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Ethnobotanical Leaflets 14: 399-405, 2010. Effect of
Cadmium on Seed Viability of Vigna unguiculata Henry Omoregie Egharevba National Institute for Pharmaceutical
Research and Development (NIPRD) Abuja, Nigeria E-mail: omorgieegharevba@yahoo.com Issued April 1,
2010 Abstract Cadmium,
a heavy metal pollutant of growing global concern, was investigated for its
effect on grain seed viability using Vigna unguiculata.
The cowpea seeds were germinated after treatment in solution
containing varying concentration of cadmium chloride (CdCl2.H2O).
The concentrations of cadmium (Cd) in the solution
used for the treatment were 0.00ppm, 0.80ppm, 8.00ppm, 40.00ppm, 100.00ppm
and 180.00ppm. The percentage germination at 120 hours germination time and
the rate of increment in shoot height between 120 and 168 hours were
determined. Results shows that the percentage germination and rate of
increment in shoot height decreased as cadmium level in the treatment
solution increased. There were however
no growth at 100 and 120 ppm. The lethal
concentration of cadmium for 50% of the viable seeds (LC50) in the
treatment solution appears to be at about 40 ppm. Key
words: Cadmium, cowpea, viability, germination. Introduction Concerns about environmental hazards
have stimulated research to evaluate the effect of pollutants on the
biosphere. Among the numerous pollutants, Cadmium, an element with no known
beneficial biological function is of major concern. With increasing
population, civilization, metallurgical advancement and world development,
industrial utilization of cadmium containing compounds has accelerated the
rate of mobilisation and distribution of cadmium
which is far in excess of natural abiotic cycling
process leading to a general increased deposition of cadmium in both aquatic
and terrestrial environment with resultant accumulation in the biota and
biosphere (Babich and Stotzky,
1978). Environment accumulation of chemical pollutants such as heavy metals
have been found to interfere with certain ecological processes and energy chain like decomposition,
growth, energy generation and transformation and transmission processes such
as photosynthetic and chemosynthetic processes through the trophic levels. The various ecological interactions like
the plant-plant, plant-microbe, plant-animal, microbe-microbe, animal-microbe,
plant-soil, microbe-soil, etc, and various biochemical cycling at the
molecular levels are however not excluded (Babich
and Stotzky, 1978). Abnormal levels of essential or nonessential
nutrient in the body of an organism have been found to impair growth and
development. In general, macronutrients are much less toxic than
micronutrients and their concentration can be raised appreciably above normal
optimum without significantly affecting growth. On the other hand, the margin
between sufficiency and toxicity is very narrow for most micronutrients and
notably for most heavy metals [Jowett, 1958 and 1964]. The sensitivity of
different plants to high concentration of individual element varies greatly
and it is the toxicity of such elements that inhibits the colonization by
many plants of slag heaps and other industrial metallurgic waste [Jowett,
1958 and 1964]. Cadmium occurs mainly as natural
components of minerals in the earth crust with an average concentration of
0.18 ppm. The most common Cadmium containing
minerals are greenockite (Cadmium sulphide) (CdS, - hexagonal), hawleyite
(CdS,- cubic), xanthochroite
(CdS – coating, amorphous), cadmoselite
(Cadmium selenide) (CdSe),
monteponite (cadmium oxide) (CdO),
octavite (cadmium carbonate) (CdCO3),
and saukovite (cadmium metacinnabar)
(Hg/CdS - cubic) (Babich
and Stotzky, 1978). Pure Cadmium metal does not
exist in nature, but minerals containing Cadmium are associated with zinc ores,
such as zinc sulphides, zinc oxides, zinc silicate
and polymetallic zinc ores (Pb-Zn,
Cu-Zn, Pb-Cu-Zn). Zinc to cadmium ratio ranges from
100:1 to 1000:1 in these ores (Babich and Stotzky, 1978). However organic compounds containing
cadmium are unstable and have not been detected in nature [Athanassiadis, 1969; Lagerwerff
1972]. Cadmium background level in the soil ranged from 0.03-0.3 ppm [Stewart et al,
1974; Onweremadu et al, 2008; Akinola
et al, 2008]. The
concentration of cadmium in fresh water is generally less than 0.001 ppm and average on about 0.00015 ppm.
In the atmosphere, cadmium exists as particulates probably with oxides of
cadmium predominating, but cadmium levels in urban atmosphere are usually
higher than nonurban atmosphere (Friberg et al, 1974). Measurable amount of cadmium occur in
many soils, animals and plant materials, and an increasing attention is been
paid to its concentration in these materials for biological, medical,
geochemical, and agricultural prospecting [Stewart et al, 1974]. Studies have shown that cadmium tend to accumulate
in plant tissues at concentrations exceeding that of the soil solution [Onweremadu et al, 2008].
Plants tissues like roots, shoots, trunks and leaves been studied and cadmium
is seen as a cumulative toxicant by most scientists. Previous studies on the
effect of cadmium on seeds have been restricted to seedlings that have
already germinated. This work was
designed to look at some effect of cadmium accumulation in polluted or
contaminated seeds with respect to germination of a quiescent seed. The
choice of V. Unguiculata,
a typical tropical/subtropical annual crop, initially staple to tropical
Africa but now to the tropical region of the world, was informed due to its
large cultivation in and consumption in most industrial cities of Nigeria,
such as Kano and Kaduna, which also are located in region where significant
metallurgic activities are ongoing. Farmers in these regions are also known
to depend heavily on fertilizers for improved crop yield. Materials
and Methods Dried cowpea (Vigna
unguiculata) seeds (white seeds) were obtained from
Oba market, Benin City, Nigeria. All reagents were analar
grade and were products of Merck Chemical Co., Darmstadt. All glassware were
products of Pyrex England. Preparation of cowpea:
The dry seeds were steeped in solutions containing 0, 0.8, 18, 40, 100, and
180 ppm of cadmium for one and a half hour.
