MODULE VI: NUTRITION IN PLANTS

Introduction

It is one of the characteristics of living organisms. It is the process where living things or organisms obtain and
utilize materials from environment for life processes such as respiration, growth, excretion and reproduction.

Modes of nutrition

Modes of feeding in living organisms can be classified into two major groups

 Autotrophic
 Heterotrophic

Autotrophic mode of nutrition

This is carried out by green plants through a process called photosynthesis and some certain bacteria through
chemosynthetic process

Photosynthetic nutrition

In photosynthesis (holophytic) nutrition. It is the process where green plant containing chlorophyll utilizes light
energy to manufacture organic food from simple inorganic raw materials such as carbon dioxide and water. It is
illustrated chemically as shown below:

Sunlight energy
6CO2+6H2O C6H12O6+6O2
Chlorophyll

From the simple sugar formed, other food substances such as proteins, oils, and vitamins are manufactured.

Chemosynthetic nutrition

Certain bacteria are autotrophic in that they are capable of manufacturing their own food from simple inorganic
substances such as carbon (iv) oxide, water, and hydrogen sulphide using the energy released during the chemical
reaction. The bacteria do not depend on sunlight, they have enzyme system with power to capture or trap chemical
energy from their environment.

Sulphur bacteria
2H2S+O2 2S+2H2O+chemical energy
Chemical energy
12H2S+6CO2 C6H12O6+6H2O+12S

Note that nitrifying bacteria (Nitrosomonas and Nitrobacter) in the soil oxidize ammonia to nitrite and nitrate
respectively

2NH3+3O2 2HNO2+2H2O+ chemical energy

2HNO2+O2 2HNO3+ chemical energy

The energy is then used in combining water and carbon (iv) oxide to form sugar, the bacteria also use the nitrate in
the soil with the sugar produced to form protein.
Note that all organisms that feed autotrophically are autotrophs.

HETEROTROPHIC NUTRITION

This type of nutrition is carried out by organisms that cannot manufacture their own food from simple organic raw
materials from their environment. Example are fungi, bacteria and few parasitic flowering plants. All organisms that
are not self-feeders or exhibit heterotrophic nutrition are called heterotrophs. They take in food items into their guts
in three different forms

i. Macrophages: take in large food items which need teeth to be broken down before digesting, e.g. mammals
ii. Microphagus feeders : ingest tiny food particles into their gut without been broken down with teeth, e.g
paramecium, tilapia, mosquito larvae.
iii. Fluid feeders: these feed on fluids, e.g adult mosquitoes, aphids and butterflies

However, there are four main types of heterotrophic nutrition.

i. Holozoic: is a mode of nutrition that involves the ingestion of liquid or solid organic material, digestion,
absorption or assimilation into the body cells.
ii. Saprophytic: the organisms obtain their food from dead and decaying organic matter of dead plants, dead
animals and other decomposing organic matter.
iii. Symbiotic; is a mode of nutrition in which two different organisms live together, share shelter and
nutrients.
iv. Parasitic: is a mode of nutrition where a parasitic organism lives on the body surface or inside the body of
another type of organism and get nutrition directly from the body of the host and the latter derive no
benefit.

Holozoic nutrition: it is the type nutrition in which food is obtained as a solid organic material is eaten, digested and
absorbed into the body. Nearly all animals are holozoic nutrition.

Photosynthesis (Capturing and Converting Light Energy)

Introduction

It is the term coined by Barnes in 1898 where green plants or autotrophic plants synthesize enormous of organic
food with the help of the light energy available from sun. Carbohydrate produced through photosynthesis
constitutes the basic raw materials, which directly or indirectly give rise to all the organic components of virtually
all plants and animals. The entire humanity depends upon the prepared food of plants through the process of
photosynthesis where light energy are captured and converted to chemical energy.

Definition of photosynthesis

It is described as a process in which green plants containing chlorophyll utilizes the sunlight energy to synthesis
organic food substances notably carbohydrate ( sugar) from inorganic raw materials such as carbon dioxide ( CO 2 )
and water ( H2O) that are obtained from the air and the soil respectively. It is indeed anabolic process.

