Full Journal - Journal of Cell and Molecular Biology - Haliç Üniversitesi

46 Gülriz Bayçu
Cd Contamination in the environment and toxic
effects on plants
Heavy metals such as cadmium (Cd), chromium (Cr),
copper (Cu), mercury (Hg), lead (Pb), aluminium
(Al), nickel (Ni), etc., are important environmental
pollutants particularly in areas where there is high
antropogenic pressure (Abrahamson et al.,1992;
Sanità di Toppi and Gabbrielli, 1999). This group of
elements is toxic to all organisms at varying
concentrations (Speiser et al., 1992). Cd is a toxic
element that normally occurs in very low
concentrations in soils (Wagner, 1993). However, it is
released into the environment by urban traffic, metalworking
industries, heating systems, power stations,
cement factories, waste incinerators and phosphate
fertilizers (Sanità di Toppi and Gabbrielli, 1999). In
the areas that have been subjected to mining, the
concentration can be high, varying from 100-600 mg
kg -1 dry weight. In addition, following the application
of sewage sludge to agricultural land, Cd can
accumulate in the top soil (Lombi et al., 2000). In
humans, Cd is suspected carcinogen it displace Ca or
Zn in proteins and can cause oxidative stress (Steffens
and Bagchi, 1995).
Photosynthetic organisms are the principal entry
point of metals into the food chain leading to animals
and man (Rauser, 1990). Heavy metals such as Cu
and Zn are essential for normal plant growth, but
elevated concentrations can result in growth
inhibition and toxicity symptoms. Non-essential
metals like Cd and Pb are potentially highly toxic and
show toxic effects even at very low concentrations
(Hall, 2002). However, a number of plants (termed
hyperaccumulators) that grow on metalliferous soils,
are able to translocate Cd from the roots and
accumulate it in high concentrations in the shoots
(Chardonnens et al., 1998). It has been suggested that
such plants would be of considerable value in the
remediation of soils that are heavily contaminated
with heavy metals (Zhu et al., 1999).
Several mechanisms are known to allow plants
and other organisms to tolerate the presence of toxic
non-essential metal ions inside the cell. Physiological
studies indicate that heavy metal tolerance is one of
the prerequisites of heavy metal hyperaccumulation
in plants (Raskin et al.,1997). Wide information is
available on ecological impact of Cd, especially as
concerns its motility in soil and uptake by plants
(Leita et al., 1991). Although Cd is not an essential
nutrient for plants, the metal ion is taken up rapidly by
the roots and on most occasions causes inhibition of
growth (Leita et al., 1991). It has been demonstrated
that Cd is not only easily available to plants from soil
or other substrates, but also it is toxic to them at much
lower concentrations than other heavy metals like Zn,
Pb or Cu. The degree to which higher plants are able
to take up Cd depends on its bioavailability,
modulated by the presence of organic matter, pH,
redox potential, temperature and concentrations of
other elements (Sanità di Toppi and Gabbrielli,
1999). Cd is believed to penetrate the root through the
cortical tissue and it is readily taken up and
transported within the plant through an apoplastic
and/or a symplastic pathway (Page et al., 1981; Salt et
al., 1997). Phytotoxicity has been observed to be
dependent upon plant species as well as Cd
concentration in substrate (Leita et al., 1991). Cd
interferes with plant metabolism and in a very general
way, causes leaf roll, chlorosis, growth and yield
reduction (Leita et al., 1991). Inhibition of
ribonuclease and nitrate reductase activity, interaction
with the water balance, decrease in chlorophyll
content, inhibition of stomatal opening, reduction of
normal H + /K + exchange, production of oxidative
stress and enhanced lipid peroxidation are the most
important phytotoxic effects of Cd (Sanità di Toppi
and Gabbrielli, 1999).
Heavy metal stress and detoxification mechanisms
All living cells are confronted with the dilemma that
on one side they need certain amounts of free heavy
metal ions (such as Zn, Cu , Ni , etc.) for their normal
metabolic function, and on the other side they have to
protect themselves from an intracellular excess of
these metal ions which would lead to cell death. This
dilemma can only be overcome by a stringent
regulation of free metal ion concentrations within the
cells (Tomsett and Thurman, 1988; Gekeler et al.,
1989).
In general, resistance to excessively available
chemical elements can be based on avoidance, i.e.
exclusion of the element from the body, or on
tolerance, i.e. the ability to survive, grow and
reproduce with the element present at elevated
concentrations in the body (Levitt, 1980; Ernst et al.,