Download In 1945, 2,4-D was introduced as one of the first

Mechanism Of Oxidative Stress In Plants And Its Regulation
INTRODUCTION
Environment is a set of relationships between livings and non-livings, when man and
other higher animals and plants began their life on this earth, there was no sign of
environmental degradation. There was perfect balance in various natural processes. Every
living species of plants and animals influences its environment and in turn gets
influenced by it. The magnitude of such influences are not usually high in these species
because of the fact that due to natural control their population cannot rise beyond certain
limits and even they cannot modify their own way of life. Stone age man was fully
dependent on this environment but the introduction of metal tool, chemicals,
industrialization and urbanization by man become the sign of extinction and exploitation
by man of environment and man got a grip on the environment. Now the environment
was at the mercy of man. Generally many health hazardous metals are found in the rock
and enter in our food chain through weathering process or anthropogenic disruption. The
use of these heavy metal in making our lifestyle goods and entrance of their waste in our
environment is the main problems at present. Otherwise many chemical such as herbicide
used to control unwanted weeds in our agricultural field and lawn vice versa is also
harmful to our environment and their biodiversity. This study was mainly based on
oxidative stress for which some monocot and dicot plants were selected and studied the
effect of heavy metal cadmium (Cd), herbicide (2,4-D) and light.
1.1 OXIDATIVE STRESS:
Molecular oxygen is generally unreactive (Elstner, 1987) due to its electron
configuration. An unbalanced metabolism of O2 leads to production of reactive oxygen
species (ROS). ROS include free radicals such as superoxide anion (O2•−), hydroxyl
radical (•OH), as well as nonradical molecules like hydrogen peroxide (H2O2), singlet
oxygen (1O2), and so forth. Stepwise reduction of molecular oxygen (O2) by high-energy
exposure or electron-transfer reactions leads to production of the highly reactive ROS.
Activation of oxygen is energy dependent and requires an electron donation. The
subsequent one electron reduction steps are not energy dependent and can occur
1
Mechanism Of Oxidative Stress In Plants And Its Regulation
spontaneously. In biological system transition metal ions (Fe2+, Cu+) and semiquinone
can act as e- donars.
In plants, ROS are always formed by the leakage of electrons onto O2 from the electron
transport activities of chloroplasts, mitochondria, and plasma membranes or as a
byproduct of various metabolic pathways localized in different cellular compartments
(Foyer and Harbinson, 1994; Foyer, 1997 and Del Rio, 2006). Environmental stresses
such as drought, salinity, chilling, metal toxicity, and UV-B radiation as well as
pathogens attack lead to enhanced generation of ROS in plants (Mittler, 2002; Sharma
and Dubey, 2005; Hu et al. 2008; Maheshwari and Dubey, 2009 and Srivastava and
Dubey, 2011). All ROS are extremely harmful to organisms at high concentrations. When
the level of ROS exceeds the defense mechanisms, a cell is said to be in a state of
“oxidative stress.” The enhanced production of ROS during environmental stresses can
pose a threat to cells by causing peroxidation of lipids, oxidation of proteins, damage to
nucleic acids, enzyme inhibition, activation of programmed cell death (PCD) pathway
and ultimately leading to death of the cells (Shah et al. 2001; Sharma and Dubey, 2005;
Maheshwari and Dubey, 2009; Srivastava and Dubey, 2011 and Meriga, et al. 2004).
1.2 THE ACTIVATION STATES OF OXYGEN:
Molecular oxygen is a biradical and can be activated by either reversing the spin on one
of the unpaired electrons to form the singlet state or by reduction. The first reduction
reaction is endothermic forming superoxide. Subsequent reductions form hydrogen
peroxide, hydroxyl radical and water. The electronic state for each activation step is
shown with the energy of the reaction in Kcal/mole.
2
Mechanism Of Oxidative Stress In Plants And Its Regulation
The univalent reduction of superoxide produces hydrogen peroxide which is not a free
radical because all of its electrons are paired. Hydrogen peroxide is noteworthy because it
readily permeates membranes and it is therefore not compartmentalised in the cell.
