Agriculture Reference
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photosynthetic organs. Our data showed that reduced growth activity estimated by the RGR
values in Cd/Cd and Ni/Ni plants were accompanied by the highest Cd
2+
(Figure 6A) and Ni
2+
(Figure 6B) shoot concentrations. However, plants grown on dual medium showed high
growth potentialities concomitant with a relatively high Ni
2+
and particularly Cd
2+
concentrations in leaf tissues. Thus, growth inhibition showed in plants subjected totally to
Cd
2+
and Ni
2+
resulted not only from the direct alteration of photosynthesis process by Cd
2+
and Ni
2+
transported to the shoots, but this effect could imply additionally the action of metals
on essential nutrients acquisition. In fact, the relationship between RGR and the nutrients (K
+
,
Ca
2+
and Fe) amounts in the shoots (Figure 7) demonstrated that the high growth potentialities
observed in B/B (control), B/Cd and B/Ni plants were accompanied by the highest K
+
, Ca
2+
and Fe uptake. Conversely, Cd/Cd and Ni/Ni plants expressed the lowest growth activities
and the reduced K
+
, Ca
2+
and Fe amounts in the shoots. On the basis of these data, we suggest
that the reduction of biomass production found in Cd/Cd and Ni/Ni plants may results, at least
partially, from the restriction of K
+
, Ca
2+
and Fe uptake by both metals Cd and Ni.
Nutritional disturbances have been reported in several species submitted to heavy metal
stress. For example, a drastic decrease in shoot potassium concentrations was observed in
white lupin (Zornoza
et al.
2002), pea (Sandalio
et al.
2001) and
Pinus sylvestris
(Kim
et al.
2003) plants under cadmium stress. Heavy metals could act indirectly or directly on nutrient
absorption, several explanations were given. Cd
2+
may limit K
+
absorption by complexing
ATP and reducing energy availability for the absorption of this nutrient. In the same context,
Kim
et al.
(2002) concluded that heavy metals compete with Ca
2+
for transport into root cells
of rice plants. They showed that the level of heavy metals bound to cell wall is much lower
than that of the total cellular level, so, this competition was not for the binding sites on cell
wall, but for the transport across the plasma membrane. Additionally, it has been
demonstrated that Cd
2+
could permeates through Ca
2+
-channels in animal (Olivi and Bressler,
2000) and plant (White 2000, Krämer
et al.
2007) cells. Furthermore, Clemens et al. (1998)
demonstrated that the plant transporter LCT1 mediates the uptake of both Ca
2+
and Cd
2+
in
yeast. In higher plants, Cd
2+
was assumed to enter root cells via either the higher affinity
uptake system for Fe2+ or low affinity system for Ca
2+
uptake (Lombi
et al.
2002; Zhao
et al.
2002; Roosens
et al.
2003). More recently, Suzuki (2005) found that elevated Ca
2+
concentration in the culture medium can greatly alleviated the toxicity of Cd
2+
in
Arabidopsis
even at high (200 µM) concentration in the medium, which is consistent with the competition
theory between the two cations during influx.
In addition, we observed that when root system was completely immersed in solution
containing metals, especially Cd
2+
, young leaves of both species suffered from chlorosis. This
symptom, which is a typical response of all plant to the lack of iron in the medium, was
attributed by (Wallace
et al.
1992, Larbi
et al.
2002) to heavy metals-induced Fe deficiency.
In this experiment some data were consistent of these explications. In fact, figure 5 shows that
when the entire root system was developed in the presence of Cd or Ni, shoot Fe
concentrations were significantly reduced. Some researchers explicated the inhibition of Fe
absorption in the presence of heavy metals in the medium by the competition between iron
and metallic cations in root plant cells transporters. Therefore, it was shown in
Arabidopsis
that uptake of both Cd
2+
and Fe may occur via members of a plant metals transporters family
with homology to
Nramp
genes that are inducible by Fe starvation (Thomine
et al.
2000).
