Mainly deals with physical functions of nutrient elements including deficiency and toxicity symptoms (item 3 of course outline)
Overview
Mineral nutrient have specific and
essential functions in plant metabolism
Main concentration will be on N, P, K, Mg
and Ca and selected micronutrients (Fe, Cu, Zn,Mn, Mo and B)
1. 0
NITROGEN
Most limiting nutrient element
1.1
N –Assimilation
·
NO3- and
NH4+ are the major sources of inorganic N taken up by
roots of higher plants
·
Most of the NH4+ has
to be incorporated into organic compounds in the roots
·
NO3- is mobile in the xyltem
and can be stored in the vacuoles of roots, shoots and storage organs
In order to be incorporated into organic
structures and fulfill its essential functions as plant nutrient it has to be
reduced to ammonia
1.1.1 Nitrate
assimilation and reduction
Path way for reduction process is
Te process is mediated by two separate
enzymes: nitrate reductase which reduces nitrate to nitrite
Nitrite reductase reduces nitrite to ammonia,
Nitrate reductase
Complex enzyme:
Ø high molecular weight
Ø contains several prothetic groups including Mo and cytochrome
Ø nitrate reductase activity can be inhibited and completely suppressed by NH4+
presence
Nitrite reductase
Ø Converts nitrate to ammonia without release of free
intermediates
Ø Has low molecular weight
Ø It is based in the chloroplast
In most plant spices both roots and shoots
are capable of nitrate reduction
1.1.2
Assimilation of ammonium
Whereas NO3- can be stored
in vacuoles without detrimental effect.
Ammonium and particularly its equilibrium partner ammonia are toxic even
at very low concentrations
Formation of amino acids, amides and
related compounds is the main pathway of detoxification of either NH4+
taken up by roots or ammonia derived from NO3- reduction
Key enzymes involved in ammonia assimilation
are glutamine synthetase and glutamate synthase
In this pathway the amino acids glutamate
acts as the acceptor for ammonia and the amide glutamine is formed
Glutamine synthetase has a very high
affinity to ammonia, and therefore capable of incorporating NH3 even
at very low concentration
The other enzyme in ammonia assimilation
glutamate synthase catalyses the transfer of the amide group (–NH2)
from glutamine to 2-oxoglutarate
The organically bond N in glutamate and glutamine
can be utilized for synthesis of other
amides as well as amino acids of high molecular weight compounds such as
proteins
1.2 ROLES
OF N
1) Amino acids and protein biosynthesis
·
Synthesis of amino acids, ureides and high molecular weight compounds such as proteins
·
Proteins are polypeptides
formed from more than 100 individual amino acids and their sequence is
determined by DNA molecules. Expression of the genetic information involves the
production of mRNA and t-RNA molecules
2) Role of low molecular weight organic N
compounds
·
These are amino acids peptides,
amides amines and ureides
These act as intermediates between the
assimilation of inorganic N and synthesis of high molecular weights compounds
Reading assignments
Deficiency and toxicity symptoms of N
2. PHOSPHORUS
General
·
Required in large amounts
·
Total concentration in
agricultural crops is 01-0.5%
·
P enters the plant through root
hairs and the outer most parts
·
P uptake is also facilitated by
mycorrhizal fungi that grows in association with the roots
·
Unlike NO3-
and sulphate, phosphate is not reduced in plants but remains in its highest
oxidized form
·
After uptake as H2PO4-
either it remains as inorganic phosphate(Pi) OR it is esterified through a
hydroxyl group to a carbon chain(C-O-P) as a simple phosphate ester OR attached
to another phosphate by the energy rich pyrophosphate bond (P)-(P)(eg in ATP)
ROLES OF PHOSPHORUS
1. As a structural element
Ø It is a constituent of macromolecular structures, example in
nucleic acids – as units of DNA as units of RNA- these are structures
responsible for the translation of genetic information
Ø In both DNA and RNA phosphate forms a bridge between
ribonucleoside units to form macromolecules
Ø Others are phosphoproteins, phospholipids enzymes and ATP
Ø P is also a component of phytin, a major storage of P in seeds.
