing bacteria,

microbes, holds water, supplies air to the roots & act as a matrix that stabilises the plant

• Deficiency of macro- nutrients (N, P, K, S, etc.) & micro- nutrients (Cu, Zn, Fe, Mn, etc.)- supplied through fertilizers as per need

Metabolism of Nitrogen• Nitrogen- macronutrient; constituent of -amino acids, proteins,

hormones, chlorophylls & vitamins

• Plant gets N through soil (limited) & from air (atmospheric N2)

• Plants have to compete with microbes for limited nitrogen present

in soil

• Metabolism of Nitrogen:

1. Nitrogen Cycle

2. Biological Nitrogen fixation

Nitrogen Cycle• Def.- A continuous series of natural processes by which nitrogen

passes successively from air to soil to organisms and back to air or soil involving principally nitrogen fixation, nitrification, decay, and denitrification

• The process of nitrogen cycle:i. Nitrogen fixationii. Ammonificationiii. Nitrificationiv. Denitrification

Nitrogen Fixation:• The process of conversion of nitrogen (N2) into ammonia (NH3)• Lightning, ultraviolet radiations converts free nitrogen to nitrogen

oxides (NO, NO2, N2O); Industrial combustion, forest fires, automobile exhausts and power-generating stations- sources of atmospheric nitrogen oxides.

• Nitrogen oxides converts to Ammonia N2 Nitrogen oxides NH3

Ammonification:• The process of decomposition of organic nitrogen of plants and

animals into ammonia (NH3)• Ammonia volatilises and re-enters the atmosphere but most of it

is converted into nitrate (NO2-)

Nitrification:• Ammonia is oxidised to nitrite (NO2

-) by the bacteria Nitrosomonas / Nitrococcus.

• The nitrite is further oxidised to nitrate (NO3-) with the help of the

bacterium Nitrobacter. • These nitrifying bacteria are chemoautotrophs.

• Nitrate- absorbed by plants & transported to leaves where nitrate reduce to form ammonia (NH3) & forms amine group of amino acids

Denitrification:• Nitrate present in the soil reduce to nitrogen by the process of

denitrification. Denitrification is carried by bacteria Pseudomonas and Thiobacillus.

Biological Nitrogen Fixation• Reduction of nitrogen to ammonia by living organisms is called

Biological Nitrogen Fixation (N2 NH3)• Certain prokaryotes (bacteria) fixes nitrogen – enzyme

nitrogenase & called N2 fixers

• Nitrogen-fixing microbes can be classified as follows:– Free living : Aerobic (Azotobacter, Beijernickia ), Anaerobic

(Rhodospirillum), Cyanobacteria (Nostoc, Anabaena), Bacillus.

– Symbiotic – with leguminous plants (Rhizobium), with non-leguminous plants (Frankia).

Symbiotic Nitrogen Fixation:• Commonly seen in legume-bacteria relationship.• Bacteria Rhizobium (rod- shaped) forms nodules at root in

legumes• Nodules- small outgrowths on roots, central portion of nodule is

pink- leguminous haemoglobin or leg-haemoglobin• Microbe, Frankia produces nitrogen-fixing nodules on the roots of

non-leguminous plants (e.g., Alnus). • Rhizobium and Frankia are free living in soil, but as symbionts,

can fix atmospheric nitrogen.

Nodule Formation:• Interaction between Rhizobium & roots of host plants• Step involved includes:1. Multiplication of Rhizobia & colonization of it around roots2. Attachment of bacteria to epidermal & root hair cells3. Root hair curls & bacteria invade root hair4. An infection thread is produced- carries bacteria to cortex5. Initiation of nodule formation in the cortex6. Release of bacteria from the thread into the cells which leads

to the differentiation of specialised nitrogen fixing cells. 7. The nodule thus formed, establishes a direct vascular

connection with the host for exchange of nutrients.

• Root nodule- contains necessary biochemical components such as the enzyme nitrogenase and leghaemoglobin.

• The enzyme nitrogenase is a Mo-Fe protein and catalyses the conversion of atmospheric nitrogen to ammonia

• Nitogenase Enzyme:• Highly sensitive to molecular oxygen, anaerobic condition• Nodule- ensures enzyme is protected from oxygen due presence

of leg- haemoglobin (oxygen scavenger)• Rhizobium- live as aerobes under free living condition

(Nitogenase- not operational) but during nitrogen fixing- anaerobic (to protect nitrogenase)

• For enzyme to catalyse reaction- 8 ATP for each NH3 produced• Energy required is obtained through respiration of host cells

Fate of Ammonia:• Ammonia (toxic) once formed is protonated to form NH4

+ (ammonium) ion

• Nitrate assimilate in most plants• Ammonia (NH4

+) used to synthesise amino acids in plants:1. Reductive amination- In these processes, ammonia reacts with α-

ketoglutaric acid and forms glutamic acid

2. Transamination

Transfer of amino group from one amino acid to the keto group of a keto acid- Transaminase catalyses all such reactions.Eg- Asparagine  and glutamine - aspartic acid and glutamic acid (asparagine synthetase  and glutamine synthetase.)