Download ch_12 - WordPress.com

Chapter-12
Mineral Nutrition




Technique of growing plants in a nutrient solution (complete absence of soil) is called as
hydroponics. It was discovered by Von Sachs (1860).
Hydroponic technique is employed to find out the elements essential for plant growth, ascertain
deficiency symptom and nowadays for commercial production of crops like tomato, seedless
cucumber etc.
Depending upon the quantity in which an essential element is required by the plants, nutrients
may be Macronutrient (required in amount more than 10 m mole/kg of dry matter).
 Micronutrient or trace element (required in amount less than 10 m mole/kg of dry
matter).
On the basis of function, essential elements may be Structural element (as components of biomolecules) like-C,H,O,N
 Components of energy related compounds like Mg in chlorophyll and phosphorus in ATP
 Activator or inhibitor of enzymes like Mo activator of nitrogenase, Zn+2 of alcohal
dehydrogenase.
 Alter the osmotic potential of a cell like Potassium .
Category
of
Nutrients
Name of
element
1-Nitrogen
Macro
Nutrients
2-Phosphorus
Form in
which
absorbed by
plants
NO3-, NO2- or
NH4+
H2PO4or
HPO42-
3-Potassium
K+
4-Calcium
Ca2+
5-Magnesium
Mg2+
6-Sulphur
SO42-
Functions/ Role played
Major constituent of Proteins, nucleic acids, vitamins
and hormones.
Constituent of cell membranes, nucleic acids,
nucleotides & required for all phosphorylation
reactions.
Maintains anion-cation balance in cells, involved in
protein synthesis, opening and closing of stomata,
turgidity of cells.
Required for permeability of cell membrane,
formation of mitotic spindle, formation of middle
lamella.
Constituent of chlorophyll, maintains structure of
ribosome, activates enzymes of respiration &
photosynthesis, involved in DNA & RNA synthesis .
Constituent of amino acid (Methionine & Cysteine),
several Co-enzymes, ferredoxin & Vitamins (Biotin,
Thiamine & CoA).
79
Micro
Nutrients
3+
1-Iron
Fe
2-Manganese
Mn2+
3-Zinc
Zn2+
4-Copper
Cu2+
5-Boron
Bo33- B4O72-
6-Molybdenum
MoO22-
7-Chlorine
Cl-
Constituent of proteins involved in electron transfer
(ferredoxin, cytochromes), activates catalase enzyme
and essential for formation of chlorophyll.
Necessary for photolysis of water in photosynthesis,
activates enzymes involved in photosynthesis,
respiration & nitrogen metabolism.
Required for synthesis of auxin, activates carboxylases
Required for over all metabolism, associated with
enzymes involved in redox reactions.
Required for uptake & utilization of Ca+2, membrane
functioning, pollen germination, cell elongation, cell
differentiation and carbohydrate translocation.
Component of nitrogenase and nitrate reductase
enzymes.
Anion-Cation balance in cell, necessary for photolysis
of water in photosynthesis.
Deficiency Symptoms Common deficiency symptoms are Chlorosis (loss of chlorophyll)- yellowing of leaves caused due to deficiency of N,K, Mg, S, Fe,
Mn, Zn & Mo.
 Necrosis (Death of tissues)-caused due to deficiency of Ca, Mg, Cu, K.
 Inhibition of cell division- caused due to deficiency of K, N, & Mo.
 Delay in flowering-caused due to deficiency in N, S, and Mo.
Micronutrients can be toxic
 Mineral ion Concn. in tissues that reduces the
dry weight of tissues by about 10% is called as
toxic.
 Moderate decrease in micronutrients causes
deficiency symptoms and moderate increase
causes toxicity.
 It may be that excess of an element (toxicity)
may inhibit the uptake of another element.
 The Concn. of the essential element below which plant
growth is retarded is called as Critical Concn.
 Morphological changes that are observed due to
deficiency (below Critical Concns.) of a particular
element are called as deficiency symptoms.
 Deficiency symptoms of some elements like S, Ca, etc.
first appear in young tissues, while other elements like N,
K, Mg. in older tissues. Because those elements which
are actively mobile can be transported from senescing
organs to younger parts and show deficiency symptoms
first in older parts, while those which are immobile
shows symptoms first in younger parts.
80
Nitrogen Cycle
Absorption and translocation of Nutrients-
NH3
 Soil acts as reservoir for many nutrients and plants
absorb them through roots.
 Absorption of elements occur in two parts-in first part
rapid uptake of ions in free space or outer space of cell
through passive process and in second part ions are
taken slowly into inner space of cell.
 Movement of ions is called as flux, which may be influx
(inward movement) or efflux (outward movement).
 Absorbed mineral salts are translocated through xylem
with ascending sap by means of transpiration pull.
Electrical N2fixation
Industrial N2 Fixation
Biological N2-fixation
Atmospheric N2
NO2-
NO3-
Soil Nitrogen pool.
Decaying
biomass
Plant
biomass
Animal biomass

Process of conversion of molecular nitrogen (N2) into compounds of nitrogen is called as nitrogen
fixation.
Nitrogen fixation may be
Industrial N2-fixation like forest fires, Electrical N2 -fixation
Automobile, Exhaust, Haber’s process Conversion of N2 into No, NO2, N2O
of NH3 synthesis etc.
by lightning and UV- radiations.


Biological N2 -fixation
Carried out by living beings
Like-bacteria, BGA (Blue,
Green Algae) etc.
Symbiotic N2 fixationMost common symbiotic association observed is between Rhizobium bacteria and roots of
leguminous plants (Gram, Pea etc.)
Small outgrowths called as nodules are formed on roots which, act as site of N2- fixation.
81
Steps of Nodule Formation
Rhizobium in soil interacts with roots of leguminous plants.
Rhizobia multiply, colonise and get attached to epidermal
& root hair cells.
Curling of root hair & invasion of bacteria into root hair.
Produce infection thread & carry bacteria into root cortex.
 Ammonification is conversion of organic
nitrogen (dead plants & animals) into
ammonia.
 Nitrification is conversion of ammonia into
Nitrate (oxidation of NH3 into NO3-). It is
carried out in two steps by Nitrifying bacteria
(chemoautotrophs).
 NH3
NO2—(by Nitrosomonas, Nitrosococcus
Nitrite
bacteria)
 NO2-
NO3— (by
Nitrate
Nitrobacter)
Initiate nodule formation in the cortex.
Mechanism of Nitrogen fixation
 An enzyme Nitrogenase (Mo-Fe protein complex) catalyses the conversion of N2 into NH3 in root
nodules.
 Enzyme Nitrogenase is highly sensitive to molecular oxygen and requires anaerobic condition to
function.
 Anaerobic condition in root nodules is provided by a pink coloured pigment called as leg
haemoglobin which acts as oxygen scavenger by binding with O2. Leg-haemoglobin is product of
interaction between Rhizobium & leguminous plant. Either of two cannot have it alone.
Nitrite
2H+
Binding of N2
2H+
2H+
Fate of AmmoniaNH3 soon get
converted into NH4+, as NH3 is
toxic to plants.
 NH4+ undergoes reductive
amination, in which it reacts
with α-ketoglutaric acid to
from glutamic acid (amino
acid).
 Then by transfer of amino
group form one amino acid
(glutamic acid) to the keto
group of a keto acid, other
amino acids are produced and
this process is called as
transamination catalysed by
an enzyme transaminase.
82