Agriculture Reference
In-Depth Information
In the others hand, our data showed also that the root part of B/Cd plants which have any
direct contact with metal presented an important Cd
2+
concentration in both species (Figure,
3B). This result suggests a possible recirculation of this metal through the phloem from the
shoots towards roots. Ni
2+
recirculation was also probable in
Mesembryanthemum
but not in
Sesuvium
. This result cannot correspond to a contamination of the roots developing in free
metal solution by an exogenous source of Cd
2+
or Ni
2+
. Indeed, in
Sesuvium
, B/Ni plants
contained 160 µg Ni /g DW in the half of roots growing on basal solution supplied with 50
µM Ni
2+
and no trace of this metal in the other half of roots maintained in the Ni
2+
free
solution.
The load of Cd
2+
in phloemic flux was shown in some studies. For example, a linear
relationship was found between the Cd
2+
concentration of the soil and those of wheat grain,
suggesting that this metal passed through the xylem towards phloemic circulation (Herren and
Feller, 1997). In the same context, Zn
2+
introduced into the cut stem of wheat plants was
shown to be removed from the transpiration stream into the peduncle, loaded into the phloem,
and transported to the maturing wheat grains (Herren and Feller, 1996). To our knowledge, no
data are available concerning the recirculation of heavy metals from shoots toward the roots
and its implication in the plant responses to heavy metal stress.
In conclusion, this study showed that the culture on dual medium improve generally plant
supply with K
+
, Ca
2+
and Fe and led to an optimal growth activities, in spite of a relatively
high Cd
2+
and at less degree Ni
2+
concentrations in the shoots. These data showed clearly that
nutritional disturbances induced by the presence of Cd
2+
or Ni
2+
in the culture medium,
contributed largely to the growth restriction in both halophytes and limit the phytoextraction
capacities of both species. Additionally, our results indicate that Cd
2+
and Ni
2+
accumulation
in the shoots is compatible with a good growth activity and suggest the possibility to enhance
plant capacity to extract heavy metals by increasing nutrient availability in soil.
A
CKNOWLEDGMENTS
This work was achieved in the Laboratory of Plant Adaptation to Abiotic Stresses. It was
supported by the PRC project
“Utilisation des halophytes pour la réhabilitation et la
valorisation des sols salins”
of the Tunisian Ministry of Scientific Research, Technology and
Competence Development.
R
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2003.
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