Description of chloroplasts

Chloroplasts are an organelles present in plant cells and some eukaryotic organisms. It the most important plastid
found in plant cells and as such the structure in a green plant cell in which photosynthesis occurs. In higher plants it
occurs in leaves, green stems, fruit and part of flower (sepal). It is abundant in leaves hence the leaves are tagged as
photosynthetic apparatus.
Chloroplasts found in higher plants are generally bioconvex or planoconvex. However, shaped such as spheroid,
filamentous saucer shaped, discoid or ovoid have been found in different plants. The size varies from species to
species and it is constant for a given cell type. In higher plants, the average size of chloroplast is 4-6 µ in diameter
and 1-3 µ in thickness.

The chloroplasts are double membrane bound organelles and are the site of photosynthesis They have a system of
three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner
membrane of the chloroplast enclose a semi-gel-like fluid known as the stroma. This stroma makes up much of the
volume of the chloroplast, the thylakoids system floats in the stroma (see below figure)

Outer membrane - It is a semi-porous membrane and is permeable to small molecules and ions, which
diffuses easily. The outer membrane is not permeable to larger proteins.

Intermembrane Space - It is usually a thin intermembrane space about 10-20 nanometers and it is
present between the outer and the inner membrane of the chloroplast.

Inner membrane - The inner membrane of the chloroplast forms a border to the stroma. It regulates
passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids
and carotenoids are synthesized in the inner chloroplast membrane.

Stroma
Stroma is a alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast.
The space outside the thylakoid space is called the stroma. The chloroplast DNA chloroplast ribosomes and the
thylakoid sytem, starch granules and many proteins are found floating around the stroma.

Thylakoid System
The thylakoid system is suspended in the stroma. The thylakoid system is a collection of membranous sacks called
thylakoids. The chlorophyll is found in the thylakoids. The membranes of these thylakoids are sites for the process
of light reactions of photosynthesis. The thylakoids are arranged in stacks known as grana. Each granum contains
around 10-20 thylakoids. .

Important protein complexes which carry out light reaction of photosynthesis are embedded in the membranes of the
thylakoids. The Photosystem I and the Photosystem II are complexes that harvest light with chlorophyll and
carotenoids; they absorb the light energy and use it to energize the electrons.

The molecules present in the thylakoid membranes use the electrons that are energized to pump hydrogen ions into
the thylakoid space. This decreases the pH and become acidic in nature. A large protein complex known as the ATP
synthase (an enzyme) controls the concentration gradient of the hydrogen ions in the thylakoid space to generate
ATP energy and the hydrogen ions flow back into the stroma.
.
Functions of chloroplast:
  In plants, all the cells participate in plant immune response as they lack specialized immune cells. The
chloroplasts with the nucleus and cell membrane and endoplasmic reticulum are the key organelles of
pathogen defense.
 The most important function of chloroplast is to make food by the process of photosynthesis. Food is
prepared in the form of sugars. During the process of photosynthesis sugar and oxygen are made using light
energy, water, and carbon dioxide.
 Light reactions takes place on the membranes of the thylakoids.
 Chloroplasts, like the mitochondria use the potential energy of the H + ions or the hydrogen ion gradient to
generate energy in the form of ATP.
 The dark reactions also known as the Calvin cycle takes place in the stroma of chloroplast.
 Production of NADPH2 molecules and oxygen as a result of photolysis of water.
 By the utilization of assimilatory powers the 6-carbon atom is broken into two molecules of
phosphoglyceric acid

Mechanism of photosynthesis

Photosynthesis consists in the building up of energy-rich carbon-containing compounds from carbon dioxide and
water by the illuminated green cells, liberating oxygen as a by-product. It is essentially an oxidation-reduction
process by which hydrogen is transferred from water to carbon dioxide through a “carrier” substance. The overall
reaction may be represented thus

6CO2 + 6H2O Light Energy and Chlorophyll C6H12O6+ 6O2

OR. NB: The two equations require light energy and chlorophylls

6CO2 + 12H2O Light energy and Chlorophyll C6H12O6 + 6H2O + 6O2

Two processes/stages of photosynthesis

Photosynthesis involves two distinct phases

 Light dependent phase ( Light reactions)

 Light independent phase ( Dark reactions)

Light reactions.