About 50% of the total P in cereal
grains is stored as phytin
3. Energy transfer
Ø P plays a vital role in every plant process that involves energy
transfer
Ø Phosphate esters (C-P) and energy rich phosphates (P)-(P)
represent the metabolic machinery of the cells
Ø Energy liberated during glycolysis, respiration and
photosynthesis is utilized for the synthesis of energy rich pyrophosphate bond
and upon hydrolysis of this bond about 30KJ per mole ATP are released
3.
Regulatory role of inorganic phosphate
Ø Inorganic phosphate (Pi) has various essential functions in the
metabolic pool
Ø
ATP ADP
+Pi
Ø Also Pi controls some key enzyme reactions eg. In fruit tissue
of tomato Pi released from the vacuoles into cytoplasm can stimulate
phosphofructokinase activity – important in fruit ripening
Ø In photosynthesis and carbohydrate metabolism low external concentration of Pi depresses the
rate of starch synthesis in isolated chloroplasts
Ø At higher (Pi) concentration the total C fixation is also
depressed
4.
Phosphorus fractions and the role of phytate
Ø The amount of P being supplied to a plant affects the various P
fractions in a typical manner
Ø With an increase in supply of P from sub optimal level all P
fractions increase. Above this level only Pi increases reflecting the storage
nature
3. POTASSIUM
General
·
K is a univalent cation
·
Characterized by high mobility
in plants at all levels (within individual cells, within tissues etc)
·
K is the most abundant cation
in making a major contribution to the osmotic potential of cells
ROLES OF POTASSIUM
1) Enzyme
activation
·
The most important function of
K+ is probably the activation of various enzyme systems
·
Over 60 different enzymes
require univalent cations for activation out of which about 50 enzymes either
completely depend on or are stimulated by K+
·
The activation is by inducing
conformational changes in the enzyme protein
·
K+ increase the rate
of catalytic reactions and some cases the affinity of the substrate
Examples
i.
Starch synthase
Catalyses
the transfer of glucose to starch molecules
In the
event of K Deficient plants the following chemical changes may occur:
·
Accumulation of soluble
carbohydrates
·
Decreased level of starch
·
Accumulation by soluble N
compounds
11. Activation
of membrane bound ATPase
This
requires Mg2+ but are further stimulated by K+
2)
Photosynthesis roles of K+
·
Dominant counter ion to light
induced H+ flux across thylakoid membrane
·
Establishment of transmembrane
PH gradient necessary for synthesis of ATP
Roles of K+ in CO2
fixation
CO2fixation
increases as K+ increases
Parameters
of Co2 exchange are closely correlated, example; increase in leaf K+
content is accompanied by:
Ø Increasing rate of photosynthesis, RUBP Carboxylase activity
Ø Decrease in dark respiration
Increasing
respiration rates are common feature of K+ deficiency
3)
Phloem transport
·
Sieve tubes have high
concentration of K+ which are likely related to the mechanism of phloem loading
of sucrose, K+ in the sieve tubes makes a vital contribution to the osmotic
pressure and therefore transport of products of photosynthesis from the source
to the sink
·
On legumes an adequate K+
supply results in greater supply of sugar in the root nodules
4) Protein synthesis
·
K+ requirements in protein
synthesis is now established in higher plants
·
It is possible that K+
is involved in several steps in the translation process including the binding
of t-RNA to ribosomes eg. In C3 species the majority of the chloroplast is RUBP
carboxylasse where this enzyme is inhibited when K+ is deficient
·
The role of K+ in protein
synthesis is reflected in the accumulation of soluble N Compounds ( eg. Amino
acids, amides etc) in the K+ deficient plants
ii.
