This document summarizes key concepts related to plant physiology and growth. It discusses nitrogen fixation by root-bacteria interactions and the importance of nitrogen for plants. Biological nitrogen fixation is described as the primary process by which atmospheric nitrogen is converted to ammonia by soil bacteria or cyanobacteria. The document also summarizes factors that influence photosynthesis like light, carbon dioxide, temperature, water, and oxygen. It describes how plant structure is influenced by both genes and the environment, and the basic root, stem, and leaf structures of plants are outlined. The tissue systems of dermal tissue, vascular tissue, and ground tissue that make up plant organs are also summarized.

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Summarized Note on Plant Physiology of Biotic stress
Compiled by Simon Peter Abah
BIOLOGICAL N FIXATION
Nitrogen Fixation: Root and Bacteria Interactions
 Nitrogen is an important macronutrient because it is part of nucleic acids and
proteins.
 Atmospheric nitrogen, which is the diatomic molecule N2, or dinitrogen, is the
largest pool of nitrogen in terrestrial ecosystems.
 However, plants cannot take advantage of this nitrogen because they do not have
the necessary enzymes to convert it into biologically useful forms.
 However, nitrogen can be “fixed,” which means that it can be converted to
ammonia (NH3) through biological, physical, or chemical processes.
 As you have learned, biological nitrogen fixation (BNF) is the conversion of
atmospheric nitrogen (N2) into ammonia (NH3), exclusively carried out by
prokaryotes such as soil bacteria or cyanobacteria.
 Biological processes contribute 65 percent of the nitrogen used in agriculture.
 The following equation represents the process:
N2 + 16ATP + 8 e− + 8 H+ → 2NH3 + 16 ADP + 16 Pi + H2
 The most important source of BNF is the symbiotic interaction between soil
bacteria and legume plants, including many crops important to humans
 The NH3 resulting from fixation can be transported into plant tissue and
incorporated into amino acids, which are then made into plant proteins.
 Some legume seeds, such as soybeans and peanuts, contain high levels of protein,
and serve among the most important agricultural sources of protein in the world.
 BNF is the process whereby atmospheric N is reduced to ammonia in the
presence of nitrogenase.
 Nitrogenase is a biological catalyst found naturally only in certain
microorganisms such as the symbiotic Rhizobium and Frankia or the free-living
Azospirillum and Azotobacter.
The biochemical mechanism of N2 fixation is written in simplified form as
N2(a) --> NH3 --> amino acids --> proteins
+ATP
+H+

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Low oxygen
tension
 ATP is the source of energy necessary for the cleavage and reduction of N2 into
ammonia.
 In general for each of gram N2 of fixed by Rhizobium, the plant fixes 1-20 grams
through photosynthesis.
 This is an indication that symbiotic N2 fixation requires additional energy.
 It is usually accepted that N2 fixing systems require more P than non-fixing
systems, P is needed for plant growth, nodule formation and development and
ATP synthesis, each process being vital for nitrogen fixation
Specificity and Effectiveness
 There are approximately 1,300 leguminous plant species in the world.Of these,
nearly 10% have been examined for nodulation 87% of which were nodulated.
Thus not all legumes are infected by Rhizobium.
 Not all symbiosis fix N2 with equal effectiveness. This means that a given
legume cultivar nodulated by different strains of the same species of Rhizobium
would fix different amount of nitrogen.
 Similarly, a given strain of Rhizobium will nodulate and fix different amounts of
N2 in symbiosis with a range of cultivars of the same plant species.


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 Interactions between the microsymbiont and the plant are complicated by edaphic,
climatic and management factors.
 A legume-Rhizobium symbionts might perform well in a loamy soil but not in a
sandy soil, in the sub-humid region but not in the Sahel.
Edaphic (Relate to the soil)
 Excessive soil moisture
 Drought
 Soil acidity
 P deficiency
 Excessive mineral N
 Deficiency of Ca,Mo,Co and B
 Excessive moisture and water logging prevent the development of root hair and
sites of nodulation, and interfere with a normal diffusion of O2 with the root
system of plants.
 Drought reduces the number of rhizobia in soils and inhibits nodulation and
N2-fixation. Prolonged drought will promote nodule decay.

