The basic needs of all living organisms are essentially the same. They
require macromolecules, such as carbohydrates, proteins and fats, and
water and minerals for their growth and development.
This chapter focusses mainly on inorganic plant nutrition, wherein
you will study the methods to identify elements essential to growth and
development of plants and the criteria for establishing the essentiality.
You will also study the role of the essential elements, their major deficiency
symptoms and the mechanism of absorption of these essential elements.
The chapter also introduces you briefly to the significance and the
mechanism of biological nitrogen fixation.
In 1860, Julius von Sachs, a prominent German botanist, demonstrated,
for the first time, that plants could be grown to maturity in a defined
nutrient solution in complete absence of soil. This technique of growing
plants in a nutrient solution is known as hydroponics. Since then, a
number of improvised methods have been employed to try and determine
the mineral nutrients essential for plants. The essence of all these methods
involves the culture of plants in a soil-free, defined mineral solution. These
methods require purified water and mineral nutrient salts. Can you
explain why is this so essential?
After a series of experiments in which the roots of the plants were
immersed in nutrient solutions and wherein an element was added /
substituted / removed or given in varied concentration, a mineral solution
12.1 Methods to
Study the
Requirements of
12.2 Essential
12.3 Mechanism of
Absorption of
12.4 Translocation of
12.5 Soil as Reservoir
of Essential
12.6 Metabolism of
suitable for the plant growth was obtained. By this
method, essential elements were identified and
their deficiency symptoms discovered. Hydroponics
has been successfully employed as a technique for
the commercial production of vegetables such as
tomato, seedless cucumber and lettuce. It must be
emphasised that the nutrient solutions must be
adequately aerated to obtain the optimum growth.
What would happen if solutions were poorly
aerated? Diagrammatic views of the hydroponic
technique is given in Figures 12.1 and 12.2.
Most of the minerals present in soil can enter plants
through roots. In fact, more than sixty elements of
the 105 discovered so far are found in different
plants. Some plant species accumulate selenium,
some others gold, while some plants growing near
nuclear test sites take up radioactive strontium.
There are techniques that are able to detect the
minerals even at a very low concentration (10
mL). The question is, whether all the diverse mineral
elements present in a plant, for example, gold and
selenium as mentioned above, are really necessary
for plants? How do we decide what is essential for
plants and what is not?
12.2.1 Criteria for Essentiality
The criteria for essentiality of an element are given
(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
(c) The element must be directly involved in the
metabolism of the plant.
Figure 12.1 Diagram of a typical set-up for
nutrient solution culture
Figure 12.2 Hydroponic plant production.
Plants are grown in a tube or
trough placed on a slight
incline. A pump circulates a
nutrient solution from a
reservoir to the elevated end of
the tube. The solution flows
down the tube and returns to
the reservoir due to gravity.
Inset shows a plant whose
roots are continuously bathed
in aerated nutrient solution.
The arrows indicates the
direction of the flow.
Based upon the above criteria only a few elements have been found to
be absolutely essential for plant growth and metabolism. These elements
are further divided into two broad categories based on their quantitative
(i) Macronutrients, and
(ii) Micronutrients
Macronutrients are generally present in plant tissues in large amounts
(in excess of 10 mmole Kg
of dry matter). The macronutrients include
carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium,
calcium and magnesium. Of these, carbon, hydrogen and oxygen are
mainly obtained from CO
and H
O, while the others are absorbed from
the soil as mineral nutrition.
Micronutrients or trace elements, are needed in very small amounts
(less than 10 mmole Kg
of dry matter). These include iron, manganese,
copper, molybdenum, zinc, boron, chlorine and nickel.
In addition to the 17 essential elements named above, there are some
beneficial elements such as sodium, silicon, cobalt and selenium. They
are required by higher plants.
Essential elements can also be grouped into four broad categories on
the basis of their diverse functions. These categories are:
(i) Essential elements as components of biomolecules and hence
structural elements of cells (e.g., carbon, hydrogen, oxygen and
(ii) Essential elements that are components of energy-related chemical
compounds in plants (e.g., magnesium in chlorophyll and
phosphorous in ATP).
(iii) Essential elements that activate or inhibit enzymes, for example
is an activator for both ribulose bisphosphate carboxylase-
oxygenase and phosphoenol pyruvate carboxylase, both of which
are critical enzymes in photosynthetic carbon fixation; Zn
is an
activator of alcohol dehydrogenase and Mo of nitrogenase during
nitrogen metabolism. Can you name a few more elements that
fall in this category? For this, you will need to recollect some of
the biochemical pathways you have studied earlier
(iv) Some essential elements can alter the osmotic potential of a cell.