Cation- anion balance
·
Charge neutrality must be
achieved in the plant metabolic activities
·
K+ is the
predominant cation in counter- balancing immobile anion in the cytoplasm and
mobile anions in the xylem and phloem thus the statement “K+ is
cation for anions”
·
Role of K+ is also
reflected in the NO3- metabolism
4. CALCIUM
Calcium
which is taken up as Ca2+ is unusual, it is considered as
macronutrient and as micronutrient, BASICALY because there is a wide range of
Ca in different parts of plant
Why macronutrient?
Required
in high amount in cell wall, outer face of plasma membrane and a vacuoles
Why micronutrients?
It is in
very low concentration in the cytoplasm
ROLES OF CALCIUM
I.
Cell wall stabilization
·
Basically due to its binding
property
·
Ca2+ cross – links polysaccharides
in the cell wall
·
Highest proportion of ca2+ in
plant tissue is located in cell wall followed by vacuoles. High concentration
of Ca2+ are found in middle lamella on the external
surface of plasma membrane. The major role is to strengthen these tissues.
II.
Membrane integrity and enzyme
modulation
·
Ca is needed in outer surface
of membranes (eg.PM) without which the cell wall will leak
·
Ca2+ role in the stabilization
of cell membrane is by bridging phosphate and carboxylate groups of
phospholipids and proteins at the membrane surface
Note:
other divalent cation eg Mg2+ cannot substitute Ca2+
role for this function
In case of
Ca deficiency there is increased leakage of low molecular weight solutes from
cells. In case of severe deficiency:
-
General disintegration of
membrane structure
-
Loss of cell compartmentation that is cannot distinguish compartments eg vacuole, cytoplasm
Ca2+
can also increase the activity of a few enzymes especially membrane bound
enzymes eg. ATPase
III.
Cell extension
Ca2+
is crucial for cell extension in plants
Low supply
or absence of exogenous Ca2+ will lead to root extension
cessation within a few hours. This is probably due to the fact that Ca2+
is essential in counter balancing the harmful effects of high concentration of
other ions at the plasma membrane.
The
cessation of root growth in the absence of exogenous Ca2+
is also due to inhibition of cell extension
Although
the role of Ca2+ in cell extension is not clear it is
probably required for the incorporation of materials into the cell wall
Pollen
tube growth also depends on the presence of Ca2+ in the
growth media and the direction of growth of the pollen tube is
chemotropically controlled by extracellular
Ca2+ gradient in which case the ca2+ level is highest in
the growing pollen tip
IV.
Cation- anion balance and
osmoregulation
Large proportion
(leaves in particular) of Ca2+is in the vacuole where it
contributes to the cation- anion balance by acting as a counter ion for
inorganic and organic anions eg. Formation of calcium oxalate in the vacuoles
helps to maintain a low level of free ca2+ in the cytoplasm and chloroplast.
This is also important in osmoregulation as calcium oxalate is sprangly soluble
hence provides a means of salt accumulation in vacuoles without increasing
osmotic pressure
V.
Enzyme activation
Ca
increases activity of only a few enzyme compared to K+. These
include amylase, phospholipases and ATPase
In general
Ca stimulates membrane bound enzymes
5. MAGNESIUM
Functions
related to its availability within the cells and capacity to interact with strongly
nucleophilic ligands eg phosphoryl groups
through ionic bonding
Ability to
act as a binding element and / or form complexes
Most bonds
involving Mg are ionic
ROLES OF MAGNESIUM
I.
Chlorophyll synthesis and cellular
Ph control
- Central
atom of chlorophyll molecule
When the
supply of Mg2+ is optimal 10-20% of the total Mg2+ of leaves is localised in the
chloroplasts
II.
Protein synthesis
Serves as
a bridging element for aggregation of ribosome subunits, This process is
necessary for protein synthesis
In case of
Mg deficiency the subunits dissociate and protein synthesis ceases
Mg is also
required for RNA polymerases synthesis
III.