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 Soil acidity and related problems of Ca deficiency and aluminium and manganese
toxicity adversely affect nodulation, N2 fixation and plant growth.
 P deficiency is common place in tropical Africa and reduces, nodulation N2
fixation and plant growth. Identification of plant species adapted to low P soils is
a good strategy to overcome this soil constraint.
 The role of mychorrizal fungi in increasing plant P uptake with beneficial effects
on N2fixation has been required.
 Mineral N inhibits the infection process and also inhibits N2fixation. The former
problem probably results from impairment of the recognition mechanisms by
nitrates, while the latter is probably due to diversion of photosynthates.
 various microelements (Cu,Mo,Co,B) are necessary for N2fixation.
Climatic factors
 Extreme temperatures affect N2fixation adversely because N2fixation is an
enzymatic process. However, there are differences between symbiotic systems in
their ability to tolerate high >35°C and low <25°C temperatures.
 Availability of light regulates photosynthesis upon which biological nitrogen
fixation depends
Biological factors
 Absence of the required rhizobia species constitute the major constraint in the
nitrogen fixation process.
Other limiting biotic factors could be:
 Excessive defoliation of host plant
 Crop competition
 Insects and nematodes.
Factor Affecting Photosynthesis
Light
It is one of the major factors affecting photosynthesis. Photosynthesis cannot occur in
the dark and the source of light for the plants is sunlight. Three attributes of light are
important for photosynthesis:

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 Intensity: Photosynthesis begins at low intensities of light and increases till it
is maximum at the brightest time of the day. The amount of light required varies
for different plants. Photosynthesis uses maximum up to 1.5 % light in the
process and so light is generally not a limiting factor at high intensity. However,
the light becomes a limiting factor in low intensity because no matter how much
water or CO2 is present, without light photosynthesis cannot occur. At high
intensities, the temperature of the plant increases which leads to increased
transpiration in the plant. This leads to the closing of the stomata which leads to a
reduced CO2 intake. Thus, leading to a reduction and finally stoppage of
photosynthesis. Therefore, excessive light inhibits photosynthesis.
 Quality: Experiments conducted by Engelmann prove that the chlorophyll
most effectively absorbs red and blue wavelengths from the entire spectrum of
light. Thus, maximum photosynthesis occurs when the plant is exposed to the
light of these wavelengths.
 Duration: The longer the plant is exposed to light, the longer the process of
photosynthesis will continue. As long as the temperature of the plant remains
balanced, photosynthesis will occur.
Carbon Dioxide Concentration
The atmosphere contains 0.03% of carbon dioxide amidst other gases. Plants take in
carbon dioxide from the air. But, since the amount of CO2 in the air is very less, it acts
as a limiting factor for photosynthesis. Experiments have been performed to study the
rate of photosynthesis on increasing the concentration of CO2 in the atmosphere.
It is seen that, when light and temperature are not the limiting factors, increasing CO2
concentration leads to an increase in the rate of photosynthesis. But, beyond a certain
limit, CO2 starts accumulating in the plant and this leads to slowing down of the
process. So, excessive CO2 inhibits photosynthesis especially when it starts to
accumulate.
Temperature
It is commonly seen in all biological and biochemical processes that they occur best in a
certain optimum range of temperature. This holds true for photosynthesis as well. It is
observed that, when CO2 and light are not limiting factors, the rate of photosynthesis
increases with increase in temperatures till the optimum level for that plant. Beyond the
optimum levels on both sides of the normal range, the enzymes are deactivated or
destroyed and photosynthesis stops.
Water
Water is considered one of the most important factors affecting photosynthesis. When
there is a reduced water intake or availability, the stomata begin to close to avoid loss of
any water during transpiration. With the stomata closing down the CO2 intake also
stops which affects photosynthesis. Therefore, the effect of water on photosynthesis is
more indirect than direct.

