59
Cadmium [Cd, 48]
by increased Cd contents of plants are growth retardation, root damage, inhibition
of photosynthesis, disturbed permeability of cell membranes, chlorosis of leaves,
and reddish-brown coloration of leaf margin and veins. At excess Cd, inhibitions of
microorganism functions and disturbed symbiosis between microbes and plants are
also observed.
Some native plants, in particular trees, shrubs, and mosses, may accumulate considerable amounts of Cd (up to about 3 mg/kg) without toxicity symptoms. The highest concentration of Cd (560 mg/kg) is reported for Thlaspi caerulescens, which is
suggested for the phytoremediation (Felix et al. 1999).
Several elements are known to interact with Cd both in elements uptake by plants
and biochemical processes. The most commonly observed interactions are as follows:
r Cd–Zn, variable effects (depressing and enhancing): most often there are
synergistic interactions, and final impacts are increased uptake of both metals under their increased levels.
r Cd–Cu: inhibitory effect of Cu on Cd absorption.
r Cd–Mn/Ni: both metals may be replaced by Cd during the uptake processes.
r Cd–Fe: interactions are related do disturbance in the photosynthesis processes.
r Cd–Se: antagonistic effects due to Se–urea complex with Cd, which
decreases Se availability.
r Cd–P: both increased and decreased uptakes of Cd are reported. Apparently,
it depends on their ratio, as well as on soil properties.
r Cd–Ca: relationship is cross-linked with soil pH. Ions like Ca 2+ are able to
replace Cd 2+ in some processes and thus inhibits Cd absorption.
r Cd interactions with several macro elements (e.g., Mg and K) are observed
and may be related to an impaired effect of Cd on cell membranes.
Cadmium concentration in plants is of a great concern as a pathway of Cd to humans
and animals. Therefore, tolerance and plant adaptation to higher Cd levels may create
a health risk. Thus, Cd contents of food and fodder plants have been studied widely.
This clearly indicates that, in general, plants from contaminated sites contain at about
1000 times higher amounts of Cd than plants grown on uncontaminated soils.
Cd background levels of common food plants that are reported for various countries are fairly similar and low. Mean Cd contents of cereal grains do not exceed
70 μg/kg. Much higher Cd amounts, up to 300 μg/kg, are present in some potato
tubers. In potato tubers produced in Quebec, its contents vary from 40 to 200 μg/kg; the
proposed tolerance content is 250 μg/kg (Fan et al. 2009). Leafy vegetables, lettuces,
and spinach may contain elevated Cd levels between 150 and 400 μg/kg.
All plants grown in soils around metal mines and industrial regions contain
highly elevated Cd levels (Table 9.3). Even grass and clover from the sites at the
ancient abandoned mine contain 1–5 mg Cd/kg. Among various native plants grown
in the polluted areas of the Northern Europe (Kola Peninsula), crowberry (Empetrum
nigrum) has the highest capacity to accumulate Cd (0.058 mg/kg) (Reimann et al.
2001). Very high Cd levels, especially in lettuces and mushrooms, from areas close
to metal industries should be of a real concern. Also, its elevated contents in lettuces
and cabbages from some fertilized farmlands should be under the control.