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INTRODUCTION
INTRODUCTION
Field view of pea
Field view of maize
Environmental stresses such as salinity, drought, heat, cold, flooding and
heavy metal toxicity are major threat to the agricultural productivity worldwide
(Gaafar et al., 2012). Heavy metals (HMs) are among the major environmental
contaminants and pose a severe threat to human and animal health by their long-term
persistence in the environment (Gisbert et al., 2003; Halim et al., 2003; Jabeen et al.,
2009), if they are excess in the food, water and in the air they may cause lot of
problems (Salama and Radwan, 2005; Itumoh et al., 2011; Omaka, 2012; Omaka et
al., 2012; Itumoh et al., 2013). They are given special attention throughout the world
due to their toxic effects even at very low concentrations (Salama and Radwan, 2005)
or at high concentrations (Stevovic et al., 2010). Today, several cases of human
diseases, disorders, malfunction and malformation of organs due to metal toxicity
have been reported (Angelin-Brown et al., 1995; Stoica, 1999; Salama and Radwan,
2005; Omaka, 2008; Itumoh et al., 2011; Itumoh et al., 2013).
Concentration of these toxic metals has accelerated dramatically since the
beginning of the industrial revolution (Ana et al., 2009) thus, posing problems to
health and environment (Nriagu, 1979). Once the heavy metals contaminate the
ecosystem, they remain a potential threat for many years. Heavy metal contaminants
causing ecological problems are of global concern. Heavy metal refers to metals and
metalloids with atomic densities greater than 4 g cm-3 or 5 times or more greater than
water and is usually associated with pollution and toxicity although; some of these
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INTRODUCTION
elements (essential metals) are required by organisms at low concentrations (Hawkes,
1997; Adriano, 2001). However, chemical properties of the heavy metals are the most
influencing factors compared to their density. Heavy metals include lead (Pb),
cadmium (Cd), nickel (Ni), cobalt (Co), iron (Fe), zinc (Zn), chromium (Cr), iron
(Fe),arsenic (As), silver (Ag) and the platinum group elements (Nagajyoti et al.,
2010).
Heavy metals are significant environment pollutants (Berry, 1986). Plants are
susceptible to heavy metal toxicity, and their toxicity is a problem of increasing
significance for ecological, evolutionary, nutritional and environmental reasons.
There are different sources of heavy metals in the environment such as:
(i)
natural sources
(ii)
agricultural sources
(iii) industrial sources
(iv) domestic effluents
(v)
atmospheric sources
Heavy metal pollution can originate from both natural and anthropogenic
sources. Activities such as mining and smelting operations and agriculture have
contaminated extensive areas of world such as Japan, Indonesia and China mostly by
heavy metals, i.e. Cd, Cu and Zn (Herawati et al., 2000), Cu, Cd and Pb in North
Greece (Zanthopolous et al., 1999) in Albania (Shallari et al., 1998) and Cr, Pb, Cu,
Ni, Zn and Cd in Australia (Smith, 1996). The most important natural source of heavy
metals is geologic parent material or rock outcroppings. The geologic plant materials
generally have high concentrations of Cr, Mn, Co, Ni, Cu, Zn, Cd, Sn, Hg and Pb
(Pacyna, 1986; Nagajyoti et al., 2010).
During the last two decades there has been an increasing awareness of the
potential adverse effects of soil pollution by trace metals (Adriano, 2001; Remon et
al., 2013). Their availability in soils depends on natural procedure, especially
lithogenic and pedogenic soils and anthropogenic factors such as mining, combustion
of fossils fuels, urban waste disposal, soil runoff, metal working industries, boating
activity, and phosphate fertilizer application. An increase in heavy metals in the soils
could also be attributed to factors such as soil properties or different agricultural
practices eg., application of sludge to agricultural land (Foy et al., 1978). The
household municipal and industrial wastes are also sources of heavy metals to soils
(Alloway, 1995). Soil contaminated with heavy metals above the permissible limit
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INTRODUCTION
lead to decreased agricultural yields (Nellessen and Fletcher, 1993; Akinola and
Ekiyoyo, 2006). There is a two way relationships between the high concentration of
heavy metals in the soil and the expression of toxicity. On the one hand, heavy metals
compete with essential mineral nutrients for uptake thereby disturbing the mineral
nutrition of plants (Clarkson and Luttge, 1989) and on the other hand after uptake by
the plant, it accumulates in plant tissue and cell compartments and hampers the
general metabolism of the plant (Taylor, 1988, Turner, 1997; Hasan et al., 2009).
