Agriculture Reference
In-Depth Information
17, 31, and 45 % respectively, as compared with the control (Panda and Choudhury
2005
). In
Nymphea alba,
decline of NR activity was positively correlated with chro-
mium concentration in the medium and with the exposure duration. The maximum
inhibition was recorded at 200 µM chromium treatment. The NR activity possibly
declines owing to the ability of chromium to affect the SH group of the NR enzyme
(Panda and Choudhury
2005
). NR activity also depends on active photosynthesis
or the production of photosynthate and requires the photosynthetically generated
reductant (NADP) and energy (Vijayraghavan et al.
1982
; Raghuram and Sopory
1995
; Ahmad and Abdin
1999
). Reduced protein content under the influence of
chromium treatments may be correlated with the decline in NR activity (Panda and
Choudhury
2005
; Vajpayee et al.
2001
; Rai et al.
1992
).
4.7 Mechanism of Chromium Toxicity to Plants
Chromium exerts its toxic effects on plants in a variety of ways. Cr(VI) is able to
pass the membrane, penetrate the cytoplasm and react with the intracellular material
(Gikas and Romanos
2006
). Negatively charged hexavalent chromium ion com-
plexes can easily cross cellular membranes by means of sulfate ionic channels, and
then undergo immediate reduction reactions leading to the formation of various re-
active intermediates. These intermediates are themselves harmful to cell organelles,
proteins and nucleic acids (Kaszycki et al.
2005
). As the chromium concentration
in plants increases, it adversely affects several biological parameters. Consequently,
there is loss of vegetation (Dube et al.
2003
). Due to high oxidation power of chro-
mium, membrane damage has been observed. Moreover, changes in the redox status
of the plant may also trigger chromium action. The redox imbalance may be caused
by the depletion of oxygen by root and microbial respiration in the water-saturated
soils (Cohen et al.
1998
). This, in turn, increases the overall accessibility of the
contaminant to the plant.
One of the common responses to a wide range of abiotic stresses is the genera-
tion of reactive oxygen species (ROS). These ROS are produced in cells as an in-
termediate product during the reduction of O
2
to H
2
O. Chromium is a toxic heavy
metal that can generate ROS like H
2
O
2
, O
2
−
, OH
−
which cause oxidative damage
to plants (Diwan et al.
2010a
; Panda and Patra
2000
; Dixit et al.
2002
; Panda and
Khan
2003
; Choudhury and Panda
2005
; Diwan et al.
2010b
). Its presence causes
oxidative damage to the biomolecules such as lipids, proteins and nucleic acids
(Kanazawa et al.
2000
). In the plants, metal-induced lipid peroxidation has been
reported (De Vos et al.
1991
), which profoundly alters the structure of membranes
and consequently modifies their enzymatic and transport activities. Lipid peroxida-
tion is considered to be an indication of oxidative damage by which the integrity
and functionality of the membrane is lost. Malondialdehyde (MDA) is the cytotoxic
product of lipid peroxidation and an indicator of free radical production and conse-
quent tissue damage (Ohkawa et al.
1979
). The enhanced production of O
2
-
anions
and hydroxyl (·OH) radicals has been demonstrated to be a cause of chromium-
