128 Elena Fernández-Miranda Cagigal
This latter technique is more convenient given its lower cost and impact on the
ecosystem. In contrast, ex situ techniques such as excavation, washing and soil
storage together with the removal of chemical or physical contaminants causes
a physicochemical alteration leading to loss of soil functionality due to the
elimination of its biological activity (including nitrogen-fixing bacteria,
mycorrhizal fungi and fauna) (Ghosh & Singh, 2005).
For all the above reasons, it is necessary to develop new techniques, more
safe and secure and with less associated costs. Among the most widely
accepted in situ biological techniques of recent years for decontamination of
polluted soils stand phytoremediation. This technique is based on the use of
plants that accumulate high concentrations of heavy metals in their tissues
(Miransari, 2011) or at least tolerate it, allowing for reforestation of these
polluted areas.
Considering the plants tolerance/intolerance to growth on contaminated
soil heavy metal, we can classify them as metallophyte or intolerant.
Metallophytes are those plants, which have developed physiological
mechanisms to resist, tolerate and survive in soils with high levels of metals
(Becerril et al., 2007). And inside of the metallophytes we can distinguish
between “strict metallophytes,” restricted entirely to metal-rich soils and
“facultative metallophytes,” with populations on both metalliferous and
nonmetalliferous soils. Among them, some support the presence of metals but
do not allow the entry into the root and therefore the translocation to other
parts of the plant. Conversely, others may accumulate metals in their aerial
tissues. This accumulation depends on type of metal and plant species. These
plants are called hyperaccumulators and are used in phytoextraction.
Not all heavy metals are considered harmful to plant development.
Toxicity varies depending on concentration, persistence and origin. Therefore,
phytoavailability of metals can be defined as the ratio of metals found in the
soil that can be absorbed by a given plant genotype (Prasad, 2004).
There are several biochemical and genetic mechanisms that allow plants
develop in habitats a priori harmful to their development. These mechanisms
can be summarized as follows (Navarro-Aviñó et al., 2007):
Cell wall: translocation of metals can be prevented by attachment to
it.
Plasma membrane: tolerant plants have a number of mechanisms that
confer protection from heavy metals to their plasma membrane.
Chelation: inside cells, plants use complexation mechanisms to
cushion the effect of heavy metals, joining with ligands to form more