The time of exposure, the degree of toxicity influenced by biological
availability of metals and interactions with other metals in the soil, nutritional status,
age and mycorrhizal infection of the plant (Anna-Maj, 1989) are some of the factors
with contribute to the phytotoxicity of the metals. Heavy metal accumulation in soils
is of concern in agricultural production due to the adverse effects on food safety and
marketability, crop growth due to phytotoxicity, and environmental health of soil
organisms (Nagajyoti et al., 2010).The heavy metal accumulation by crop species
decreases in the following order: leaf vegetables>root vegetables>grain crops
(Puschenrieter et al., 2005; Korkmaz et al., 2010). The vegetables absorb and adsorb
these metals from the ground as well as from the parts of vegetable exposed to air
from polluted environment (Vausta et al., 1996; Oti Wilberforce and Nwabue, 2013).
There differences can occur between different parts of the crops and the edible parts
are the most relevant as heavy metals can be easily transferred from them to human
food chain. Except for roots, the highest concentrations are found in leaves, whereas
the lowest are typically observed in seeds (Ivanova et al., 2003; Korkmaz et al.,
2010). Heavy metals are accumulated in the environment and lead to reduced root and
shoot growth, low yield production, low nutrient uptake and impaired homeostasis.
Growth inhibition is a general phenomenon associated with most of heavy metals
(Peralta et al., 2001), although the tolerance limits for heavy metals toxicity are
specific not only for species but also for each variety of crop plants (Metwally et al.,
2008). The acidification of the rhizosphere and exudation of carboxylase are
considered to be potential targets of enhanced metal accumulation (Clemens et al.,
2002; Yang et al., 2005). Heavy metals taken up by plants from contaminated soil and
water are toxic to growth performance of plants and posses a hidden threat to
consumers.
The toxic levels of heavy metals change the pattern of biomass productivity,
plant growth, photosynthetic pigments, protein, amino acids, starch, soluble sugars,
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INTRODUCTION
and essential nutrients uptake. Plants have multiple direct and indirect effects on plant
growth and alter many physiological functions due to heavy metals toxicity (Wool
House, 1983) by forming complexes with O, N and S ligands (Van Assche and
Clijsters, 1990). Thus, heavy metals interfere with mineral uptake, protein
metabolism, membrane functioning (Azevedo et al., 2005) seed germination and
water relations (Kastori et al., 1992).
Many heavy metals like Fe, Cu, Cd, Cr, Zn. etc have been shown to cause
oxidative damage in higher plants (Prasad et al., 1999; Panda and Patra, 2000).
Highly reactive free radicals are produced with the exposure of plants to a range of
abiotic stresses which include water, salt and heavy metal toxicity etc. These reactive
oxygen species (ROS) have been implicated directly with molecular damage in plant
cells. The heavy metal toxicity is also attributed to the generation of ROS leading to
oxidative injury (Hendry et al., 1992; Gallego et al., 1996; Chaoui et al., 1997; Goel,
2012). As, certain metals are known to act as catalysts for the production of Free
radicals in radicals in biological system. Reactive oxygen species (ROS) can damage
biological molecules including DNA, RNA, protein and lipid by inducing
peroxidation (Shah et al., 2001). Lipid peroxidation occurs in plants as a consequence
of high ROS level when excessive ROS cannot be scavanged immediately and
effectively and finally result in the disruption of plant growth and development.
