Agriculture Reference
In-Depth Information
have been seen to adsorb chromium successfully; the process being pH-dependent
(Turan et al.
2007
). The Cr(VI) is effectively reduced in acidic soils because acidic
conditions enhance the rate of release of iron species from soil minerals for re-
action with agricultural species (Fendrof et al.
2004
). Early research established
that calcium (Ca) stimulates the uptake of sulphate, orthophosphate and rubidium
(Leggett and Epstein
1956
; Rains et al.
1964
). On the other hand, chromium inhib-
its calcium uptake by plants. Terry (
1981
) observed that at toxic concentrations of
Cr(VI) ( > 2 mg kg
−1
Cr), sugar beet plants absorbed very little calcium and were
calcium-deficient.
The uptake of chromium results into its accumulation in plant parts, especially
in roots (Cary
1982
; WHO
1988
; Zayed et al.
1998
). Among the aerial parts, leaves
usually contain more chromium than other parts like seeds. High chromium accu-
mulation in roots might be because of immobilization of chromium in the vacuoles
of the root cells. Translocation of chromium in the plant is governed by its translo-
cation potential, which is dependent on chromium forms, underlying chemistry in
the plant and chromium complexation with some ligands. The uptake of Cr(III) is
higher than that of Cr(VI) (Mishra et al.
1995
). This could be due to passive trans-
port of Cr(III) in the plant, dissipating no metabolic energy in this process, and also
because Cr(III) has a role of in amino acid and nucleic acid metabolisms (Richard
and Bourg
1991
). However, restriction in chromium translocation irrespective of
the chromium forms, despite the differential accumulation in roots and shoot can be
attributed to the non-essential behaviour of the metal with no key role in the plant
metabolism. However, in Salsola kali, Cr(VI) was found to move from roots to the
aerial parts more easily than Cr(III); the anionic form of Cr(VI) possibly moves fast,
whereas Cr(III) can interact with the cell wall easily [14]. Also, Cr(VI) is taken up
by plants actively and thus forms a metabolically driven process, whereas Cr(III)
is taken up passively and is retained by the cation exchange sites of the cell wall
(Marchner
1995
; Gardea-Torresday et al.
2005
).
Chromium levels in plants growing in 'normal' soils are usually less than 1 mg g
−1
Cr (DW), rarely exceed 5 mg kg
−1
, and typically in the order of 0.02-0.2 mg kg
−1
DW (Kabata-Pendias and Pendias
1992
). The lowest chromium concentration in
above ground plant tissues is always observed in the fruit, with increases in the stem
and the highest in the leaf. Leaves usually contain higher concentrations of chro-
mium than grains (Cary and Kubota
1990
). In general, chromium concentrations in
shoots of various plants are considered very low and may not meet the nutritional
requirements for human diet. This is largely because chromium is a relatively im-
mobile element in both soils and plants and it would appear that this is due to the
prevalence of the more insoluble Cr(III) form. Some plant species (especially those
growing on serpentine soils), however, can accumulate relatively large amounts of
the element in their shoots. These are termed 'Cr accumulators'. Leaf contents in
certain accumulator plants, namely
Leptospermum scoparium,
were reported to be
as high as 20,000 mg kg
−1
(Lyon et al.
1969
). Peterson (
1975
) measured in mg kg
−1
(DW): 48,000 for
Sutera fodina
; 30,000 for
Dicoma niccolifera
; and 2,470 for
Lep-
tospermum scoparium
. In general, plant to soil concentration ratios vary widely
with some very low values such as 0.01 (Adriano et al.
1986
).