Potassium plays an important role in the opening and closing of
stomata. You may recall the role of minerals as solutes in
determining the water potential of a cell.
12.2.2 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
Nitrogen: This is the essential nutrient element required by plants in the
greatest amount. It is absorbed mainly as NO
though some are also taken
up as NO
or NH
. 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
2 4
). 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
). 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
. 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
( )SO
. 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
). 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 Fe
to Fe
during electron
transfer. It activates catalase enzyme, and is essential for the formation of
Manganese: It is absorbed in the form of manganous ions (Mn
). 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 Zn
ions. It activates various enzymes,
especially carboxylases. It is also needed in the synthesis of auxin.
Copper: It is absorbed as cupric ions (Cu
). 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 Cu
Boron : It is absorbed as
4 7
. Boron is required for uptake
and utilisation of Ca
, membrane functioning, pollen germination, cell
elongation, cell differentiation and carbohydrate translocation.
Molybdenum: Plants obtain it in the form of molybdate ions
( )MoO
. 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
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.
12.2.3 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 mobilised
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
mobilising 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 sulphur and
calcium are a part of the structural component of the cell and hence are
not easily released. This aspect of mineral nutrition of plants is of a great
significance and importance to agriculture and horticulture.
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.
12.2.4 Toxicity of Micronutrients
The requirement of micronutrients is always in low amounts while their
moderate decrease causes the deficiency symptoms and a moderate increase
causes toxicity. In other words, there is a narrow range of concentration at
which the elements are optimum. Any mineral ion concentration in tissues
that reduces the dry weight of tissues by about 10 per cent is considered
toxic. Such critical concentrations vary widely among different
micronutrients. The toxicity symptoms are difficult to identify. Toxicity levels
for any element also vary for different plants. Many a times, excess of an
element may inhibit the uptake of another element. For example, the
prominent symptom of manganese toxicity is the appearance of brown
spots surrounded by chlorotic veins. It is important to know that
manganese competes with iron and magnesium for uptake and with
magnesium for binding with enzymes. Manganese also inhibit calcium
translocation in shoot apex. Therefore, excess of manganese may, in fact,
induce deficiencies of iron, magnesium and calcium. Thus, what appears
as symptoms of manganese toxicity may actually be the deficiency
symptoms of iron, magnesium and calcium. Can this knowledge be of some
importance to a farmer? a gardener? or even for you in your kitchen-garden?
Much of the studies on mechanism of absorption of elements by plants
has been carried out in isolated cells, tissues or organs. These studies
revealed that the process of absorption can be demarcated into two main
phases. In the first phase, an initial rapid uptake of ions into the ‘free
space’ or ‘outer space’ of cells – the apoplast, is passive. In the second
phase of uptake, the ions are taken in slowly into the ‘inner space’ – the
symplast of the cells. The passive movement of ions into the apoplast
usually occurs through ion-channels, the trans-membrane proteins that
function as selective pores. On the other hand, the entry or exit of ions to
and from the symplast requires the expenditure of metabolic energy, which
is an active process. The movement of ions is usually called flux; the
inward movement into the cells is influx and the outward movement, efflux.
You have read the aspects of mineral nutrient uptake and translocation
in plants in Chapter 11.
Mineral salts are translocated through xylem along with the ascending
stream of water, which is pulled up through the plant by transpirational
pull. Analysis of xylem sap shows the presence of mineral salts in it. Use
of radioisotopes of mineral elements also substantiate the view that they
are transported thr
ough the xylem. You have already discussed the
movement of water in xylem in Chapter 11.
Majority of the nutrients that are essential for the growth and
development of plants become available to the roots due to weathering
and breakdown of rocks. These processes enrich the soil with dissolved
ions and inorganic salts. Since they are derived from the rock minerals,
their role in plant nutrition is referred to as mineral nutrition. Soil
consists of a wide variety of substances. Soil not only supplies minerals
but also harbours nitrogen-fixing bacteria, other microbes, holds water,
supplies air to the roots and acts as a matrix that stabilises the plant.
Since deficiency of essential minerals affect the crop-yield, there is often
a need for supplying them through fertilisers. Both macro-nutrients
(N, P, K, S, etc.) and micro-nutrients (Cu, Zn, Fe, Mn, etc.) form
components of fertilisers and are applied as per need.
12.6.1 Nitrogen Cycle
Apart from carbon, hydrogen and oxygen, nitrogen is the most
prevalent element in living organisms. Nitrogen is a constituent of
amino acids, proteins, hormones, chlorophylls and many of the
vitamins. Plants compete with microbes for the limited nitrogen that