Malondialdehyde (MDA) is one of the ultimate products as a result of lipid
peroxidation (Bailly et al., 1996). Therefore, antioxidant enzymes activities and MDA
content often serve as important physiological indicators for the resistant abilities of
plants under stress conditions (Yin et al., 2009).
The over production of ROS is a common response of plants to different stress
factors. An oxidative stress is defined as a shift of the balance between prooxidative
and antioxidative reactions. The sources of reactive oxygen includes various
environmental and biological factors such as hyperoxia, light, draught, high salinity,
cold, metal ions, pollutants, xenobiotics, toxins, reoxygenation after anoxia,
experimental manipulations, pathogen, infection and aging of plant organs. The
production of ROS during environmental stress is one of the main cause of decreases
in productivity, injury and death that accompany these stresses in plants. Generation
of reactive oxygen species (ROS) has been identified as an inevitable process of
normal aerobic metabolism in plants and the four major types of ROS are singlet
oxygen (1O2), superoxide (.O2-), hydrogen peroxide (H2O2) and hydroxyl radicals
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INTRODUCTION
(OH-) (Dinakar et al., 2010). The reactive oxygen species are also generated in plant
cells during normal metabolic processes. When plants are exposed to toxic metals,
apoplastic transport followed by cytosolar uptake and distribution of metals to
organelles which causes ROS generation which affects sub-cellular metabolism
(Sharma and Dietz, 2009).
Fig. 1. Occurrence of heavy metals in India (www.google.com)
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INTRODUCTION
1.1 Cadmium
The regulatory limit of Cadmium (Cd) in agricultural soil is 100 mg/kg soil
(Salt et al. 1995). Cd is a heavy metal that normally occurs in low concentrations in
soils and up to 100-120 mg/kg dry weight was reported by Lombi et al. (2002) and
Nwaichi et al. (2010). In plants, the accumulation of Cd can cause numerous
morphological and physiological changes. Plants grown in soil containing high levels
of Cd show visible symptoms of injury reflected in terms of chlorosis, necrosis, leaf
roll, growth inhibition, browning of root tips and finally death (Sanita di Toppi and
Gabbrielli, 1999; Benavides et al., 2005; Guo et al., 2008).
Cadmium structure
At the physio-logical level, excess Cd results in an inhibition of
photosynthesis and transpiration (Mobin and Khan, 2007; Shi and Cai, 2008; Shi et
al., 2010), imbalance of mineral nutrients, induction of oxidative stress (Sandalio et
al., 2009), changes in enzyme activity (Hasan et al., 2009), and modifications to gene
expression (Herbette et al., 2006). Photosynthesis, one of the major determinants to
biomass production, has been shown to be very sensitive to Cd in higher plants
(Ekmekci et al., 2008; Liu et al., 2011).
The inhibition of root Fe (III) reductase induced by Cd led to Fe (II)
deficiency, and it seriously affected photosynthesis (Alkantara et al., 1994). In
general, Cd has been shown to interfere with the uptake, transport and use of several
elements (Ca, Mg, P and K) and water by plants (Das et al., 1997). Cd also reduced
the absorption of nitrate and its transport from roots to shoots, by inhibiting the nitrate
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INTRODUCTION
reductase activity in the shoots (Hernandez et al., 1996). Cadmium treatments have
been shown to reduce ATPase activity of the plasma membrane fraction of wheat and
sunflower roots (Fodor et al., 1995). Cadmium produces alterations in the
functionality of membranes by inducing lipid peroxidation (Fodor et al., 1995) and
disturbances in chloroplast metabolism by inhibiting chlorophyll biosynthesis and
reducing the activity of enzymes involved in CO2 fixation (De Filippis and Ziegler,
1993; Nagajyoti et al., 2010).