Numerous enzymes (peroxidases) use hydrogen peroxide as a substrate in oxidation
reactions involving the synthesis of complex organic molecules. The well-known
reactivity of hydrogen peroxide is not due to its reactivity per second, but requires the
presence of a metal reductant to form the highly reactive hydroxyl radical which is the
strongest oxidizing agent known and reacts with organic molecules at diffusion-limited
rates. Fenton, 1894; 1899 described in the late nineteenth century, the oxidising potential
of hydrogen peroxide mixed with ferrous salts. Forty years later, Haber and Weiss (1934)
identified the hydroxyl radical as the oxidising species in these reactions:
(1)
In biological systems the availability of ferrous ions limits the rate of reaction, but the
recycling of iron from the ferric to the ferrous form by a reducing agent can maintain an
ongoing Fenton reaction leading to the generation of hydroxyl radicals.
3
Mechanism Of Oxidative Stress In Plants And Its Regulation
(2)
(3)
(4)
Therefore, in the presence of trace amounts of iron, the reaction of superoxide and
hydrogen peroxide will form the destructive hydroxyl radical and initiate the oxidation of
organic substrates. Metals other than iron may also participate in these electron transfer
reactions by cycling between oxidised and reduced states. The oxidation of organic
substances may proceed by two possible reactions A addition of OH to the
organic molecule or abstraction of a hydrogen atom from it. In the addition reaction
(reaction 5), the hydroxyl radical adds to an organic substrate forming a hydroxylated
product that is further oxidised by ferrous ions, oxygen or other agents to a stable,
oxidised product (reactions 6 and 7). The hydroxylated products can also dismutate to
form cross-linked products (reaction 8).
(5)
(6)
(7)
(8)
In the abstraction reaction, the hydroxyl radical oxidises the organic substrate forming
water and an organic radical (reaction 9). The latter product has a single unpaired
electron and thus can react with oxygen in the triplet ground-state (reaction 10). The
addition of triplet oxygen to the carbon radical can lead to the formation of a peroxyl
radical which can readily abstract hydrogen from another organic molecule leading to the
formation of a second carbon radical (reaction 11). This chain reaction is why oxygen
free radicals cause damage far in excess of their initial concentration.
4
Mechanism Of Oxidative Stress In Plants And Its Regulation
(9)
(10)
(11)
1.3 REGULATION OF ROS THROUGH ENZYMATIC AND NON-ENZYMATIC
ANTIOXIDATIVE COMPONENTS:
Rather than destructive activity, ROS are well-described as second messengers in a
variety of cellular processes including tolerance to environmental stresses (Neill, et al.
2000 and Yan et al. 2007). Whether ROS will act as damaging or signaling molecule
depends on the delicate equilibrium between ROS production and scavenging. To control
the level of ROS cell possess a complex set of enzymes and non-enzymatic antioxidative
component which protect cells from oxidative injury (Yan et al. 2007). The enzymic
antioxidants include superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase
(GPX), enzymes of ascorbateglutahione (AsA-GSH) cycle such as ascorbate
peroxidase(APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate
reductase (DHAR), and glutathione reductase (GR) (Noctor and Foyer, 1998). Ascorbate
(AsA), glutathione (GSH), carotenoids, tocopherols, amino acids and phenolics serve as
potent nonenzymic antioxidants within the cell. Various workers have reported increased
activities of many enzymes of the antioxidant defense system in plants to combat
oxidative stress induced by various environmental stresses. Maintenance of a high
antioxidant capacity to scavenge the toxic ROS has been inked to increased tolerance of
plants to these environmental stresses (Zaefyzadeh et al. 2009 and Chen et al. 2010).
Considerable progress has been made in improving stress-induced oxidative stress
tolerance in crop plants by developing transgenic lines with altered levels of antioxidants
(Allen, et al. 1997 and Faize, et al. 2011). Simultaneous expression of multiple
antioxidant enzymes has been shown to be more effective than single or double
expression for developing transgenic plants with enhanced tolerance to multiple
environmental stresses (Lee, et al. 2007).
5
Mechanism Of Oxidative Stress In Plants And Its Regulation
1.4 LIGHT AND OXIDATIVE STRESS:
Sun light is the key factor in photosynthesis, a process vital for life on earth. Besides
photosynthesis, it helps in the synthesis of melanin and vitamin D in animals. It also
helps in the development and differentiation. Beside beneficial role, high intensities of
light perform several degenerative roles by the production of reactive oxygen species.
