Agriculture Reference
In-Depth Information
Fig. 14.1
Schematic sum-
mary of uptake of major
heavy metals by plants,
generation of reactive oxygen
species, induction of major
components of plant anti-
oxidant defense system for
the maintenance of ascorbat-
eglutathione-homoeostasis
and the control of plant stress
tolerance.
AsA
ascorbate,
GSH
reduced glutathione,
ROS
reactive oxygen
treated plants may be the result of the de-repression of that system. The fact that
both anions are transported by the same system may, also explain the increase of
chromate uptake by the lack of competition when sulphate is not present (Kleiman
and Cogliatti
1997
). Iron (Fe), sulphur (S) and phosphorous (P) are also known
to compete with chromium for carrier binding (Wallace et al. 1976). Independent
uptake mechanisms for Cr(VI) and Cr(III) have been reported in barley. The use
of metabolic inhibitors diminished Cr(VI) uptake whereas it did not affect Cr(III)
uptake, indicating that Cr(VI) uptake depends on metabolic energy and Cr(III) does
not (Skeffington et al.
1976
). In contrast, an active uptake of both chromium spe-
cies, slightly higher for Cr(III) than for Cr(VI), was found in the same crop (Ram-
achandran et al.
1980
).
Uptake of chromium from soil to plant is also a function of soil type and reten-
tion time (Stewart et al.
2003
). Iron oxides are known to have a high affinity for
Cr(III), with increase in pH favouring the sorption. The sorption edge for Cr(III) is
highly proportional to the hydrolyzable constant with specific pH. Cr(III) is said to
be the most available form in the soil under reducing conditions (Dzomback and
Morel
1990
). In contrast, the Cr(VI) is very unstable and is easily mobilized in
acidic as well as alkaline medium (Banks et al.
2006
). Cr(VI) solubility is related
to the formation of oxides and hydroxides, chemical secretion in the rhizosphere
and immobilization (Ball and Nordstrom
1998
). Oxides of aluminum or kaolinite