1.2 Chromium
Chromium (Cr) is one of the most toxic heavy metals found abundantly in the
earth’s crust (Panda and Choudhory, 2005; Nematshahi et al., 2012), which attenuates
the environment. Although toxic, it has nutritive importance too (Kabata-Pendias and
Pendias, 2001).
Chromite structure
Chromium can exist in several chemical forms, displaying oxidation numbers
from 0 - VI. Cr compounds are highly toxic to plants and are detrimental to their
growth and development. Although some crops are not affected by low Cr (3.8 x 104
μM) concentrations (Huffman and Allaway, 1973a,b), Cr is toxic to higher plants at
100 kg-1 dry weight (Davries et al., 2002). Since seed germination is the first
physiological process affected by Cr, the ability of a seed to germinate in a medium
containing Cr would be indicative of its level of tolerance to this metal (Peralta et
al., 2001). High levels (500 ppm) of hexavalent Cr in soil reduced germination up to
48% in the bush bean Phaseolus vulgaris (Parr and Taylor, 1982). Peralta et al.
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INTRODUCTION
(2001) found that 40 ppm of Cr (VI) reduced by 23% the ability of seeds of Lucerne
(Medicago sativa cv. Malone) to germinate and grow in the contaminated medium.
Reductions of 32–57% in sugarcane bud germination were observed with 20 and 80
ppm Cr, respectively (Jain et al., 2000). The reduced germination of seeds under Cr
stress could be a depressive effect of Cr on the activity of amylases and on the
subsequent transport of sugars to the embryo axes (Zeid, 2001). Protease activity, on
the other hand, increases with the Cr treatment, which could also contribute to the
reduction in germination of Cr-treated seeds (Zeid, 2001). Decrease in root growth
is a well-documented effect due to heavy metals in trees and crops (Tang et al.,
2001). Chromium stress is one of the important factors that affect photosynthesis in
terms of CO2 fixation, electron transport, photophosphorylation and enzyme
activities (Clijsters and Van Assche, 1985). In higher plants and trees, the effect of
Cr on photosynthesis is well documented (Van Assche and Clijsters, 1983).
However, it is not well understood to what extent Cr-induced inhibition of
photosynthesis is due to disorganization of chloroplasts ultra structure (Vazques et
al., 1987), inhibition of electron transport or the influence of Cr on the enzymes of
the Calvin cycle. Chromate is used as a Hill reagent by isolated chloroplast (Desmet
et al., 1975). The more pronounced effect of Cr(VI) on PS I than on PS II activity in
isolated chloroplasts has been reported by Bishnoi et al. (1993a, b) in peas.
Nevertheless, in whole plants, both the photosystems were affected. Chromium
stress can induce three possible types of metabolic modification in plants as follows:
(i)
alteration in the production of pigments, which are involved in the life
sustenance of plants (e.g., chlorophyll, anthocyanin) (Boonyapookana et al.,
2002).
(ii)
increased production of metabolites (e.g., glutathione, ascorbic acid) as a direct
response to Cr stress, which may cause damage to the plants (Shanker et al.,
2003b).
(iii) alterations in the metabolic pool to channelize the production of new
biochemically related metabolites, which may confer resistance or tolerance to
Cr stress e.g., phytochelatins, histidine (Schmoger, 2001).
Induction and activation of superoxide dismutase (SOD) and of antioxidant
catalase are some of the major metal detoxification mechanisms in plants (Shanker et
al., 2003a). Gwozdz et al. (1997) found that at lower heavy metal concentrations,
activity of antioxidant enzymes increased, whereas at higher concentrations, the SOD
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INTRODUCTION
activity did not increase further and catalase activity decreased (Nagajyoti et al.,
2010).
1.3 C3 and C4 Plant System
In this study two plants such as pea (a C3 plant) and maize (a C4 plant) were
taken as test plants. Their physiological and morphological details are discussed
below:
In nature three different plant systems exist viz., C3, C4 and CAM,
characterized on the basis of CO2 trapping mechanisms, however; C4 and CAM plants
essentially follow C3 pathway to trap CO2 as an initial step. C4 is a characteristic
photosynthesis
syndrome
of
angiosperms.
