Agriculture Reference
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induced oxidative stress, DNA damage, and apoptotic cell death in cultured human
chronic myelogenous leukaemic K562 cells and promyelocytic leukaemic HL-60
cells (Bagchi and Bagchi 2000 ). Chromium reactivity is apparent from its interac-
tion in cell-free systems with glutathione, NADH and hydrogen peroxide to form
hydroxyl radicals (Aiyar et al. 1991 ). The participation of chromium in redox re-
actions is, however, not clearly understood (Strlic et al. 2003 ), but one study has
demonstrated its participation in redox or Fenton reactions just as copper or iron
(Shi and Dalal 1989 ). Production of H 2 O 2 , OH and O 2 under chromium stress has
been demonstrated in many plants, generating oxidative stress leading to damage
of DNA, proteins and pigments as well as initiating lipid peroxidation (Panda and
Patra 2000 ; Choudhury and Panda 2005 ).
5   Tolerance Mechanism of Plant against Chromium
Plants are known to generate a host of defense responses to cope with the oxidative
stress and develop tolerance to metal exposure. Tolerance can be defined as the
ability of plants to cope with stresses which they are exposed to and which are lethal
or inhibitory to non-tolerant individuals. Plants have several lines of defense that
help in protecting them from metal toxicity.
5.1   Induction of Metal Homeostasis Network System
The first protective barrier against heavy metalis of a highly non-specific nature.
The cation exchange capacity (CEC) of the cell walls and the presence of root exu-
dates and root-tip mucilage enable the roots to moderate the heavy metal concen-
tration reaching the cytoplasm (Banks et al. 2006 ). However, filtering capacity in
plants against chromium is extremely limited and does not prevent chromium from
reaching the interior of the cell, satisfactorily. A complex metal homeostasis net-
work system has evolved in plants to regulate heavy metal uptake and distribution,
thus protecting the metabolic process. Several strategies/mechanisms have been
proposed to describe how plants tolerate heavy metals in the soil/growing media
environment. These include exclusion, restricting heavy metal uptake, sequestrat-
ing and compartmentalizing metal in organs and organelles. The root exudates are
very important agents that form complexes with trace metals and affect their re-
dox behaviour (Caltado et al. 1988 ). Root exudates containing organic acids can
form complexes with chromium compounds, making them available for plant up-
take (Bartlett and James 1988 ). Studies on the role of organic acids in chromium
toxicity in Lycopersicon esculentum showed that in the presence of organic acids
like carboxylic acid and amino acids, chromium uptake in roots is enhanced (Sriv-
astava et al. 1999a ). However, of these types of organic acids, amino acids have
been found to be less effective in mobilizing chromium (Srivastava et al. 1999b ).
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