It is the first phase of photosynthesis and comprises the following

 Conversion of radiant or solar energy to chemical energy i.e formation of adenosine triphosphate (ATP)
and reduced nicotinamide adenine dinucleotide phosphate (NADPH)
 Photolysis or ionization of water
 Evolution of molecular oxygen from water which escapes into the atmosphere

Dark reactions (Calvin cycle)

It is the second phase of photosynthesis. It consists of the following

 Reduction of carbon dioxide (CO2) by NADPH ( reduced nicotinamide adenine dinucleotide phosphate
 Participation of ATP
 Series of enzymes
 Ribulose-di-phosphate (carbon dioxide acceptor) to give rise to hexose sugar
 Regeneration of Ribulose-di-phosphate (RUDP)
It should be noted that the term dark reactions is misnomer as the reactions do not take place in the dark as it
appears. This is because enzymes in the dark reactions required light to activate them.

Light reactions (Photosystems I and II)

These reactions take place in the thylakoid membrane of the chloroplast. Light energy absorbed by chlorophyll
molecule is converted into compounds namely adenosine triphosphate (ATP) and reduced nicotinamide adenine
dinucleotide phosphate. There are two ways in which ATP is formed. These are Cyclic Photophosphorylation and
Non-cyclic photophosphorylation. The NADPH is formed only in one way. Note that the production of ATP
using light energy be it in cyclic and non-cyclic manner is called Photophosphorylation

CyclicPhotophosphorylation: It requires photosystem I (700 nm) but not photosystem II. Light-dependent electron
transport occurs in the thylakoid membranes, where electrons follow a cyclic pathway, returning to the photosystem
I reaction center. The energy of this electron transport results in a H + gradient formation, the energy source for ATP
synthesis. ATP is formed from ADP and Pi, but NADP+ is not reduced ( see below illustration)

Note: Chl = Chlorophyll; FD=Ferredoxin; PQ= Plastoquinone; PC= Plastocyanin; ADP= Adenosine
diphosphate; ATP= Adenosine triphosphate and Pi= Phosphate

Non-cyclic photophosphorylation: This also occurs in the thylakoid membrane. In this electron flow, two pigment
systems are involved. These are Photosystems I and II and they operate in tandem. The absorbing maxima of
Photosystems I and II are 700 nm and 680 nm respectively. When photosystem II absorbs light, an electron is exited
to higher energy level in the reaction center chlorophylls (P 680) is captured by the primary electron acceptor. The
light energy absorbed by photosystem II is used to split water molecule into hydrogen ions (H +) and the hydroxyl
ions (OH-). The hydroxyl ions combine with one and another to generate oxygen molecules and water. The
electrons released by photosystem II pass from the primary acceptor to photosystem I via electron transport chain.
As the electrons moved down the chain, the energy release is used to produce ATP. As electrons pass through the
non-cyclic pathway, they do not return to the original photosystem. This does not create a cycle, hence the name
non-cyclic. The electrons that are released from PS II pass on to PS I and are used to reduce NADP + to NADPH for
the synthesis of glucose in the Calvin cycle.

Photolysis provides H+ ions to replace those lost in the photosystems. The excited electrons provide energy for
a proton pump to actively transport additional H+ into the thylakoid. The high concentration of H+ diffuse
past ATP synthase as they pass out of the membrane to the lower H+ concentration. The energy created as
H+ passes the ATP synthase to forms ATP. The Diffusion of H+ ions from high to low concentration through
ATP synthase to form ATP is called chemiosmosis. The schematic diagram is shown below
.

Dark reactions ( Calvin Cycle)

The Calvin cycle or Calvin-Benson-Bassham cycle or carbon fixation) is a series of biochemical reactions that take
place in the stroma of chloroplast in photosynthetic organisms. It was discovered by Melvin Calvin, James Bassham
and Andrew Benson at the University of California, Berkeley. It is one of the light-independent reactions or dark
reactions. The reactions are broken down to four basic processes

 Carboxylation
 Reduction( reversal of glycolysis)
 Isomerization/condensation
 Phosphorylation

1.4.2.1. Overview

It should be noted that, during photosynthesis, light energy is used to generate chemical free energy, stored in
glucose. The light-independent cycle also misleadingly known as the “dark reaction” or “dark stage” uses the energy
from short-lived electronically-excited carriers to convert carbon dioxide and water into organic compounds that can
be used by organism and by animals which feed on it). This set of reactions is also carbon fixation. The key enzyme
of the cycle is called RuBisCO.