Generally,
phosphoenolpyruvate
carboxylase (EC 4.1.1.31, PEPC) enzyme is widespread among all plants, including
C3 (e.g., Pisum sativum, Gossipium hirsutum, Oryza sativa, Brassica campestris,
Triticum aestivum, Avena sativa) C4 (e.g., Zea mays, Saccharum officinarum,
Sorghum spp., Vetiveria zizanioides, Cyanadon dactylon) and CAM (e.g., members of
Orchidaceae, Polypodiaceae (ferns)) species and is responsible for the initial carbon
fixation in C4 and CAM plants. C3 and C4 plants have unique carbon trapping
mechanisms. The general enzymatic system involves in CO2 fixation in C3 and C4 are
Ribulose-1-5-bisphosphate carboxylase oxygenase (Rubisco EC 4.1.1.39) and
phosphoenolpyruvate carboxylase (PEPC), NADP-malic enzyme (NADP-ME),
Pyruvate, phosphate dikinase (PPDK) respectively. The leaves of C4 plants display
Kranz anatomy whereby an outer layer of mesophyll cells containing chloroplast
surrounds vascular bundles with an inner layer of bundle sheath cells (Dengler and
Nelson, 1999). In C3 plants, mesophyll cells are devoid of chloroplast and CO2 is
fixed in bundle sheath cells by Rubisco.
The chloroplasts of C3 plant contain a complete Calvin cycle and are able to
assimilate CO2 to convert it to the principle 3 carbon compound (triose phosphate), on
the other hand CO2 is distributed in two cells viz., mesophyll and bundle sheath in C4
plants and converted primarily in 4 carbon compound (acid oxaloacetate) by the
action of PEPC in mesophyll cells which is then transported in bundle sheath cells
where by the acids from mesophyll cells provide carbon dioxide. Extensive research
literature is available on the comparative account of C3 and C4 plant systems (Du and
Fang, 1982; Derner et al., 2003; Sage, 2004; Edwards et al., 2005; Caird et al., 2007;
Brautigam et al., 2008; Doubnerova and Ryslava, 2011). However, very few and
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INTRODUCTION
scattered information is available on the comparative account of C3 and C4 plants and
their growth performances under extreme environmental conditions.
Photosynthesis in C 4 plants does not saturate but increases at high light
intensities and can continue at very low CO 2 concentrations (Sage, 2004; Brautigam
et al., 2008). Subsequently, these plants have rapid growth rates and higher biomass
and economic yields as compared to the C 3 plants. There are evidences from
researches that C 4 plant such as vetiver grass (Vetiveria zizanioides L. Nash) can
withstand harsh environmental conditions (Chen et al., 2004, Chiu et al., 2006). A
comparative study performed on two separate species belonging to C 3 and C4
systems respectively show that the environmental tolerance depends on the high
biomass production which is higher in case of C 4 plants (Ali et al., 2002). However;
there is lack of information regarding the biochemical differences among C 3 and C4
plant systems exposed to toxic environment for e.g., the extent of detoxification
mechanism, mycorrhization, proteomes (expression of genes). Toxic response of
particular plant variety belonging to C 3 and C4 type indicate that there is a high
tolerance in C4 plants as compared to the C 3 plants which may or may not be true for
the entire group of plants belonging to these systems (Chapin, 1991; Ali et al.,
2002). C4 photosynthesis allows fast biomass accumulation with high nitrogen and
water use efficiency (Leegood and Edwards, 1996; Sage, 2004) which is desired set
of traits to increase the productivity of crop plants (Matsuoka et al., 1998) and a
required character for successful phytoremediation (Srivastava et al., 2012).
Pea (Pisum sativum L.)