Floating seeds were skimmed off. The
steeped grains were sown on wet cotton wool in corresponding pre-labeled
Petri-dishes. 14 seeds were sown in each Petri-dish and experiment carried
out in replicate of three. Germination was done in the light at room
temperature (about 250C) for 168 hours and analyses were done at
intervals as germination progressed. Growth Parameters:
Percentage germination was computed from the number of seeds that germinated
per replicate. The germination rate was determined by measuring the height of
shoots at 120 and 168 hours of germination. Results
and Discussion The results of cadmium effect on the
seed are as shown in Table 1. The results show more than 50% reduction in the
number of seeds that germinated from treatment with up to 40 ppm, with close to 0% germination at 100 ppm. The same pattern was observed in the rate of growth
of the shoot. The decrease in percentage germination obtained from this study
may be due to loss of viability because of decreased energy generation by the
embryo. Energy generation is very important for seed germination and its
blockage could affect protein, RNA and DNA synthesis as well as mitosis,
since energy is required for these processes to occur (John and van-Laerhoven 1976). Growth retardation as observed has
been reported for many organisms including plants (Babich
and Stotzky, 1978, John and van-Laerhoven
1976). Growth retardation is caused by decrease in cell enlargement and
division. The inhibition of cell enlargement and division by cadmium is
probably due to depression of growth promoters mainly through enzyme
inactivation and depression or direct inhibition of cell division by
interfering with the cell membrane integrity. The inhibition of cell division
may be by the inhibition of mitosis through interference with the formation
of mitotic apparatus [Brachet and Mirsky, 1961]. Inhibition of protein synthesis, DNA
replication and energy generation could also inhibit cell division. Cadmium
probably inhibited mitosis by binding to the –SH groups in the protein
molecules which build up the spindles or mitotic apparatus. Previous studies
on the inhibition of –SH groups of such proteins by inhibitors such as
fluoride, iodoacetic acid, chloroacetophenone
and colchicines have been found to inhibit mitosis in meta-phase because the
formation of the spindles is prevented [Brachet and
Mirsky 1961]. Cadmium could also affect the
plasticity and fluidity necessary for division of meristematic
cells. Cadmium ion, Cd2+, like other divalent ions like Ca2+,
could reduce cell memberane permeability and
fluidity which is necessary for cell division. At 40 and 100 ppm
the emerged radicles were brownish and stunted with
swellings from base to tip. The elongation of the radicles
soon stopped without emergence of the shoot or withering of the emerged
shoot. A similar effect has been reported for copper (Kopittke
and Menzies 2006). In general, cadmium concentration above 18 ppm reduces viability by close to 20% while concentration
close to 40 ppm appears to reduce seed viability or
inhibit germination by more than 50% and this could represent the ecological
dose 50% (EcD50) of Cadmium for cowpea farm. Worse is the fact that cadmium
as most heavy metals is a cumulative toxicant. A concentration of up to 0.39 ppm has been reported for the Warri
River, Nigeria, 5.88 ppm for some Amaranth plants
in Benin city and 0.65m -0.81 in the tissues of Talinum
triangulare in Lagos (Ayenimo
et al 2005, Onweremadu et al 2008, Akinola
et al 2008). These are well
over WHO permissible level of 0.1 mg/Kg or ppm (Onweremadu et al 2008). In conclusion, it is very apparent
that cadmium pollution or contamination in seeds is very detrimental to seed
viability and germination. In this age of world economic crisis and food
insecurity, the threat of heavy metal pollution or contamination of
agricultural produce is of paramount consideration. Cadmium with no know beneficial
use to man is of major concern and effort should be made my government of
agrarian metallurgic communities practicing urban and peri-urban
agriculture to ensure strict monitoring of cadmium emission and contamination
of soil and farm produce in order to minimise its
threat to increase agricultural yield and food production. Further work is recommended in area of
mechanisms of action in the inhibition of enzymes and cell division. Acknowledgement I wish to acknowledge all the staff of
the Department of Biochemistry, University of Benin Benin
City, Nigeria, for their support in carrying out this work. I also wish to
acknowledge Professor E.A.C. Nwanze, of the
University of Benin, for his contribution to the work. References Akinola,
M.O., Njoku, K. L. and Ekeifo,
B. E. (2008). Determination of Lead, Cadmium and Chromium in the Tissue of an
Economically Important Plant Grown Around a Textile Industry at Ibeshe, Ikorodu Area of Lagos
State, Nigeria, Adv. Environ. Biol., 2(1): 25-30 Athanassiadis,
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O. (1958). Population of Agrostis species tolerance
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P. M. and Menzies, N. W. (2006). Effect of Cu
toxicity on growth of cowpea (Vigna unguiculata). Plant
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J. V. (1972). Micronutrients in
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Pp. 593-635. Mirsky,
A. E. (1961). The Cell: Meiosis and Mitosis. Academic Press Inc., London. pp:
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of two contaminant metal in soils proximal to three Nigerian roads. New York Science Journal. 1(2):1-9 Stewart,
E. A., Grimshaw, N. M., Parkinson, J. A. and Quarmby, C. (1974). Chemical analysis of ecological material.Blackwell Scientific Publication, Oxford,
London, pp 38. Table
1: Effect of Cadmium on seed germination (Growth parameters).
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