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Oxygen
Optimum levels of oxygen are favourable for photosynthesis. Oxygen is needed for
photorespiration in C3 plants and the by-product of photorespiration is CO2 which is
essential for photosynthesis. Also, the energy generated during the oxygen respiration is
needed for the process of photosynthesis as well. However, an increase in the oxygen
levels beyond the optimum for the plant leads to inhibition of photosynthesis.
This is because oxygen tends to break down the intermediaries that are formed in
photosynthesis. Oxygen also completes with CO2 to combine with RUBISCO which a
part of the dark reaction of photosynthesis and photorespiration. Therefore, increased
levels of O2 would mean that RUBISCO will combine with O2 to initiate
photorespiration and photosynthesis will slow down.
Blackman’s Principle of Limiting Factors
This principle states that when a process is governed by more than one factor, the rate
of the process is governed by that factor which is closest to its minimum value.

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(Image source: harennotes4u.blogspot.com)
For example, if a leaf is exposed to a certain amount of light intensity with constant
temperature but the CO2 available is less, the rate of photosynthesis will not increase
with an increase in the light intensity. Therefore, in this case, CO2 is the limiting factor.

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PLANT STRUCTURE AND GROWTH
 Both genes and environment affect plant structure. A plant’s structure reflects
interactions with the environment on two time scales.
 Over the long term, entire plant species have, by natural selection, accumulated
morphological adaptations that enhance survival and reproductive success in the
environment in which they grow. For example, some desert plants such as Cacti
have leaves that are so highly reduced that the stem is actually the primary
photosynthetic organ.
 This reduction in leaf size, and thus, in surface area, is a morphological
adaptation that reduces water loss. Over the short term, individual plants, exhibit
structural responses to their specific environment.
 The architecture of a plant is a dynamic process, continuously shaped by the
plant’s genetically directed growth pattern along with the fine-tuning of the
environment.
 Plants physiological adjustments to environmental changes are even faster than a
plant’s structural response.
 Unlike cacti, most plants are rarely exposed to severe drought conditions and rely
mainly on physiological adaptations to cope with drought stress. In the most
common response, the plant produces a hormone that causes stomata to close.
 Plants have three basic organs: roots, stems and leaves. The plant body consists of
organs that are composed of different tissues; and these tissues are teams of
different types of cells.
 The basic morphology of plants reflects their revolutionary history as terrestrial
organisms that must simultaneously inhabit and draw resources from two very
different environments – soil and air.
 Soil provides water and minerals, but air is the main source of CO2, and light
does not penetrate far into the soil.
 The revolutionary solution to this separation of resources was differentiation of
the plant with two main systems: the subterranean root system and an aerial shoot
system consisting of leaves and stems highly modified for sexual reproduction.
Roots anchor the plant in the soil, absorb minerals and water, and store food.
 Monocots, including grasses, generally have fibrous root systems consisting of a
mat of thin roots that spread out below the surface
 The fibrous root system extends the plant’s exposure to soil, water and minerals
and anchors it tenaciously to the ground because their root systems are
concentrated to the upper few centimeters of soil, grasses hold the top soil in
place and make excellent ground cover for preventing erosion.

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 Many dicots have tap root system, consisting of one larger vertical root (the tap
root) that produces many smaller lateral or branch, roots.
 Tap roots anchor the plant in the soil. In addition, tap roots often store food. The
plant consumes the food reserves during flowering and fruit production. Tap roots
are particularly long in certain desert plants, which tap water sources located far
below the ground.
 Most absorption of water and minerals in both monocots and dicots occur near
the root tips where vast numbers of tiny root hairs increase the surface are of the
root enormously. Root hairs are extensions of individual epidermal cells on the
root surfaces.
 In addition to roots that extend from the base of the shoot, some plants have roots
arising above ground from stems or even leaves. Such roots are said to be
adventitious. The adventitious roots of some plants including corn, function as
props that help support tall stems.
STEMS
 A stem is an alternating system of nodes, the point at which leaves are attached
and internodes, the stem segment between nodes. In the angle (axil) formed by
each leaf and stem is an axillary bud, a structure that has the potential to form a
vegetative branch.
 Most axillary buds of a young shoot are dominant thus growth of shoot is usually
concentrated at its apex where there is a terminal bud with developing leaves and
a compact series of nodes and internodes.
 The presence of terminal buds is partly responsible for inhibiting the growth of
axillary buds, a phenomenon called apical dominance.
 By concentrating resources on growing taller, apical dominance is an
evolutionary adaptation that increases the plant’s exposure to light. A growing
axillary bud gives rise to a vegetative branch complete with its own terminal bud,
leaves, axillary buds. This is the rationale for prunning trees and shrubs.
 Modified shoots with diverse function have evolved in many plants. These
modified shoots include stolons, rhizomes, and bulbs.
LEAVES
 They are the main photosynthetic organs of most plants, although green plants
also perform photosynthesis.
 Leaves vary extensively in forms, but they generally consist of a flattened blade
and a stalk, the petiole which joins the leaf to the node of the stem. Grasses and
many other monocots lack petiole, instead the base of the leaf forms a sheath that
envelopes the stem.

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 The leaves of monocots and dicots differ in the arrangement of their major veins
that run the length of the leaf blade. In contrast, dicot leaves generally have a
multi branched network of major veins.
 Each organs of a plant has three tissue systems:
The dermal tissue or epidermis
 The dermal tissue or epidermis is generally a single layer of tightly packed cells
that covers and protects all young organs. The epidermis of leaves and most
stems secrete a waxy coating called the cuticle that helps the aerial parts of the
plant to retain water. An important adaptation to living on land.
Vascular tissue
 Vascular tissue is continuous throughout the plant and is involved in the transport
of materials between roots and shoots.
 The two types of vascular tissue are xylem, which conveys water and dissolved
minerals upwards from the roots into the shoots, and phloem which transports
food made in mature leaves to the roots and to non-photosynthetic parts of the
shoot system such as developing leaves and fruits.
 Both xylem and phloem are composed of a variety of cell types.
 The water-conducting element of xylem, the tracheids and vessel elements, are
elongated cells that are dead at functional maturity.
 In the phloem, sucrose, other organic compounds and some minerals are
transported through tubes formed by chains of cells called sieve-tube members.
 In angiosperms, the end walls between sieve-tube members called sieve plates,
have pores that presumably facilitate the flow from cell to cell along the sieve
tube.
 Alongside each sieve-tube member is a non-conducting cell called a companion
cell.


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

Ground tissue
 Ground tissue is tissue that is neither dermal tissue nor vascular tissue. In dicot
stems ground tissue is divided into pith and cortex.
 Plant tissues are composed of three basic cell types; parenchyma, collenchymas
and sclerenchyma.

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 A multicellular organism is characterised by a division of labour among cell types
specialized for different functions.
Parenchyma cells
 Mature parenchyma cells have primary walls that are relatively thin and flexible,
and most lack secondary walls. The protoplast generally has a large vacuole.
 Parenchyma cells are often depicted as “typical” plant cells because they
generally are the least specialized but there are exceptions e.g. the highly
specialized cells that function in the transport of sugar sap in the phloem- the
sieve tube members.
 Parenchyma cells perform most of the metabolic functions of the plant,
synthesizing and storing various products e.g. photosynthesis occurs within the
chloroplast of parenchyma cells in the leaf.
 Some parenchyma cells in stems and roots have colourless plastids that store
starch. And the fleshy tissue of most fruits is composed mostly of parenchyma
cells.
 Developing plant cells of all types are parenchyma cells before specializing
further in structure and function.
 Those cells that retain the less specialized condition and become mature
parenchyma cells do not generally undergo cell division.
 Most of them, however retain the ability to divide and differentiate into other
types of plant cells under special conditions- during the repair and replacement of
organs after injury to the plant.
Parenchyma cells
Collenchyma cells
 Collenchyma cells have thicker primary walls than parenchyma cells, though the
walls are unevenly thickened.
 Grouped in strands or cylinders, collenchyma cells help support young parts of
the plant shoot. Because they lack secondary walls and the hardening agent lignin

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is absent in their primary walls, collenchyma cells provide support without
restraining growth.
 Mature functioning collenchyma cells are living and flexible and elongate with
the leaves and stems they support.
Collenchyma cells
Sclerenchyma cells
 They function as supporting elements in the plant. They have thick secondary
walls usually strengthened by lignin. Mature sclerenchyma cells cannot elongate
and they occur in regions of the plant that have stopped growing in length.
 So specialized are sclerenchyma cells for support that many are dead at functional
maturity, but they produce secondary walls before the protoplast dies.
 The water-conducting vessel elements and tracheids in the xylem are
sclerenchyma cells with dual functions: support and transport. There are also two
types of sclerenchyma cells called fibres and sclereids that specialize entirely in
support.
Sclerenchyma cells

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 The growth and development of plants is not fast limited to an embryonic or
juvenile period, but occurs throughout the life of the plant. At any given instance,
a typical plant consists of embryonic organs, developing organs and mature
organs.
 Growth is the irreversible increase in mass that results from cell division and cell
expansion. Development is the sum of all of the changes that progressively
elaborate an organism’s body.
 Most plants continue to grow as long as the plant lives, a condition known as
indeterminate growth.
 In contrast, certain plant organs such as flowers and leaves undergo determinate
growth i.e. they cease growing after reaching a certain size.
 For as long as it survives, a plant is capable of indeterminate growth because it
has perpetually embryonic tissues called meristems in its regions of growth.
Meristematic cells divide to generate additional cells.
 Some of the products of this division remain in the meristematic region to
produce still more cells while others become specialized and are incorporated into
the tissues and organs of the growing plant.
 The pattern of plant growth depends on the locations of the meristems. Apical
meristems, located at the tips of roots and in the buds of shoots, supply cells for
the plant to grow in length.
 This elongation called primary growth enables roots to increase their exposure
to light and carbon dioxide.
 In herbaceous plants, only primary growth occurs. In woody plants, however,
there is also secondary growth, a progressive thickening of the roots and shoots
formed earlier by primary growth.
 Secondary growth is the product of lateral meristems, cylinders of dividing cells
extending along the length of roots and shoots


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Nutrient Elements
The term essential element (or mineral nutrient) was proposed by Arnon and Stout
(1939).
They concluded three criteria must be met for an element to be considered essential.
These three criteria are:
i. A plant must be unable to complete its life cycle in the absence of mineral
element.
ii. The function of the element must not be replaceable by another mineral element.
iii. The element must be directly involved in plant metabolism.
 These criteria are important guidelines for plant nutrition but exclude beneficial
mineral element.
 Beneficial elements are those that can compensate for toxic effects of other
element or may replace mineral nutrients in some other less specific function
such as the maintenance of osmotic pressure.
 The omission of beneficial nutrients in commercial production could mean that
plants are not being grown to their optimum genetic potential but are merely
produced at a subsistence level.
 There are about 20 mineral elements necessary or beneficial for plant growth.
C,H,O are supplied by air and water. The six macro nutrients: N,P,K,Ca,Mg,S are
required by plants in large amounts.
 The rest of the elements are required in trace amounts (micronutrients).Essential
trace elements include B, Cl,Cu,Fe,Zn, Mo and Ni.
 Beneficial elements include Si,Co
 The absorption of mineral ions is dependent on a number of factors in addition to
weather conditions. These include the action exchange, pH of the growing
medium and the total alkalinity of the irrigation water
 Nitrogen
 Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and
enzymes essential for plant life.
 Nitrogen metabolism is a major factor in stem and leaf growth (vegetative
growth).Too much can delay flowering and fruiting.
 Deficiencies can reduce yield, cause yellowing of the leaves and stunt growth.
Macro nutrients

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Nitrogen
 Nitrogen is a major component of proteins, hormones, chlorophyll, vitamins and
enzymes essential for plant life.
 Nitrogen metabolism is a major factor in stem and leaf growth (vegetative
growth).Too much can delay flowering and fruiting.
 Deficiencies can reduce yield, cause yellowing of the leaves and stunt growth.
Phosphorus
 Phosphorus is necessary for seed germination, photosynthesis, protein formation
and almost all aspects of growth and metabolism in plants.It is essential for
flower and fruit formation
 Low PH(<4) results in phosphate being chemically locked up in organic soils.
 Deficiency symptoms are purple stems and leaves; maturity and growth are
retarded.
 Yields of fruit and flowers are poor. Premature drop of fruits may often occur.
 Phosphorus must be applied close to the plant's roots in order for the plant to
utilize it.
 Large application of phosphorus without adequate level of zinc can cause a zinc
deficiency.
Potassium
 Potassium is necessary for formation of sugars, starches, carbohydrates, protein
synthesis and cell division in roots and other parts of the plant.
 It helps to adjust water balance, improves stem rigidity and cold hardiness,
enhances flavor and color on fruit and vegetable crops, increases the oil content
of fruits and is important for leafy crops.
 Deficiencies result in low yields, mottled, spotted or curled leaves, scorched or
burned look to leaves.
Sulfur
 Sulfur is a structural component of amino acids, proteins, vitamins and enzymes
and is essential to produce chlorophyll. It imparts flavour to many vegetables.
 Deficiencies show as light green leaves. Sulfur is readily lost by leaching from
soils and should be applied with a nutrient formula.
 Some water supplies may contain sulfur.
Magnesium

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 Magnesium is a critical structural component of the chlorophyll molecule and is
necessary for functioning of plant enzymes to produce carbohydrates, sugars and
fats.
 It is used for fruit and nut formation and essential for germination of seeds.
 Deficient plants appear chlorotic,show yellowing between veins of older leaves;
leaves may droop.
 Magnesium is leached by watering and must be supplied when feeding. It can be
applied as a foliar spray to correct deficiencies.
Calcium
 Calcium activates enzymes ,is a structural component of cell walls, influences
water movement in cells and is necessary for cell growth and division.
 Some plants must have calcium to take up nitrogen and other minerals
 Calcium is easily leached. Calcium, once deposited in plant tissue, is immobile
(non- translocatable) so there must be a constant supply for growth.
 Deficiency causes stunting of new growth in stems,flowers and roots.
 Symptoms range from distorted new growth to black spots on leaves and fruit.
Yellow leaf margins may also appear.

Micronutrients
Iron
 Iron is necessary for many enzyme functions and as catalyst for the synthesis of
chlorophyll. It is essential for the young growing parts of plants.
 Deficiencies are pale leaf color of young leaves followed by yellowing of leaves
and large veins.
 Iron is lost by leaching and is held in the lower portions of the soil structure.
Under conditions of high pH (alkaline) iron is rendered unavailable to plants.
 When soils are alkaline,iron may be abundant but unavailable.
 Application of an nutrient formula containing iron chelates,held in soluble form,
should correct the problem.
Manganese
 Manganese is involved in enzyme activity for photosynthesis, respiration, and
nitrogen metabolism.
 Deficiency in young leaves may show a network of green veins on a light green
background similar to an iron deficiency.

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 In the advanced stages the light green parts become white, and leaves are
shed.Brownish, black, or grayish spots may appear next to the veins.
 In neutral or alkaline soils plants often show deficiency symptoms.In highly acid
soils, manganese may be available to the extent that it results in toxicity.
Boron
 Boron is necessary for cell wall formation, membrane integrity, calcium uptake
and may aid in the translocation of sugars. Boron affects at least 16 functions in
plants.
 These functions include flowering, pollen germination, fruiting, cell division,
water relationships and the movement of hormones.
 Boron must be available throughout the life of the plant. It is not translocated and
is easily leached from soils.
 Deficiencies kill terminal buds leaving a rosette effect on the plant.
 Leaves are thick, curled and brittle. Fruits, tubers and roots are discolored,
cracked and flecked with brown spots.
Zinc
 Zinc is a component of enzymes or a functional cofactor of a large number of
enzymes including auxins (plant growth hormones). It is essential to carbohydrate
metabolism, protein synthesis and internodal elongation (stem growth).
 Deficient plants have mottled leaves with irregular chlorotic areas. Zinc
deficiency leads to iron deficiency causing similar symptoms.
 Deficiency occurs on eroded soils and is least available at a pH range of 5.5 -
7.0.Lowering the pH can render zinc more available to the point of toxicity.
Copper
 Copper is concentrated in roots of plants and plays a part in nitrogen metabolism.
It is a component of several enzymes and may be part of the enzyme systems that
use carbohydrates and proteins.
 Deficiencies cause die back of the shoot tips, and terminal leaves develop brown
spots.
 Copper is bound tightly in organic matter and may be deficient in highly organic
soils. It is not readily lost from soil but may often be unavailable. Too much
copper can cause toxicity.

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Molybdenum
 Molybdenum is a structural component of the enzyme that reduces nitrates to
ammonia. Without it, the synthesis of proteins is blocked and plant growth
ceases.
 Root nodule (nitrogen fixing) bacteria also require it. Seeds may not form
completely, and nitrogen deficiency may occur if plants are lacking molybdenum.
 Deficiency signs are pale green leaves with rolled or cupped margins.
Chlorine
 Chlorine is involved in osmosis (movement of water or solutes in cells), the ionic
balance necessary for plants to take up mineral elements and in photosynthesis.
 Deficiency symptoms include wilting, stubby roots, chlorosis (yellowing) and
bronzing. Odors in some plants may be decreased. Chloride, ionic form of
chlorine used by plants, is usually found in soluble forms and is lost by leaching.
 Some plants may show signs of toxicity if levels are too high
Nickel
 Nickel has just recently won the status as an essential trace element for plants
according to the Agricultural Research Service Plant, Soil and Nutrient
Laboratory in Ithaca, NY.
 It is required for enzyme urease to break down urea to liberate the nitrogen into a
usable form for plants. Nickel is required for iron absorption. Seeds need nickel
in order to germinate.
 Plants grown without additional nickel will gradually reach a deficient level at
about the time they mature and begin reproductive growth. If nickel is deficient
plants may fail to produce viable seeds.
Sodium
 Sodium is involved in osmosis (water movement) and ionic balance in plants.
Cobalt
 Cobalt is required for nitrogen fixation legumes and in root nodules of
nonlegumes.

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 The demand for cobalt is much higher for nitrogen fixation than for ammonium
nutrition. Deficient level could result in nitrogen deficiency symptoms.
Silicon
 Silicon is found as a component of cell walls. Plants with supplies of soluble
silicon produce stronger, tougher cell walls making them a mechanical barrier to
piercing and sucking insect.
 This significantly enhances plant heat and drought tolerance. Foliar sprays of
silicon have also shown benefits reducing population of aphids on field crops.
 Test have also found that silicon can be deposited by the plants at the site of
infection by fungus to combat the penetration of cell wall by attacking fungus.
 Improved leaf erectness, stem strength and prevention or depression of iron and
manganese toxicity have all been noted as effects from silicon.
 Silicon has not been determined essential for all plants but beneficial for many.