Enzyme activation and energy
transfer
-Mg2+
catalyses a long list of enzymes
-Can be
categorized by the type of reactions to which they conform the transfer of phosphate eg. ATPase OR
carboxyl group
Many enzymes
require or are promoted by Mg2+
Examples;
Transfer
of phosphate ( eg. Phosphotases and ATPases)
The
substrate for most ATPases is Mg.ATP
In the
case of ATP Mg.ATP complex is formed
Another
key reaction of Mg2+ is the modulation of RUBP carboxylase in the stroma of
chloroplasts
MICRONUTRIENTS
An overview
A
micronutrient is an element that plants must have to complete their life cycle
but only a small amount is required
The
potency Of micronutrient is impressive
Yields of
most crops have been found to be limited by micronutrients
There is
increasing research interest on micronutrients
There is
still a room for more discovery of other micros
GENERAL ROLES OF MICROS
Micronutrient
cannot serve as building blocks of major plant components, the amounts involved
are quite inadequate for such a role
Micros
function in the enzymes systems of plants
-
Anion forming micros eg.B and
Mo can be part of the structure of the enzyme molecule
-
Cation forming micros eg. Cu2+,
Zn2+ are more likely to serve as co-enzymes, that is they
activate enzymes but they are not an integral part of the molecules
Some
micros eg. Fe, Cu, and Mn function in oxidation reduction processes
1. IRON (Fe)
Chelates
of Fe (iii) and occasionally of Fe(ii) are the forms of soluble Fe in soil and
nutrient solutions
As a rule,
Fe (ii) is the form taken up. Iron (iii) therefore has to be reduced at the
root before transportation into the cytoplasm
The
formation of coordination of chelates and its action as a reversible oxidation
–reduction system
FUNCTIONS OF IRON
Fe
containing constituents of redox system;
There are
two groups of well defined iron containing proteins
1)
Hemoproteins
2)
Iron sulfur proteins
Hemoproteins
Well known
hemoproteins are cytochromes which are constituents of the redox system in
chloroplasts and mitochondria
Iron sulfur proteins
They are non
heme proteins
Fe is coordinated
to the thiol group of inorganic S
The most
prominent one is ferradoxin which acts as an electron transmitter
Other iron containing enzymes
They are
either metal components in redox
reactions OR as a bridging element between enzyme and substrate
Under
conditions of iron deficiency riboflavin accumulates in the roots of many dicotyledons
Deficiency and Fe toxicity
Reading assignment
2. MANGANESE
Adsorbed
mainly as Mn2+ and translocated predominantly as a
divalent cation in the xylem from the roots
Manganese
has the lowest complex stability compounds thus forms the weakest bonds
It can
therefore replace Mg2+ in many reactions eg. In its role
as a bridge between ATP and enzyme complexes eg, Phosphokinases
FUNCTIONS OF MANGANESE
1. Photosynthesis and oxygen evolution
-
In Mn deficient plants low level
of phosphothetic activity eg. In Algae
-
oxygen evolving system
2. Mn
containing enzymes
-
Very few Mn containing enzymes
have been isolated
-
In leaf extracts of peas a superoxide dismutase
(SOD) with one atom of Mn per enzyme molecule has been isolated
3.
Modulation of enzyme activities
In vitro
reactions Mn can replace Mg and in some cases is even more effective in enzyme
activation example dehydrogenase
4.
Synthesis of proteins and lipids
-
Mn is structural constituent of
ribosomes
-
Mn activat RNA polymers
-
Mn distinct changes are
observed in lipid content and composition of Mn deficient plants
5.Cell
division and extension
-All
affected in Mn deficiency plants
3. COPPER
- Cu2+
is strongly bond in soils to humic and fulvic acids forming copper organic
matter complexes
- Conflicting
reports as to whether Cu is taken as Cu2+ or as cu
chelate
Most of
the functions of Cu as a plant nutrient are based on the participation of enzymetically
bond Cu in redox reactions
ROLES OF COPPER
1)
Copper proteins
Cu is
present in proteins in three different forms
a.