The enzymes in the Calvin cycle are found in the chloroplast stoma. They are activated in the light (which is why
the name “dark reaction is misleading), and also by products of the light-dependent reaction.

The sum of reactions in the Calvin cycle is shown below

3 Co2 +6 NADPH + 5 H2O + 9 ATP C3H5O3-PO32- + H+ + 6 NADP+ + 9 ADP + 8 Pi

It should be noted that hexose (six carbon) sugars are not a product of the Calvin cycle. Although many texts list a
product of photosynthesis as C6 H12 O6, this is mainly a convenience to counter the equation of respiration, where
six-carbon sugars are oxidized in mitochondria. The carbohydrate products of the Calvin cycle are three-carbon
sugar phosphate molecule, or triose phosphate specifically, glyceraldehyde -3-phosphate.
Steps of the Calvin Cycle

1. The enzyme RuBisCO ( Ribulose-bisphosphate carboxylase and oxygenase) catalyses the carboxylation of
Ribulose-1,5-biphosphate, a 5 carbon compound, by carbon dioxide ( a total of 6 Carbons) in a two-step
reaction. Two molecules of glycerate 3-phosphate, a 3 carbon compound , are created ( 3-
phosphoglycerate, 3-phosphoglyceric acid, 3PGA)
2. The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by ATP ( which wa
produced in the light dependent stage), 1,3-bisphosphoglycerate ( glycerate-1,3- bisphosphate) and ADP
are the products( Note that two PGAs are produced for every CO 2 that enters the cycle, so this step utilizes
2 ATP per CO2 fixed
3. The enzyme G3P dehydrogenase catalyses the reduction of 1,3 BPGA by NADPH ( which another product
of the light-dependent stage). Glyceraldehyde -3- phosphate ( also G3P, GP) is produced, and the NADPH
itself was oxidized and become NADP+. Again, two NADPH are utilized per CO2 fixed
4. Triose phosphate isomerase converts a G3P reversibly into dihydroxyacetone phosphate (DHAP) also a 3-
carbon molecule.
5. Aldolase and fructose-1,6-bisphosphate convert a G3P and a DHAP into fructose -6-phosphate (6C). A
phosphate ion is lost into solution.
6. The fixation of another CO2 generates two more G3P.
7. F6P has two carbons removed by transketolase, giving erythrose-4-phopshate. The two carbons on
transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (XU5P)
8. E4P and a DHAP ( formed from one of the G3P from the second CO 2 fixation) are converted into
sedoheptulose-1,7-bisphosphate by aldolase enzyme.
9. Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle which are unique to
plants) cleaves sedoheptulose-1,7-bisphospahte into sedoheptulose-7-phosphate, releasing an inorganic
phosphate ion into solution
10. Fixation of a third CO 2 generates two more G3P. The ketose S7P has two carbons removed by
transketolase, giving ribose -5- phosphate (R5P), and the two carbons remaining on transketolase are
transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3
CO2, with generation of three pentoses which can be converted to Ru5P
11. R5P is converted into ribulose-5-phopshate (Ru5P, RuP) by phosphpentoseisomerase. Xu5P is converted
into RuP by phosphopentoseepimerase
12. Finally, phosphoribulokinase( another plant unique enzyme of the pathway) phosphorylates RuP into
RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.See the
figure

Thus, of 6 G3P produced, three RuBP (5C) are made totaling 15 carbons, with only one available for
subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 CO2

Note that RuBisCO also reacts competitively with O 2 in photorespiration and this higher at high temperatures.
Photorespiration has very negative consequences for the plants, because rather fixing CO 2, this process leads to
loss of CO2,. C4 carbon fixation evolved to circumvent photorespiration but can only occur in certain plants
living in very warm or tropical climates

Products of the Calvin Cycle

The immediate product of Calvin cycle is glyceraldehyde-3-phosphate (G3P) and water. Two G3P molecules
(or one F6P molecule) that have exited the cycle are used to make larger carbohydrates. In simplified versions
of the Calvin cycle they may be converted to F6P or F5P after exist, but this conversion is also part of the cycle
Hexose isomerase converts about half of the F6P molecules into glucose-6-phosphate. The glucose can be used
to form starch, which is stored in, for example, potatoes, or cellulose used to build of cell walls. Glucose, with
fructose, forms sucrose, a non-reducing sugar which is a stable storage sugar, unlike glucose (see both simplied
and detailed Calvin Cycle below)

Simplified form of Calvin Cycle.

