194
B
IOLOGY
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.
12.1
M
ETHODS
TO
S
TUDY
THE
M
INERAL
R
EQUIREMENTS
OF
P
LANTS
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 this is 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 /
removed or given in varied concentration, a mineral solution suitable for
M
INERAL
N
UTRITION
C
HAPTER
12
12.1 Methods to
Study the
Mineral
Requirements of
Plants
12.2 Essential
Mineral
Elements
12.3 Mechanism of
Absorption of
Elements
12.4 Translocation of
Solutes
12.5 Soil as Reservoir
of Essential
Elements
12.6 Metabolism of
Nitrogen
M
INERAL
N
UTRITION
195
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.
12.2
E
SSENTIAL
M
INERAL
E
LEMENTS
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
-8
g/
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
below:
(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.
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.
Nutrient
solution
Pump
196
B
IOLOGY
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
requirements.
(i) Macronutrients, and
(ii) Micronutrients
Macronutrients are generally present in plant tissues in large amounts
(in excess of 10 mmole Kg
–1
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
2
and H
2
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
–1
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
nitrogen).
(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
Mg
2+
is an activator for both ribulose bisphosphate carboxylase-
oxygenase and phosphoenol pyruvate carboxylase, both of which
are critical enzymes in photosynthetic carbon fixation; Zn
2+
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
M
INERAL
N
UTRITION
197
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.
Nitrogen : This is the essential nutrient element required by plants in the
greatest amount. It is absorbed mainly as NO
3
–
though some are also taken
up as NO
2
–
or NH
4
+
. 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
H PO
2
4
−
or
HPO
4
2−
). 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
(
)
SO
4
2−
. 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 Fe
2+
to Fe
3+
during electron
transfer. It activates catalase enzyme, and is essential for the formation of
chlorophyll.