Blue proteins without oxidase activity
-
functions in one electron
transfer (eg phytocynin)
b.
Non blue proteins
Produce peroxides
c.
Multicopper proteins
d.
Contains at least four copper
atoms per molecule, acts as oxidases
2)
Photosynthesis
Component
of chloroplast enzyme and is required for the synthesis of quinones. In case of
efficiency?
3)
CHO and nitrogen metabolism
In plants
suffering from Cu deficiency soluble carbohydrates is low
Given the
role of Cu in photosynthesis a lower content of soluble carbohydrates would be
expected during vegetative growth
In legumes
receiving low Cu supply, nodulation and N2 fixation are depressed
4)
Lignification
Impaired
lignification of cell walls is a result of Cu deficiency
Cu has a
marked effect on the formation and chemical composition of cell walls
5)
Pollen formation and
fertilization
Cu
deficiency affects grain, seed and fruit formation
How, why?
Cu
deficiency and toxicity
Reading assignment
4. ZINC (Zn)
Predominantly
taken as Zn2+
At high Ph
it is taken up as monovalent cation (ZnOH+)
ROLES OF ZINC
1)
Zinc containing enzymes
At least
four plant enzymes contain Zn
-
Alcohol dehdrogenouses
-
RNA polymerase
-
Cu-Zn super oxide
-
Carbonic anhydrase
2)
Enzyme activation
Required for
activation of various enzymes
Eg - dehydrogenases
-
isomerases
3)
protein synthesis
Zn
deficient plants lower levels of protein synthesis
You get
accumulation of amino acids and amides
4)
P-Zn interactions
Large
applications of P fertilizer to soils low in Zn may induce Zn deficiency
5.
MOLYBDENUM (Mo)
Although Mo
is a metal it occur in acquous solution as molybdate oxyanion, MoO42-
The
requirement of the plant for Mo is lower than of any other nutrient
The
function of Mo as a plant nutrient are related to the valency changes it
undergoes as a metal component of enzymes
In its oxidized
stage it exists as Mo(vi) it is reduced to Mo(v) and eventually to Mo(iv)
FUNCTIONS OF Mo
1.
Mo containing enzymes
In plants
only a few enzymes have been found to contain Mo as a co-factor. These include
nitrate reductase and nitrogenase
i)
Nitrogenase
All
biological systems fixing N2 requre nitrogenase, each nitrogenase
molecule contains two Mo atoms
The Mo
requirement of root nodules in legumes is relatively high
In case of
deficiency?
ii)
Nitrate reductase NR
Catalyses
the reduction of NO3 to Nitrite
Deficiency?
Reading assignment
6. BORON (B)
In acquous
solution B occurs mainly as basic acid H3BO3, this is a
very weak acid that accepts OH- rather than H+
There is
no indication that B is an enzyme component
B is an
essential mineral nutrient for all vascular plants
ROLES OF BORON
1.
B complexes with organic
structures
A
substantial proportion of total B content of higher plants seems to be
complexed as stable cis-borate esters in the cell walls
2.
Cell elongation, cell division
and nucleic acid metabolism
In case of
deficiency of B inhibition or cessation of elongation of roots occurs
RNA
synthesis is affected by B deficiency
3.
Carbohydrate and protein
metabolism
Role of B
in CHO metabolism is focused on synthesis of cell wall material and transport
of sugars
Utilization
of CHO for synthesis of cellulose or RNA is impaired; therefore in case of B
deficiency protein metabolism is impaired
4.
Membrane permeability
B has a
direct effect on membranes, probably by the formation of cis-diol borate
complexes with membrane constituents
5.
Pollen germination and pollen
tube growth
The supply
of B required for seed and grain production is usually higher than that needed
for vegetative growth only
Higher B
levels in the stigma and style are required for pollen tube growth.