 Detailed form of Calvin cycle

Importance of photosynthesis

During photosynthesis, plants take in carbon dioxide (i.e purifies the atmosphere) and give off oxygen as a by-
product. Photosynthesis can therefore be considered as the reverse of respiration. Without green plants performing
photosynthesis there would be no way for nature to replace all the oxygen being consumed in processes such as
respiration and combustion. Furthermore the levels of carbon dioxide in the atmosphere would increase. Thus, the
balance of atmospheric gases is kept stable by photosynthesis.Green plants are called producers because they
produce all their own food from the raw materials around them via photosynthesis. Animals and humans on the
other hand are consumers and all the food they eat comes directly or indirectly from plants. Most of the world’s
population obtains more than 80% of their food directly from plants, for e.g. rice, potatoes, wheat, corn etc. The
remaining source comes from animals and these animals are part of the food chain which always begins with plants.

Summary on the importance of photosynthesis

i. All life forms depend on the process of photosynthesis to carry out their life processes.
ii. Glucose, a product of photosynthesis is a starting material for the synthesis of protein, fats and oils, and
vitamin, these are various food for both plants and animals.
iii. It provide energy for all life forms.
 iv. Oxygen which is released as a by-product is necessary for an aerobic respiration.
v. This process purifies the atmosphere because it constantly removes carbon oxide

Experiment on photosynthesis

The easiest proof that photosynthesis has taken place is the test for the presence of starch in the green leaves. Other
experiment include those show the importance of carbon (iv)oxide , sunlight energy, and chlorophyll in
photosynthesis

Testing for Starch

The usual laboratory test for starch is to bring the testing material such as yam or potato in contact with iodine
solution. A blue black colour confirm the presence of starch. In green leaf, the test is not so straight forward, it
involves the following steps

i. Detach the leaf to be tested from the plant after 4-6 hours of exposure to light. Boil the leaf for 10-15
minutes to kill the protoplasm and stop enzyme activities.
ii. Put the boiled leaf into 70% boiling ethanol or methylated spirit to remove the chlorophyll or decolorize.
Note that the alcohol is heated to a boiling point in a water to avoid explosion. Decolourized leaf becomes
white and brittle then dropped into hot water to soften the tissue.
iii. Spread the bleached leaf on that surface and tile and add few drops of iodine solution and allow to stand for
a few minutes, then observe the colour of the leaf against a light source.
iv. If the tested leaf appears bluish-black in colour, it contains starch.
v. If the colour is yellow-brown, it contains no starch.

Note that any experiment in photosynthesis, it is necessary to destarch the leaf. Green leaves can be destarched by
keeping a potted plant in a dark cupboard for 48-72 hours

Factors affecting photosynthesis

Based on the equation, it is probably not surprising that carbon dioxide and sunlight are two major external factors
that affect photosynthesis. But, there are other factors such as temperature and oxygen.

Light intensity is one factor that affects photosynthesis. Without the energy from light, the chemical reaction cannot
happen. If plants have less light, they photosynthesize more slowly. The speed of photosynthesis increases with
greater light intensity, though it ultimately levels off once the plant has as much light as it needs.

Light quality is another factor since photosynthesis only occurs in the visible part of the spectrum of light, i.e
between wavelengths of about 400nm and 700 nm which are tagged as photosynthetically active radiation (PAR).
These correspond to blue and red regions of the electromagnetic spectrum

Light duration that is duration of daily light period has also a significant effect on the total photosynthetic yield of
plant. Generally speaking a plant carries on a good rate of photosynthesis if it is given light for 10 to 12 hours per
day.

Carbon dioxide concentration is another factor that affects photosynthesis. Generally, there is plenty of carbon
dioxide in the air so there is little impact on photosynthesis in different environments. It is still a requirement,
however, and if we deprive plants of carbon dioxide in the lab, we find a similar relationship to that of light. The rate
of photosynthesis increases with carbon dioxide concentration until the plant has enough or more carbon dioxide
than it needs; at that point, the rate doesn't increase much further.