The pea is a cool-season annual vine that is smooth and has a bluish-green waxy
appearance. Vines can be up to 9 ft long, however modern cultivars have shorter
vines, about 2 ft long. The stem is hollow, and the taller cultivars cannot climb
without support (Elzebroek and Wind, 2008). Leaves are alternate, pinnately
compound, and consist of two large leaf like stipules, one to several pairs of oval
leaflets, and terminal tendrils (McGee, 2012).Pea is an important vegetable in India;
the crop is generally cultivated for its green pods. It is highly nutritive and is rich in
protein. It is used as a vegetable or in soup, canned frozen or dehydrate. It is cooked
as a vegetable along or with potatoes. Split grains of pea are widely used for dal. Pea
straw is a nutritious fodder.
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INTRODUCTION
Fig.
2.
The global cultivation of Pisum sativum
(http//en.wikipedia.org/wiki/file:peayield.png)
with
seed
yields
Peas are adapted to many soil types, but grow best on fertile, light-textured,
well-drained soils (Hartmann et al., 1988; Elzebroek and Wind, 2008). Peas are
sensitive to soil salinity and extreme acidity. Pea is a cool season crop and performs
best at 10°C to 18°C. The flower and young pods are badly affected by frost. The
germination of seeds takes place at 3.3°C soil temperature. Boswell (1920) reported
that as the temperature increases during the growing season the yield decline sharply.
The optimum mean monthly temperature for pea is 12.8°C to 18°C. Pea is currently
grown in temperate regions, at high elevations, or during cool seasons in warm
regions throughout the world (Elzebroek and Wind, 2008). Major pea producers are
China, India, Canada, Russia, France and the United States (FAO, 2012) as shown in
Fig. 2.
1.4 Harvesting
Peas are harvested for table use when the pods are filled and the young tender
peas changing in colour from dark to light green. Peas may be picked in 45 to 60
days, 75 days and 100 days according to early, mid and late season. Airtimes
respectively, 3 to 4 pickling are done within the interval of 2 to 10 days. Fresh
unshielded peas may be kept two at °C and 90-95 percent relative humidity.
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INTRODUCTION
1.5 Commercial Crop
Peas are a cool-season crop grown for their edible seed or seed pods. Garden
or green peas are harvested before the seeds are mature for the fresh or fresh-pack
market (Elzebroek and Wind, 2008). Sugar snap peas and snow peas lack the inner
pod fiber and are also harvested early for the fresh or fresh-pack market (McGee,
2012). Field peas, including fall-sown Austrian winter peas, are harvested when seeds
are mature and dry, and are primarily blended with grains to fortify the protein
content of livestock feed. Dried peas are also sold for human consumption as whole,
split or ground peas. Peas are a nutritious legume, containing 15 to 35% protein, and
high concentrations of the essential amino acids lysine and tryptophan (Elzebroek and
Wind, 2008).
1.6 Forage Crop
Peas are grown alone or with cereals for silage and green fodder (Elzebroek
and Wind, 2008). Peas can also be grazed while in the field. Young Austrian winter
pea plants will regrow after being grazed multiple times (Clark, 2007).
1.7 Rotational Crop
Peas and other legumes are desirable in crop rotations because they break up
disease and pest cycles, provide nitrogen, improve soil microbe diversity and activity,
improve soil aggregation, conserve soil water, and provide economic diversity.
1.8 Green Manure and Cover Crop
Peas are grown as green manures and cover crops because they grow quickly
and contribute nitrogen to the soil (Clark, 2007). Pea roots have nodules, formed by
the bacteria Rhizobium leguminosarum, which convert atmospheric nitrogen to
ammonia. Peas also produce an abundance of succulent vines that breakdown quickly
and provide nitrogen (Clark, 2007). Austrian winter peas are the most common type
of pea used as a green manure or cover crop because they are adapted to cold
temperatures and fit in many rotations.
