Environmental Engineering Reference
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heavy metals present in the mine residues (Cu, Zn, Pb, Mn and Cd).
Nevertheless, dahlia survived the highest amount of residues (60%), but did
not bloom or flower for 117 days after transplantation (dat). In contrast,
marigold did not tolerate more than 40% of mine residues in the growth
substrate, but produced blooms and flowers. These authors observed that these
ornamental plants behaved different in their Cd foliar Cd accumulation
capacity when growing in mine waste residues. While dahlia accumulated Cd
between 5 to 21 mg Cd/kg foliar dry weights, marigold presented much higher
Cd concentrations (13 to 336 mg Cd/kg), which represented an interesting
prospect in phytoextraction with leaf Cd concentrations higher than 100
mg/kg. The AM fungus induced significantly higher foliar Cd accumulation in
both plants. In addition, fungus influenced protection against oxidative stress
caused by metals. For example, high concentrations of ascorbate peroxidase
(APX) were observed in early growth stages in dahlia in all substrate mixtures,
while in marigold, higher APX in leaves was shown at 40% of mine residues
at all stages of plant growth (vegetative, flowering and seeding stages), and
higher peroxidase (POD) at the seeding stage. Therefore, t he enhanced
tolerance of these ornamentals by the arbuscular fungus highlights the
potential to increase phytoremediation. An additional advantage of using
ornamental species is the low risk of metal exposition.
Echinocloa polystachya (H.B.K.) Hitchcock is another interesting
prospect to use in phytoremediation of tropical areas. This plant is a fast-
growing plant with high biomass production; it grows on soils or flooded areas
and is also able to tolerate hydrocarbon contamination. Knowing that many
soils polluted with hydrocarbons also contain high concentrations of metals, in
a first research, Solis-Dominguez et al. (2007) studied Cd tolerance in this
plant. Under hydroponics conditions, these authors found that E. polystachya
had the ability to tolerate and hyperaccumulate high Cd concentrations. No
metal-toxicity symptoms were observed at any of the Cd concentrations added
(0, 0.25, 1, 2, 10, 50 and 100 mg of Cd/mL). Higher Cd concentrations were
found in roots than in leaves (299±13.93 and 233±8.77 mg/kg dry weight,
respectively). Following electron microscopy studies, these authors also
mentioned that Cd was mainly observed in the root xylem and in the bulliform
cells of the epidermis of leaves.
Solis-Dominguez (2007) also showed that the inoculation of E.
polystachya with rhizospheric microorganisms such as Glomus mosseae BEG-
25 and Pseudomonas fluorescens 2-4 significantly increased Cd accumulation
in roots (24 and 61%) and in leaves (29 and 35%, respectively). However, the
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