Light rays produce excited states in molecules due to the absorption of one or more
photons. Excited-states molecules can react with adjacent molecules and excite them. So
a number of chain reactions, producing reactive oxygen species are generated. High
irradiance of light on plants reduces the capacity of photosynthesis and cause oxidative
stress through the formation of reactive oxygen species (ROS). These include superoxide
radicals (O-2), singlet oxygen, hydrogen peroxide (H2O2) and hydroxyl radicals, which
cause tissue injury (Foyer et al., 1994). As described by Elstner (1991), there are at least
four sites within the chloroplast that can activate oxygen. Plants have evolved various
protective mechanisms to eliminate or reduce ROS. These ROS species are highly toxic
for biomolecules such as lipid, protein and nucleic acid through lipid peroxidation,
protein denaturing and DNA mutation, respectively (Scandalions, 1993; Van Breusegen
et al., 2003; Quiles and Lopez, 2004; Foyer and Noctor, 2005; Guo et al., 2006 and
Moller et al., 2007). When plants are exposed to high light intensities, high or low
temperature, or ozone or air pollution, more reactive oxygen species are produced than
the scavenging mechanisms can detoxify (Alscher, 1997). Moreover ROS can also be
formed in metabolic pathways. The chloroplast possesses an elaborate system for
scavenging ROS, which comprises both enzymatic and non-enzymatic compound (Foyer
et al.1994, Asada, 1996). APX scavenges the hydrogen peroxide generated by the action
of SOD and thereby prevents the chemical formation of other toxic oxygen species
(Asada, 1994). The accumulation of H2O2 is prevented by the activity of catalase. In
addition, one of the most damaging effect of activated oxygen and their product in cells is
the peroxidation of membrane lipids which leads to ion leakage (Gratao et al., 2005 and
Lee et al., 2007). Evidence suggests that membranes are one of the primary sites of cells
and organelle injuries (Candan and Tarhan, 2003). This is because ROS can react with
unsaturated fatty acids to cause peroxidation of essential membrane lipids (Karubal et al.,
2003). Besides, antioxidative responses plants display the ability to adjust their
6
Mechanism Of Oxidative Stress In Plants And Its Regulation
performance by alteration in morphology and physiology in response to environmental
variations (Sultan, 1995). Moreover, ROS are inevitable by-product of normal cell
metabolism (Martinez et al., 2001). Although under normal conditions, production and
destruction of ROS are well regulated in cell metabolism (Mittler, 2002). When a plant is
encountered with harsh conditions, ROS production will overcome scavenging system
and consequently oxidative stress will dominate. Varying intensities of light affect plant
growth and development. Reduction in chlorophyll content, due to breakdown of the
structural integrity of chloroplast on exposure to excess light was found in some
experiments (Rhizopaulou, 1991). This result was also testified in similar experiment by
(Valladares and Pearcy, 1998). Given the role of increased chlorophyll content in
improving light harvesting capacity, the increased chlorophyll content in shades leaves
has been interpreted as an adaptation to low light environment. However, in some
experiments (Boardman, 1977) it is reported that the leaf chlorophyll content in responses
to shade acclimation might decrease. Light affects the net photosynthesis of plants
subsequently plant height, fresh weight, dry weight, as well. Anderson (1986) reported
decreased rate of net photosynthesis in seedlings grown in shade. Plants growing in high
light intensity in their native habitat have a high capacity for photosynthesis at a
saturative light intensity, and they show lower rates of net-photosynthesis than shaded
plants at low light intensities.
1.5
2,4-DICHLOROPHENOXYACETIC
ACID
(2,4-D)
AND
OXIDATIVE
STRESS:
In 1945, 2,4-D was introduced as one of the first selective herbicides. 2,4-D, a member of
the phenoxy family of herbicides, rapidly became the most widely used herbicide in the
world. After 50 years of use, 2,4-D is still the third most widely used herbicide in the
United States and Canada, worldwide. The most common use of 2,4-D, is post-emergent
weed control in agricultural crops. Its major uses in agriculture are on wheat and small
grains, sorghum, corn, rice, sugar cane, soybeans, rangeland, and pasture. 2,4-D can enter
the environment through discharge and spills arising from a variety of different
manufacturers, transporters and through direct application as a weed control agent. Injury
to off-target vegetation is a major problem associated with this herbicide. Herbicides used
7
Mechanism Of Oxidative Stress In Plants And Its Regulation
in agriculture are the classic example of anthropogenic chemicals that enter the Earth’s
environment in large amounts via non-point sources. These compounds can adversely
affect non-target organisms, and may be detrimental to human health if people are
exposed by direct contact or to residues of the molecules in soil, water and agricultural
products Water, Sanitation and Health (1998). Occupational exposure to 2,4-D has
produced serious problems like diarrhea, temporary loss of vision, respiratory tract
irritation, confusion, numbness and tingling, bleeding and chemical hypersensitivity
Prescott and John, (1996). 2,4-D enters the body through inhalation and the skin during
occupational exposure. Its mechanism of action is related to uncoupled oxidative
phosphorylation and decreased oxygen consumption in tissues, as well as to disturbances
in carbohydrate and other metabolic processes Water, Sanitation and Health (1998). Once
absorbed 2,4-D is translocated within the plant and accumulates at the growing points of
roots and shoots where it inhibits growth Top et al. (1998). 2,4-D destroyed plants,
through over producing reactive oxygen species, by inhibiting antioxidativ defense
system of the plants and by membrane damage and their dysfunction. Symptoms of 2,4-D
vary with the different commercial products because of the specific amounts and types of
additives such as surfactants and solvents. Only poor occupational practices make
possible massive dermal and inhalation overexposure with signs and symptoms of acute
or chronic intoxication. 2,4-D was a major component (about 50%) of the product Agent
Orange used extensively throughout Vietnam war. However most of the problems
associated with the use of Agent Orange were associated with a contaminant (dioxin) in
the 2,4,5-T component of the defoliant. Vietnam estimates 400,000 were killed, and
500,000 children born with birth defects as a results of Agent orange use York,
(2008).The association of 2,4-D with Agent Orange has prompted a vast amount of study
on the herbicide. The project was undertaken to examine 2,4-D toxicity in black gram
plants and its mode of action.
1.6 CADMIUM (CD) AND OXIDATIVE STRESS:
Cadmium (Cd) is a widespread trace pollutant released into the soil by industrial
processes or by the utilization of fertilizers and pesticide content (Prasad, 2004). The
main source of soil and water Cd contamination are especially fertilizers (phosphate
8
Mechanism Of Oxidative Stress In Plants And Its Regulation
fertilizers), but also pesticides, fungicides, and sewage (Mirlean and Roisenberg 2006;
Zarkovic and Blagojevic 2007; Zhao and Masaihiko 2007; Yildiz et al., 2008; Hadlich
and Ucha 2010; McGrath and Tunney 2010). Plants like crops and vegetables cultivated
for human consumption uptake the Cd from contaminated soil and water (Jafarnejadi et
al. 2011; Khodaverdiloo et al., 2011 and Moustakas et al., 2011). Cadmium is one of the
most toxic heavy metal due to its great solubility in water (Lockwood, 1976). At higher
concentration, it characteristically inhibits growth of different plant species such as maize
(Lagriffoul et al., 1998), barley and wheat (Talanova et al., 2001), and garlic (Liu et al.,
2003/2004). Cd can be accumulated in leaves and caused phytotoxic effects on plant,
such as chloroplast structure change, reactive oxygen species (ROS) production and cell
death (Cuypers et al., 2011). Cd was found to produce oxidative damage of lipids and
nucleic acids (Sandalio et al., 2001; Romero-Puertas et al., 2003; Lee and Shin, 2003 and
Watanabe et al., 2003). The damage caused ROS is known as oxidative stress. Life under
aerobic condition is intimately linked with ROS production. Indeed, ROS are generated
as by-products of the most essential energy generating processes such as photosynthesis
and respiration. Chen et al. (2011) have reported declined net photosynthetic rate and
stomatal conductance in musterd following to Cd exposure. Together with an extensive
battery of oxidases, chloroplast, peroxisomes, and mitochondria are the main organelles
of ROS producer (Vranova et al., 2002; Apel and Hirt, 2004). Despite efficient
antioxidant machinery in these organelles, subtle changes in ROS homeostasis are
inevitable (Foyer and Nocter, 2005). When the increase in ROS is relatively small, the
housekeeping antioxidant capacity is sufficient to reset the original balance between ROS
production and scavenging, thus re-establishing redox homeostasis. Under stress
conditions, the concentration of ROS generated as by-products of normal metabolism
increases to toxic levels and antioxidant machinery unable to maintain homeostasis.
