Ideally, the plant chosen for phytoextraction should be
• Contaminant tolerant
• Adapted to the site conditions
• Able to produce a large biomass quickly with high metal concentrations in the shoots
• Easy to manage, dispose of, and/or recycle
• Able to be grown continually or repeatedly
No plants that possess all these characteristics are available; however, crop plants and bioenergy plants usually produce a large biomass in a short time. Crop plants are usually annual but tree species — including willows — are perennial and thus do not require annual planting. In addition, their agronomic requirements are well known and their harvest is mechanized. They accumulate metals in their tissues moderately, but could be used for multiple contamination because they may be able to take up more than one metal. In the end, the choice will depend on the final total extraction obtained multiplying the yield with the metal concentration in the shoots. This will require preliminary studies to estimate these two values.
A large genetic variability exists in willows and active breeding programs for the bioenergy production have already produced a broad range of clones, especially of S. viminalis . They have been selected for rather rich and fertilized agricultural soils and may thus not adapt easily to contaminated sites (see earlier comments). However, clones also exist that are adapted to extreme environments and grow spontaneously at contaminated sites [54-56]. They may have developed and enhanced metal tolerance to the metal present in their original soil [38,57] and may also be able to stand a limited nutrient availability. Vandecasteele et al.  proposed to use Salix species as bioindicators because the foliar content in Zn and Cd of Salix spp. grown on contaminated dredged sediments reflected the soil metal content, indicating that metal accumulation is probably a common trait to many Salix species. However, they also stressed the risk of metal transfer to the food chain. This is a recognized risk of phytoextraction, but is probably less problematic with willows than with hyperaccumulators.
FIGURE 30.5 Cadmium visualization in leaves of S. viminalis (clone 78980): autoradiographs of S. viminalis leaves grown in hydroponics and after 30 days exposure to 10 mtf Cd spiked with 2.2 10-3 mtf of 109 Cd; (a) leaves n°1-3; (b) leaf n°11; (c) leaf n°16; (d) leaf n°21; and (e) leaf n°27. Leaves were counted from the top to the base of the stem; leaf n°1 is the first identifiable leaf. The analyzed plant had a total of 29 identifiable leaves. (After Cosio, C., Ph.D. thesis n°2937, Swiss Federal Institute of Technology, Lausanne, Switzerland, 2004.)
Thus, because of the wide range of wild species that can be collected at contaminated sites, as well as the existing nurseries for biomass production, a large choice of species and clones can be tested according to site specifications and metal tolerance.
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