Therefore, Pea plants were chosen to reveal consequences of Cadmium
toxicity in-vivo and in vitro. Pea is a self pollinated crop plant, commonly known as
seed-pod of the legume. Botanically it is a fruit, treated as a vegetable in cooking,
grown once in a year in cool climatic surrounding depending on location altitude. The
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INTRODUCTION
seeds are used as a vegetable, fresh, frozen or canned, and are also dry peas in the
form split pea as pulses. They do not thrive in the summer heat of warmer temperate
and tropical climates but do grow well in cooler and high altitude tropical areas.
Generally, many cultivars reach maturity in about 60 can also be grown outdoors
during the winter. The protein concentration of pea seeds ranges from 15.5-39.7%
(Bressani and Elias, 1988) along with other nutritional characteristics as shown in
Table 1.
Table 1. Nutritional values of raw green pea
Raw Green Pea
(Nutritional value 100g seeds)
Energy
339 kJ (81 kcal)
beta-carotene
449 μg (4%)
Carbohydrates
14.5 g
Thiamine (Vit. B1)
0.3 mg (23%)
Sugars
5.7 g
Riboflavin (Vit. B2)
0.1 mg (7%)
Dietary fiber
5.1 g
Niacin (Vit. B3)
2.1 mg (14%)
Fat
0.4 g
Pantothenic acid (B5)
0.1 mg (2%)
Protein
5.4 g
Vitamin B6
0.2 mg (15%)
Vitamin A equiv.
38 μg (4%)
Folate (Vit. B9)
65 μg (16%)
Vitamin C
40.0 mg (67%)
Iron
1.5 mg (12%)
Calcium
25.0 mg (3%)
Phosphorus
108 mg (15%)
Magnesium
33.0 mg (9%)
Potassium
244 mg (5%)
Zinc
1.2 mg (12%)
lutein and zeaxanthin
2593 μg
(Source: USDA Nutrient database; www.google.com)
Maize (Zea may L.)
Maize (Zea mays L.) is the most important grain crop. Maize is widely
cultivated throughout the world, and a greater weight of maize is produced each
year than any other grain. Maize (Zea mays L.), is world’s one of the three most
popular cereal crops for both livestock feed and human nutrition. With its high
content of carbohydrates, fats, proteins, some of the important vitamins and
minerals, maize is the main food of the lower-socio-economic populations and in
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INTRODUCTION
some regions it represents more than 80% of daily food consumption. The United
States produces 40% of the world's harvest; other top producing countries include
China, Brazil, Mexico, Indonesia, India, France and Argentina (Fig. 3). Maize is
the most widely grown grain crop throughout the Americas, with 332 million
metric tons grown annually in the United States alone. Approximately 40% of the
crop 130 million tons is used for corn ethanol. Maize kernels are used in cooking
as a starch. The most suitable soil for maize is one with a good effective depth,
favourable morphological properties, good internal drainage, an optimal moisture
regime, sufficient and balanced quantities of plant nutrients and chemical
properties that are favourable specifically for maize production. Although largescale maize production takes place on soils with a clay content of less than 10 %
(sandy soils) or in excess of 30 % (clay and clay-loam soils), the texture classes
between 10 and 30 % have air and moisture regimes that are optimal for healthy
maize production.
Therefore, maize plants were chosen to reveal consequences of Chromium
toxicity in-vivo and in vitro. Maize (Zea mays L.) is one of the most important
cereal crops and comprises some heavy metal tolerant genotypes (Clark, 1977; Liu
et al., 2001). Some maize cultivars with capability of absorbing and accumulating
extraordinarily high amounts of heavy metals from soil (Liu et al., 2006). Zea
mays L. is one of the most important agricultural crops. Being rich source of
nutrition (72% starch, 10% protein, 8.5% fiber & 4.8% edible oil), maize is one of
the major sources of food, sugar, cooking oil and animal feed all over the world
(Dowswell et al., 1996; Hussain et al., 2012). The nutritional values are given to
Table 2.