Recently the production of ROS at the sub-cellular level was demonstrated in the leaves
of Pisum sativum plants treated with Cd (Romero Puertas et al., 2004 and Rodriguez et
al. 2006). Plants possess several antioxidative defense systems to scavenge toxic free
radicals in order to protect themselves from the oxidative stress induced by heavy metals.
The antioxidative defense system falls into two categories: (1) low molecular weight
antioxidants, like (Non-protien thiol group, proline, cystien etc); and (2) antioxidative
enzymes: superoxide dismutase (SOD), peroxidase (POX), catalase (CAT), and
9
Mechanism Of Oxidative Stress In Plants And Its Regulation
glutathione reductase (GR) etc. Previous studies have shown that NP-SH and cysteine are
involved in the synthesis of a number of polypeptide which are involved in the metal
detoxification by binding and compartmentalizing them in vacuoles. Such polypeptide,
are the metalothionine (MTs) and phytochyletins (PCs). MTs are small gene encoded
cysteine-rich polypeptide, and the PCs, which in contrast are enzymatically synthesized,
cysteine rich peptides (Cobbett, 2000). MTs were first identified as Cd-binding proteins
(Robinson et al., 1993; Cobbett and Goldsbrough, 2002). PCs were first identified in
1983 in the yeast (Cobbett, 2000) and numerous physiological and biochemical studies
have confirmed that were synthesized from a number of glutathione variant such as
(GSH; γ-Glu-Cys-Gly). Therefore the synthesis of cysteine is correlated with plant
tolerance against the heavy metal toxicity. Proline is an important amino acid which acts
as an osmoregulator whose is synthesis increased under stress conditions. The role of
proline in cell osmotic adjustment, membrane stabilization and detoxification of injurious
ions in plants exposed to salt stress is widely reported (Hare et al., 1999; Kavi Kishor et
al., 2005; Ashraf and Foolad, 2007). In response to heavy metal toxicity plants have
developed antioxidative enzymatic defense system which include catalase (CAT),
peroxidase (POX) and superoxide dismutase (SOD). There is numbers of studies that
suggested toxic nature of Cd in plants (Shaw, 1995; Luo et al., 1998; Schickler and
Caspi, 1999; Hegedus et al., 2001; Schutzendubel et al., 2001 and Olmos et al., 2003).
Superoxide anion radicals produced in different cell compartments are rapidly converted
into H2O2 in a reaction catalyzed by SOD (Noctor and Foyer, 1998). ROS generated
under stress condition are very harmful for plant membrane and, membrane damage is
calculated by estimating malondialdehyde contents. A no. of reports on membrane
damage Mazhoudi et al. (1997); Nouairi et al. (2006) and Ouariti et al. (1997),
suggesting that Cd metal negatively affects the plant growth and development. Catalases
are synthesized in a tissue specific and age dependent manner and scavenge H2O2
generated during the photorespiration and β-oxidation of fatty acids (Lin and Kao, 2000).
The free proline has been found to chelate Cd ion in plants and form a non toxic Cd
proline complex (Sharma et al., 1998).The cumulative capacity of free proline is a
manifestation of the self protection ability of plants exposed to different metal stresses
(Sun et al., 2007). The higher proline production has recently been demonstrated to
correlate with increased metal tolerance in a transgenic alga (Siripornadulsil et al., 2002).
