Agriculture Reference
In-Depth Information
and Ni
2+
concentrations. Ca
2+
status was not modified by Cd
2+
or Ni
2+
in
Sesuvium
, but it
was decreased by Cd/Cd and Ni/Ni treatments in
Mesembryanthemum
. However, for the
latter species, B/Cd and B/Ni plants showed appropriate Ca
2+
shoot amounts. Hence, our
results indicate that nutritional disturbances induced by Cd
2+
or Ni
2+
, contributed largely
to the growth restriction in both halophytes leading to limitation of heavy metals
extracted in the shoots. So, we suggest the possibility to enhance the capacity of both
species to extract these metals by increasing nutrient availability in soil.
Keywords:
Cd and Ni accumulation, halophytes, nutritional disturbances, phytoextraction,
split root system.
I
NTRODUCTION
Human activities, particularly industry, urbanism and agricultural practices, have resulted
in increased mobilisation and deposition of potentially toxic heavy metals, posing a major
threat to the environment and human health (Cunningham
et al.
1995; Lu
et al.
2004; Ait Ali
et al.
2004). In the United States for example, Approximately 63% of the sites on the National
Priority List (NPL) for the treatment of contaminated soils include contamination with toxic
heavy metals (Hazardous Waste Consultant, 1996), indicating the extensiveness of this
problem. Some heavy metals (Zn, Ni, Co) are essential micro nutrients for plant growth.
However, excess of these metals and the presence of non essential heavy metals like Cd, Pb,
Hg, even at low concentrations, may be toxic and always induce growth reduction and leaf
chlorosis (Sanità di Toppi and Gabbrielli, 1999; Ghnaya
et al.
2005; Wojas
et al.
2007). In
Tunisia, saline depressions, which are less populated, often constitute sites of accumulation of
industrial and urban effluents contaminated by heavy metals. Preliminary studies achieved in
various saline industrial sites in Tunisia showed that these zones are contaminated by Cd, Ni
and Pb (Nouairi
et al.
2002; Ghnaya
et al.
2004).
These elements can not be biodegraded and must be extracted from contaminated sites.
The clean up of heavy metals contaminated soils is one of the most difficult tasks for
environmental engineering. For these reasons, several techniques have been investigated for
the removal of these pollutants from polluted soils; but such efforts necessitated intensive
labor and were costly (Chen
et al.
2000). Interest in using plants for soil rehabilitation has
increased due to natural capacity of particular species to accumulate various heavy metals.
For phytoextraction high biomass yielding plants are to be chosen, which are able to transfer
the heavy metals efficiently and rapidly from soil via the root system to the aerial part
(Arduini
et al.
2004). Thus, the two major factors that determine the total amount of metal
extracted by plants are: (a) the metal concentrations in dry biomass and (b) the total biomass
produced by the plant. (Ghosh and Singh, 2005). Thus, plants used in this process should be
fast growing, easily propagated and able to accumulate the target metals.
Most plants used for metal accumulation are crop plants, including sunflower (
Helianthus
annus
), corn (
Zea mays
), pea (
Pisum sativum
) and mustard (
Brassica juncea
). These plants
are glycophytes (salt sensitive species) and cannot be used in phytoextraction of metals from
salty zones characterized by a higher salinity levels. Halophytes, supporting high salinity, are
promising candidates for soil desalination by producing higher biomass concomitant to
elevated salt concentration in harvestable parts. For example, Glenn
et al.
(1999) showed that