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INTRODUCTION
Fig.3.
The global cultivation of Zea mays
(http//en.wikipedia.org/wiki/file:Zeayield.png)
L.
with
seed
yields
Table 2. Nutritional values of raw yellow corn
Raw Yellow corn
(Nutritional value 100g seeds)
Energy
360 kJ (86 kcal)
Thiamine (Vit. B1)
0.200 mg (17%)
Carbohydrates
19.02 g
Niacin (Vit. B3)
1.700 mg (11%)
Sugars
3.22 g
Vitamin B6
0.093 mg (7%)
Dietary fiber
2.7 g
Folate (Vit. B9)
0.093 mg (7%)
Fat
1.18 g
lutein and zeaxanthin
644 μg
Protein
3.22 g
Potassium
270 mg (6%)
Vitamin A equiv.
9 μg (1%)
Iron
0.52 mg (4%)
Vitamin C
6.8 mg (8%)
(Source: USDA Nutrient database; www.google.com)
1.9 Harvesting and Uses
It is very important to harvest sweet corn at the proper stage of maturity. The
critical time is the milk stage, a stage when the juice in the kernel appears milky when
you puncture the kernel with your thumbnail. Sweet corn remains in the milk stage for
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INTRODUCTION
a relatively short period, so check the ears frequently. Corn that is too young will ooze
a watery material, while ears that are too old will have a tough, doughy kernel. During
the milk stage, the unhusked ear should feel firm, have full kernels at the tip of the
ear, and have brown, dry silks. Generally, ears should be ready about three weeks
from silking time.
Field corn in the U.S. is used mainly to feed livestock, but in other countries is
used for human consumption as well. Indian corn was originally the term applied to
what we now know as maize or corn, to differentiate it from the generic term of
“corn” Europeans used for all grains at that time. Now, it usually refers to any corn
that has different colored kernels. Usually it is dried and used for ornamental
purposes. Popcorn, the ability of maize kernels to “pop” and expand upon heating,
was also discovered by the Native Americans. Maize is able to pop because, unlike
other grains, its kernels have a hard moisture-sealing hull and a dense starchy filling.
When heated, pressure builds inside the kernel until an explosive "pop” results, and
the starch expands and then hardens in the cooler air. Many maize varieties will pop,
but some varieties have been specifically cultivated for this purpose.
Maize flour, or meal, is made into a thick porridge in many cultures (polenta,
Italy; angu, Brazil; mãmãligã, Romania; sadza, nshima, ugali, and mealie pap,
Africa). Maize meal is also used as a replacement for wheat flour, to make cornbread
and other baked products.
Popcorn
Maize flour
Fig. 4. The various maize products used by the society
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Corn syrup
INTRODUCTION
Corn syrup is used as a sweetener instead of sugar in thousands of products, including
soda, candy, cookies and bread as shown in Fig. 4.
The effects of chromium on some metabolic activities of maize (Zea mays L.)
have caused visible lesions of interval chlorosis. The young leaves showed vein
clearing margins of leaves with curling and appearance of pale colour due to loss in
chlorophyll (Sharma et al., 2003; Labra et al., 2006; Zou et al., 2009). The growth of
pea (Pisum sativum L.) plants treated with CdCl2 for 15 days produced a reduction in
the number and length of lateral roots and changes in structure of the principal roots
affecting the xylem vessels. Cadmium induces reduction in glutathione and ascorbate
content, and also reduces activities of catalase, glutathione reductase and guaiacol
peroxidase (Serrano et al., 2006; Popova et al., 2008; Smiri et al., 2010; Bavi et al.,
2011; Januskaitiene, 2012).
Keeping above details in view, our study is aimed to investigate to the
following:
1.
The effect of heavy metals on seed germination, growth and biomass
characteristics of pea and maize plants.
2.
The effect of heavy metals (Cadmium and Chromium) on physiological and
biochemical characteristics on pea and maize crop plants.
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