10
Mechanism Of Oxidative Stress In Plants And Its Regulation
1.7 PHOSPHORUS AND COPPER:
Phosphorus is one of the 17 essential elements required for plant growth (Bieleski, 1973
and Raghothama, 1999). The P concentration in plants ranges from 0.05 to 0.50% dry
weight. This element plays a role in an array of processes, including energy generation,
nucleic acid synthesis, photosynthesis, glycolysis, respiration, membrane synthesis and
stability, enzyme activation/inactivation, redox reactions, signaling, carbohydrate
metabolism and nitrogen fixation. Through various chemical reactions, it is incorporated
into organic compounds, including nucleic acids (DNA and RNA), phosphoproteins,
phospholipids, sugar phosphates, enzymes, and energy-rich phosphate compounds for
example, adenosine triphosphate (ATP). Movement of nutrients within the plant depends
largely upon transport through cell membranes, which requires energy to oppose the
forces of osmosis. Here again, ATP and other high energy P compounds provide the
needed energy. The most important chemical reaction in nature is photosynthesis. It
utilizes light energy in the presence of chlorophyll to combine carbon dioxide and water
into simple sugars, with the energy being captured in ATP. The ATP is then available as
an energy source for the many other reactions that occur within the plant, and the sugars
are used as building blocks to produce other cell structural and storage components (Zhou
et al., 1993). Copper is an essential metal for plants. It plays key roles in photosynthetic
and respiratory electron transport chains, in ethylene sensing, cell wall metabolism,
oxidative stress protection and biogenesis of molybdenum cofactor (Inmaculada, 2009).
1.8 THE SOIL:
Soil is a component of the lithosphere and biosphere system. It is a vital natural resource
on which supporting life systems and socio-economic development depends. The interest
in soil as a natural body originated from its ability to produce and sustain crops. Soils are
formed as a result of weathering of rocks and minerals. Soils are the surface mineral and
organic formations; always more or less colored by the humus, which constantly manifest
themselves as a result of the combined activity of the following agencies: living and dead
organisms (plant and animals), parent materials, climate and relief. Soil is a complex
medium, which provides minerals (organic and inorganic) and water to the flora. The
mineral components of soil derived from rocks. Chemically, soils have inorganic and
11
Mechanism Of Oxidative Stress In Plants And Its Regulation
organic components. The inorganic rich soil is also described as mineral soil. Mineral
soils are low in organic matter (1-10%). Soil containing more than 20% organic matters
is called organic soil. The major component of soil is air (20-30%), mineral (45%),
organic matter (5%), water (20-30%) and rest is pore space. The size groups are
arbitrarily chosen, for instance by the International Society of Soil Science (ISSS), as
clay (>2 microns), silt (2-20 microns), fine sand (20-200 microns), and coarse sand (2002000 microns). Organic components are major soil resources of phosphorus and sulphur
and also nitrogen. The organic components consist of organic tissue and partially
decomposed equivalents (un-decomposed roots and higher tops of plants) and humus
(decomposed products formed by the activity of microorganisms). Plants need sunlight as
well as nutrients for growth. The compounds of Al, Si, Ca, Mg, Fe, K and Na are chiefly
founds as inorganic components in soil. Besides these other inorganic components such
as B, Mn, Cu, Zn, Mo, Co, I, Cl, Fe etc. These components may be categorized as macronutrients (C, H, O, N, S, P, K, Mg and Ca) and micro-nutrients (Mo, Cu, Cl, B, Mn and
Zn). These inorganic components are the constituents of plant body which influence the
osmotic pressure of plant cells, pH, permeability etc. Macronutrients such as N, P, K, Ca
and Mg etc. are very essential for normal growth and yield of plants. The sources of
nitrogen are soil and atmosphere. Nitrogen is absorbed from the soil in the form of
nitrates, nitrites and ammonium salts. Atmospheric nitrogen can be fixed and made
available to the plants only by microbial activities. Nitrogen is an important component
of amino acids, purines, pyrimidines and co-enzymes and essential for the synthesis of
protein, chlorophyll and many enzymes. Nitrogen deficiency actually causes foliage
which turns sickly yellow in colour. Potassium is also an important component of soil. It
is an activator of enzymes, easily moves through membranes, involved in stomatal
opening and closing, uptake of water by roots, and essential for photosynthesis, starch
formation, translocation of sugar, protein synthesis, necessary for chlorophyll formation,
grain formation and also associated with disease and cold resistance. A plant which lacks
potassium will have stunted roots and leaves. Calcium is an essential nutrient which
contributes to cell growth in a plant. It is used in cell metabolism and has a high
contribution to cell division and cell wall strength. Magnesium is another essential
nutrient which plays a critical role in the chlorophyll. It helps plant to produce
chlorophyll which is used for the process of photosynthesis. Trace elements are required
12
Mechanism Of Oxidative Stress In Plants And Its Regulation
in a very small amount and also known as micronutrients e. g. copper, zinc, manganese,
sulphur, chloride, molybdenum, boron and iron. Copper is necessary for carbohydrate
and nitrogen metabolism Inadequate copper results in stunting of plants. Copper is also
required for lignin synthesis which is needed for cell wall strength and prevention of
wilting. Copper deficiency is mainly reported on sandy soils which are low in organic
matter and causes dieback of stems and twigs, yellowing of leaves, stunted growth and
pale green leaves. Copper uptake decreases if soil pH increases. Increased phosphorus
and iron availability in soils decreases copper uptake by plants. Zinc is an essential
component of various enzymes for energy production, protein synthesis, and growth
regulation. Zinc deficient plants also exhibit delayed maturity. Zinc is not mobile in
plants so zinc deficiency symptoms occur mainly in new growing plants. The most
visible symptoms of Zn deficiency are short internodes and a decrease in leaf size.
