proposed to additionally set
hyperaccumulation threshold criteria at a level that is (i) 2–3 orders of
magnitude higher than in plant leaves on normal soils, and (ii) at least
one order of magnitude greater than the usual range in plant leaves
on metalliferous soils. Taking this into account, despite not reaching
1000 μg g−1 DW both species T. minuta and B. pilosa showed values
that make them candidates for further studies about their capabilities
as phytoremediators [T. minuta meeting requirement (i) and B. pilosa
meeting requirement (ii)]. Nevertheless, as it was pointed by van der
In addition, both species are annual and fast-growing herbs, features
considered to be optimal for phytoremediation (Blaylock et al., 1997).
Other species from genera Tagetes and Bidens were reported for
phytoextraction of heavy metals from soils, but T. minuta and B. pilosa
have never been reported for this purpose. Most of the other studied
species also showed higher concentrations than plants growing on uncontaminated
soils, but with relatively low differences, indicating that
lead was transferred to plant tissues but not accumulated at concentrations
to be of interest for phytoremediation.