|
Ethnobotanical Leaflets 14: 413-19. 2010. Effect
of Cadmium on Seed Viability of Vigna unguiculata Egharevba, Henry Omoregie National
Institute for Pharmaceutical Research and Development (NIPRD) E-mail: omorgieegharevba@yahoo.com Issued March 1, 2010 Abstract Cadmium a heavy metal pollutant of
growing global concern was investigated for its effect on grain seeds
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
range 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 averages 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 Materials and
Methods Dried cowpea (Vigna unguiculata) seeds (white seeds) were
obtained from Oba market 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 result showed 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 The contributions of all staff of the
Department of Biochemistry, References Athanassiadis, Y. C. (1969). Preliminary air
pollution survey of cadmium and its compounds. APTD No. 69-32. US Dept. of Health,
Education and Welfare, 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 Ayenimo, J. G., Adeeyinwo,
C.E. and Amoo, Babich, H. and Stotzky,
G. (1978). Effect of cadmium on the biota: Influence of environmental
factors. Appl. Environ.
Microbiol.
23,55-117 Friberg, L., Piscator,
M., Nordberg, G. F. and Kjellstrom, T. (1974). Cadmium
in the environment. 2nd Ed. Cleveland, CRC Press, John, M. K. and van-Laerhoven, C. J. (1976). Differential effects of cadmium
on Lettuce varieties. Environ.
Pollution 10(3), 163-173. Jowett, O. (1958). Population of Agrostis species tolerance of heavy metals. Nature 182: 812-817. Jowett, O. (1964). Population studies
on lead tolerance: Agrotis termus.
Evolution 18:70-80. Kopittke, P. M. and Menzies,
N. W. (2006). Effect of Cu toxicity on growth of cowpea (Vigna unguiculata). Plant and Soil 279, 287–296. Lagerwerff, J. V. (1972). Micronutrients in Agriculture. Soil Sci. Soc. Am. Madison, Wisconsim. Pp. 593-635. Onweremadu, E. U., Chukwucha,
N. B. A. C. and Idoko, M. A. (2008). Distribution
and remediation of two contaminant metal in soils
proximal to three Nigerian roads. Stewart, E. A., Grimshaw,
N. M., Parkinson, J. A. and Quarmby, C. (1974). Chemical analysis of ecological
material. Blackwell
Scientific Publication, Table 1: Effect of Cadmium on seed
germination (Growth parameters)
|