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GROWTH ANALYSIS
 The collection of a series of growth measurements from an individual plant or a
crop and their subsequent examination and interpretation is called Growth
Analysis.
The measurements taken from an individual plant are generally different to those
taken from a crop (a plant community).
Those measurements taken from an individual plant include
 Relative or Absolute Growth Rate
 Unit Leaf Area Rate or Net Assimilation rate.
 Leaf Area Ratio
 Specific Leaf Area
 Specific Leaf Weight.
The measurements taken from a crop include
 Leaf area index
 Leaf area duration
 Net assimilation Rate
 Crop Growth Rate
 Harvest Index
RELATIVE GROWTH RATE
Relative Growth expresses for an individual plant , the dry weight increases in a time
interval in relation to calculated using the equation
W=W₀ e^rt = (log(w2) - log(w1))/(t2 - t1)
Where
W= Weight at end of Time Interval
W₀ = Weight at beginning of time interval
e = natural logarithm base (2.71828)
r = RGR
t = time interval

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Since the growth of the plant follows a simoid curve pattern,PGR is highest near the
beginning of the plant’s life.It continues to slow after reaching its innitial peak.
LEAF AREA INDEX(LAI)
Leaf Area Index is determined by taking a certain area of ground (occupied by atleast
two plants of a crop) and then finding the leaf area on the plants in that certain area of
Ground.Thus
LAI = Area of Leaf (LA)/Area of Ground (AG)
Leaf Area can be measured by a number of methods but the two main methods
currently in use are
 A photoelectric device that computes leaf area as individual Leaves are fed into
it.
 A mathematical model can be obtained by correlating Length X width (LW),
Length squared(L^2) or Width^2 to the actual area(A) of a sample of leaves using
Regression Analysis. Mathemathical Models have been worked out for a number
of species including Taro and Corn.
LEAF AREA DURATION(LAD)
 If Leaf area index (LAI) is plotted against Time , the area under the curve is the
measure of the magnitude and persistent (duration) of Leaf Area or Leafiness
during the period of Crop Growth. It reflects the Extent of Light interception in a
crop and can be highly correlated with yield.
 Leaf Area duration can be measured exactly by calculus methods or by using a
computer program. It can be measured simply but not quiet exactly as follows.
LAD=(LA2+LA1)(T2-T1)/2
LAD=(L2 +L1)(T2-T1)/2
NET ASSIMILATION RATE(NAR)
 In Growth analysis studies , leaf area of a crop is measured because the leaves
receive energy from the sun . If we know leaf area and the change in crop weight
over a limited period , then it can be calculated how effective these leaves have
been in converting energy to assimilate.
 In an experiment or on the farm , it can then be seen whether some treatments
applied to a crop are more efficient than others in converting energy or whether

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efficiency merely changes with time . This measure of efficiency is termed Net
Assimilation Rate(NAR).
NET ASSIMILATION RATE CONT’D
 It is usual to use Leaf Areas when calculating NAR but other measures
photosynthetic potential can be used ;Leaf Weight , Leaf protein or Nitrogen
content which give a slightly different NAR value.
The Equation used for calculating NAR is
NAR = (W2-W1)/T2-T1 X loge LA2-loge LA1/LA2-LA1
Note: the log transformation is necessary because Crop growth should be considered
as being exponential over its own life.
 NAR gives a measure of the rate (per day , week)of increase in dry weight per
unit leaf area.eg.gm/cm^2(of leaf)/day ;Kg/m^2(of leaf)/week.
CROP GROWTH RATE(CGR)
 As net Assimilation rate(NAR) is the rate of increase in weight per unit area of
leaf ,and leaf area index (LAI) is the leaf area per unit area of ground ,it follows
that the gain in weight of a community of plants on a unit of land in a unit of time
i.e. crop growth rate (CGR) is equal to NAR X LAI.
HARVEST INDEX
 The Harvest index is expressed as a ratio of seed weight/plant weight. It has many
similarities to the partitioning coefficient mentioned just above. The harvest index
appears sometimes to be expressed as a percentage.