Uptake of Zinc is also adversely affected by high levels of available phosphorus and iron
availability in soils. Mn is necessary in photosynthesis, nitrogen metabolism and to form
other compounds required for plant metabolism. Interveinal chlorosis is a characteristic
of Mn deficiency. In severe Mn deficiency cases, brown necrotic spots appear on leaves,
resulting in premature leaf drop. White/ gray spots on leaves of some cereal crops are a
sign of Mn deficiency. Chloride is a mobile anion in plants, most of its functions relate to
salt effects (stomatal opening) and electrical charge balance in physiological functions in
plants. Chloride also indirectly affects plant growth by stomatal regulation of water loss.
Wilting and highly branched root systems are the main chloride deficiency symptoms,
which are found mainly in cereal crops. Most soils contain sufficient levels of chloride
for adequate plant nutrition. However, chloride deficiencies have been reported on sandy
soils in high rainfall areas. Mo is associated with enzyme systems relating to nitrogen
fixation by bacteria growing symbiotically with legumes. Mo has a significant impact on
pollen formation, so fruit and grain formation are affected in Mo deficient plant. Nitrogen
metabolism, protein synthesis and sulphur metabolism are affected by molybdenum. The
characteristic Mo deficiency symptom in some vegetable crops is irregular leaf blade
formation known as whiptail. But interveinal mottling and marginal chlorosis of older
leaves is also reported. Mo deficiencies are found mainly on acid, sandy soils in humid
regions. It is essential for cell growth in plants. A primary function of boron is related to
cell wall formation, so boron deficient plants may be growth stunted. Sugar transport in
13
Mechanism Of Oxidative Stress In Plants And Its Regulation
plants, flower retention and pollen formation, and germination are also affected by boron.
Seed and grain production are reduced with low boron supply. Boron deficiency
symptoms first appear at the growing points. Which results in the stunted appearance
(rosetting), barren ears, hollow stems and fruit and brittle, discolored leaves and loss of
fruiting bodies. Boron deficiencies are more pronounced during drought periods when
root activity is restricted. Fe is involved in the production of chlorophyll, and iron
chlorosis is easily recognized on Fe-sensitive crops growing on calcareous soils. Iron is
also a component of many enzymes associated with energy transfer, nitrogen reduction
and fixation, and lignin formation. Iron deficiencies are mainly manifested by yellow
leaves due to low levels of chlorophyll. Severe iron deficiency causes leaves to turn
completely yellow or almost white, and then brown to death. Fe deficiencies are found
mainly on high pH soils, although some acid, sandy soils low in organic matter may also
be Fe-deficient.
1.9 AIMS AND OBJECTIVES:
On the basis of literature surveyed and some preliminary experiments, followings
objectives were decided to be undertaken:
 Studying the antioxidative factors during early seedling growth when given
oxidative stress (through light, herbicide, heavy metals).
 Evaluating the plant growth and yield under variable potential of oxidative stress
in plants.
 Evaluating certain remedial measure to minimize the oxidative stress in plants.
 Exploring the combination of variable micronutrients to minimize the heavy metal
caused oxidative stress.
14