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24 M. Barbafieri et al.

production and the metal concentration in the biomass are fundamental success
factors for the practical efficiency of phytoextraction (McGrath and Zhao 2003;
Robinson et al. 2003b).

2.2.2 Protocols for Enhancing Metal Phytoextraction

Several strategies for achieving more efficient heavy metal removal have been
recently developed such as the enhancing concentration of soluble heavy metals in
the soil with the application of synthetic chelate agents (e.g., EDTA). This then
leads to an increase in the metal uptake of high biomass crop plants (e.g., Brassica
juncea, Helianthus annuus, Zea mays, and Nicotiana tabacum) (Meers et al. 2005;
Di Gregorio et al. 2006; Pedron et al. 2009).
An alternative strategy, to increase the efficiency of the assisted phytoextraction,
is to use plant growth regulators (PGRs) to counteract the negative effects of heavy
metal stress in growing plants and boost the shoot biomass (Ouzounidou and Ilias
2005; Lopez et al. 2007; Barbafieri and Tassi 2010; Zhao et al. 2011; Barbafieri et al.
2012). PGRs play a major role in cell division and cell differentiation. They can
stimulate shoot initiation, bud formation, the growth of lateral buds, leaf expansion,
and chlorophyll synthesis. They can also delay leaf senescence, enhance resistance
to salinity, low temperature and drought, and induce stomatal opening in some
species (Letham et al. 1978; Barciszewski et al. 2000; Pospisilova et al. 2000). The
combined effects of EDTA and cytokine resulted in an increase in the Pb and Zn
phytoextraction efficiency (up to 890 % and 330 %, respectively, compared to
untreated plants) and up to a 50 % increase in foliar transpiration (Tassi et al.
2008). Cytokinins have also showed potential use for the increasing of Ni
phytoextraction capability in Alyssum murale, a well-known Ni hyperaccumulator
(Cassina et al. 2011). Application of exogenous PGRs was examined as a viable
technique to increase the efficiency of plant metal extraction from contaminated
soils. However, further experiments are needed to increase the knowledge of the
dynamics of the transport mechanism involving metal uptake, since this mechanism
is dependent on plant characteristics and environmental parameters. In order to
increase the efficiency of phytoextraction, fertilizers can be used to enhance the
productivity of selected plants; positive results have been reported recently in the
case of the boron-contaminated soils (Giansoldati et al. 2012).

2.2.3 Experimental Protocols for Phytoextraction: Applicability
Test at Different Scales

In practice there are always many variables that render each situation “site-specific,”
so cases in which it is possible to skip feasibility test and proceed to large scale field
projects are very rare. In general, the following sequential test steps are applied: