Agriculture Reference
In-Depth Information
stitution of magnesium by chromium may be a contributing damage mechanism.
Chromium possesses the capacity to degrade d-aminolevulinic acid dehydratase,
an important enzyme involved in chlorophyll biosynthesis, thereby affecting the d-
aminolevulinic acid (ALA) utilization resulting in the buildup of ALA and finally
reducing the level of chlorophyll (Vajpayee et al. 2001 ). Chromium, mostly in its
hexavalent form, can replace magnesium ions from the active sites of many en-
zymes. d-aminolevulinic acid dehydratase is a metalloenzyme and its activity in
plants is dependent on the availability of magnesium (Ilag et al. 1994 ). Moreover,
Cr(VI) also causes a iron deficiency in stressed plants of L. perenne , leading to
an interruption in chlorophyll biosynthesis at the level of the oxidation step from
coporphyrinogen (Barcelo et al. 1986 ). In this study, the chlorophyll a/b ratio de-
creased when the plants were exposed to an excess of chromium, which may have
been due to a faster breakdown or decreased synthesis of chlorophyll a compared to
chlorophyll b, although chlorophyll b also decreased. These effects were reported
by many researchers (Vajpayee et al. 2001 ; Appenroth et al. 2003 ). The decrease in
the chlorophyll a/b ratio by chromium could induce a reduction in the size of the
peripheral part of the antenna complex (Shanker et al. 2005 ). This decrease in the
chlorophyll b could be due to the destabilization and degradation of the protein of
the peripheral part. The observed reduction of photosynthetic pigment concentra-
tions in chromium-stressed L. perenne and Salvinia plants involves a reduction in
the light harvesting capacity of the plants as observed in plants treated with Cr(VI)
at wavelengths ranging between 400-450 and 650-700 nm (Nichols et al. 2000 ).
Synthesis of d-ALA is the first identified step in tetrapyrrole biosynthesis, leading
to the formation of heme, chlorophyll, billins, vitamin b and specialized products
(Garnick and Sassa 1971 ). ALA synthetic ability plays a key role in the regulation
of chlorophyll biosynthesis in higher plants (Beale 1978 ). However, Naito et al.
( 1980 ) suggested that chlorophyll biosynthesis might be regulated not only by ALA
synthetic ability but also by d-aminolevulinic acid dehydratase (ALAD) activity.
Altered ALAD activity concomitant with reduced chlorophyll content has been
reported in many terrestrial plants exposed to various concentrations of lead, cad-
mium and mercury (Prasad and Prasad 1987 ).
4.6   Chromium Toxicity to Nitrogen Assimilationin Plants
Nitrogen is not only a constituent of protein but also a core element of a number of
biomolecules such as nucleic acids, purines, pyrimidines, porphyrins, coenzymes
and other derivatives. Nitrate reductase (NR), an important enzyme of nitrate as-
similation pathway, is a rate-limiting enzyme as it catalyzes the first step of nitrate
assimilation (Kandlbinder et al. 2000 ). So, NR activity can be seen as a stress index
for plants grown in soils exposed to abiotic stress (Caravaca et al. 2003 ; Caravaca
et al. 2005 ). A decrease in NR activity is witnessed with exposure of plants to chro-
mium. Inhibition of NR activity has been observed in Polytricum commune at chro-
mium concentration of 1, 10, and 100 mM after 24 h; the activity dropped by about
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