They are fast growing plants that can be easily cultivated using established agronomic practices which compensate for their relatively low capacity of metal accumulation. Their metal uptake capacity can further be enhanced by adding

conditioning fluid containing a chelator or another agent to soil to upsurge metal solubility or mobilization so that the plants can absorb them more easily. This is known as chemically induced/assisted phytoextraction. Afterwards, the soluble metal (desorbed from soil particles) is easily transported to roots surface via diffusion and translocated from roots to shoots. Complexing with organic ligands, which may occur at any point along the transport pathway, converts the metal into less toxic form thus conferring high metal tolerance in biomass plants [5]. A wide range of synthetic chelates [e. g.Ethylenediaminetetraacetic acid (EDTA), 1,2-cyclohexylenedinitrilotetraacetic acid (CDTA), diethylenetriamine- pentaacetic acid (DTPA), EGTA, EDHA, hydroxyethylethylenediaminetriacetic acid (HEDTA), nitriloacetic acid (NTA), and organic acids (e. g. citric acid, oxalic acid, malic acid) are used for enhancing root uptake and translocation of metal contaminants from soil to biomass plants, thereby improving phytoex­traction [6] (Fig. 14.3, 14.4, 14.5).

Fig. 14.4 Willows

Fig. 14.5 Poplars

Example: Indian mustard, sunflower, and maize as high biomass crop plants, willows, and poplars as high biomass trees

Advantages. The main advantage of phytoextraction is environmental friend­liness. Traditional methods that are used for cleaning up heavy metal contaminated soil disrupt the soil structure and reduce its productivity, whereas phytoextraction can clean up the soil without causing any kind of harm to soil quality. Add on benefit of phytoextraction is that it is less expensive than any other clean-up process (Fig. 14.6) (Table 14.1).

Fig. 14.6 Phytoextraction

Table 14.1 Main characteristics of the two categories of plants for phytoextraction of metals [4]: Chemically assisted phytoextraction Natural phytoextraction