Temperature is also a major factor that affects photosynthesis, although this one works differently. There is an
optimum temperature for the reaction to happen at the highest possible rate. This is true of a lot of chemical
reactions, especially biological ones. If the temperature is too cold or too hot, plants struggle to photosynthesize.
However, plants can be adapted to different climates and handle different temperatures.

Oxygen has been found to have considerable effect on photosynthesis. It is a competitive inhibitor of carboxylase
activity of RuBP and, therefore inhibits photosynthesis of C 3 plants over a wide range of concentration. It causes
loss of Co2 and in this case, the plant is said to undergo photorespiration as against photosynthesis if the volume of
oxygen is more than that of CO2.

Internal factors that affect the photosynthesis are chlorophyll, hydration of the protoplasm, enzymes and
accumulation of End-product of photosynthesis

MINERAL NUTRITION

Introduction

In most natural soils, the unavailability of mineral nutrients limits plants growth and primary productivity. It should
be noted that plant nutrition does not refer to photosynthesis, it implies essential elements that are necessary for
plant growth and development.

Classification of minerals

On the basis of the amounts found in plants, mineral elements are classified as macronutrients- N, K, Ca, Mg, P, S,
and Na- and micronutrients Cl, Fe, B, Mn, Zn, Cu, Ac, Mo, Ni

What is an essential element?

It is essential because in its absence, the plant cannot complete a normal life cycle. It formed an essential molecule
inside plants (macromolecule metabolite).

Hydroponic culture can determine which mineral elements are actually essential nutrients.

Macronutrients are elements required by plant in relatively large quantity. They are carbon, oxygen, hydrogen,
nitrogen, sulphur, and phosphorus. The other three are potassium, calcium, and magnesium.

Micronutrients elements are nutrients that plant needs in very small amount. They are iron, chlorine, copper, zinc,
magnesium, molybdenum, boron and nickel. They mostly function as co-factors of enzymatic reactions.

Role of Macro- and Micro-nutrients

Essential elements perform several functions. They participate in various metabolic processes in the plant cells such
as permeability of cell membrane, maintenance of osmotic concentration of cell sap, electron transport systems,
buffering action, enzymatic activity and act as major constituents of macromolecules and co-enzymes. Various
forms and functions of essential nutrient elements are given below.

Carbon: It is the basic structural elements of life, It occurs in plants combined with hydrogen and oxygen. It also
occurs in atmosphere as CO2 and in rocks as carbonate minerals such as limestone. Deficiency symptoms: no growth

Oxygen: It is a powerful oxidizing agent. It occurs in the free state in the atmosphere as O 2, it is one of the most
abundant elements in the earth crust (21% of air by mass). It occurs in combined state in mineral, living thing, and
water. Deficiency- no respiration

Hydrogen :It is the lightest element and powerful reducing agent. It is the most abundant element in the universe. It
is not usually free i.e uncombined hydrogen is very rare. It occurs in combined state as water, and organic
compounds.
Nitrogen :This is the essential nutrient element required by plants in the greatest amount. It is absorbed mainly as
NO3– though some are also taken up as NO 2– or NH4+. Nitrogen is required by all parts of a plant, particularly the
meristematic tissues and the metabolically active cells. Nitrogen is one of the major constituents of proteins, nucleic
acids, vitamins and hormones.

Phosphorus: Phosphorus is absorbed by the plants from soil in the form of phosphate ions (either as HPO 24− or
HPO42− ). Phosphorus is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides, and is
required for all phosphorylation reactions.

Potassium: It is absorbed as potassium ion (K+). In plants, this is required in more abundant quantities in the
meristematic tissues, buds, leaves and root tips. Potassium helps to maintain an anion-cation balance in cells and is
involved in protein synthesis, opening and closing of stomata, activation of enzymes and in the maintenance of the
turgidity of cells.

Calcium: Plant absorbs calcium from the soil in the form of calcium ions (Ca 2+). Calcium is required by
meristematic and differentiating tissues. During cell division it is used in the synthesis of cell wall, particularly as
calcium pectate in the middle lamella. It is also needed during the formation of mitotic spindle. It accumulates in
older leaves. It is involved in the normal functioning of the cell membranes. It activates certain enzymes and plays
an important role in regulating metabolic activities.

Magnesium: It is absorbed by plants in the form of divalent Mg 2+. It activates the enzymes of respiration,
photosynthesis and are involved in the synthesis of DNA and RNA. Magnesium is a constituent of the ring structure
of chlorophyll and helps to maintain the ribosome structure.

Sulphur: Plants obtain sulphur in the form of sulphate (SO42−). Sulphur is present in two amino acids – cysteine and
methionine and is the main constituent of several coenzymes, vitamins (thiamine, biotin, Coenzyme A) and
ferredoxin.

Iron: Plants obtain iron in the form of ferric ions (Fe 3+). It is required in larger amounts in comparison to other
micronutrients. It is an important constituent of proteins involved in the transfer of electrons like ferredoxin and
cytochromes. It is reversibly oxidised from Fe2+ to Fe3+ during electron transfer. It activates catalase enzyme, and is
essential for the formation of chlorophyll.

Manganese: It is absorbed in the form of manganate ions (Mn 2+). It activates many enzymes involved in
photosynthesis, respiration and nitrogen metabolism. The best defined function of manganese is in the splitting of
water to liberate oxygen during photosynthesis.

Zinc: Plants obtain zinc as Zn2+ ions. It activates various enzymes, especially carboxylases. It is also needed in the
synthesis of auxin.

Copper: It is absorbed as cupric ions (Cu 2+). It is essential for the overall metabolism in plants. Like iron, it is
associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu + to Cu2+.

Boron :It is absorbed as BO33− or BO472− . Boron is required for uptake and utilisation of Ca 2+, membrane
functioning, pollen germination, cell elongation, cell differentiation and carbohydrate translocation.

Molybdenum :Plants obtain it in the form of molybdate ions (MoO ) 22+ . It is a component of several enzymes,
including nitrogenase and nitrate reductase both of which participate in nitrogen metabolism.

Chlorine :It is absorbed in the form of chloride anion (Cl –). Along with Na+ and K+, it helps in determining the
solute concentration and the anion-cation balance in cells. It is essential for the water-splitting reaction in
photosynthesis, a reaction that leads to oxygen evolution.

Deficiency Symptoms of Essential Elements

Whenever the supply of an essential element becomes limited, plant growth is retarded. The concentration of the
essential element below which plant growth is retarded is termed as critical concentration. The element is said to
be deficient when present below the critical concentration. Since each element has one or more specific structural or
functional role in plants, in the absence of any particular element, plants show certain morphological changes. These
morphological changes are indicative of certain element deficiencies and are called deficiency symptoms. The
deficiency symptoms vary from element to element and they disappear when the deficient mineral nutrient is
provided to the plant. However, if deprivation continues, it may eventually lead to the death of the plant. The parts
of the plants that show the deficiency symptoms also depend on the mobility of the element in the plant. For
elements that are actively mobilized within the plants and exported to young developing tissues, the deficiency
symptoms tend to appear first in the older tissues. For example, the deficiency symptoms of nitrogen, potassium and
magnesium are visible first in the senescent leaves. In the older leaves, biomolecules containing these elements are
broken down, making these elements available for mobilizing to younger leaves. The deficiency symptoms tend to
appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the
mature organs, for example, element like calcium is a part of the structural component of the cell and hence is not
easily released.

The kind of deficiency symptoms shown in plants include chlorosis, necrosis, stunted plant growth, premature fall of
leaves and buds, and inhibition of cell division. Chlorosis is the loss of chlorophyll leading to yellowing in leaves.
This symptom is caused by the deficiency of elements N, K, Mg, S, Fe, Mn, Zn and Mo. Likewise, necrosis, or
death of tissue, particularly leaf tissue, is due to the deficiency of Ca, Mg, Cu, K. Lack or low level of N, K, S, Mo
causes an inhibition of cell division. Some elements like N, S, Mo delay flowering if their concentration in plants is
low.
You can see from the above that the deficiency of any element can cause multiple symptoms and that the same
symptoms may be caused by the deficiency of one of several different elements. Hence, to identify the deficient
element, one has to study all the symptoms developed in all the various parts of the plant and compare them with the
available standard tables. We must also be aware that different plants also respond differently to the deficiency of
the same element.