As mentioned previously, phytoextraction uses plants to absorb heavy metals. The term is also
confused with tolerance. Studies of evolutionary trajectories suggest that tolerance and metal accumulation
are genetically different responses of plants against excess metal. The plants used for such purposes must have certain characteristics that
can help them in phytoextraction of heavy metals. For example, they should have a high rate of growth
and high production of above- and below-ground organs such as stems, leaves, and roots so that
efficient translocation of heavy metals to all parts can be accomplished. They should also be tolerant to
high concentrations of heavy metals as well as adaptable to biotic and abiotic stresses so that they can
be easily cultivated and harvested. The plants that have all of these characteristics and
are able to tolerate and accumulate high levels of heavy metals in their above-ground parts are termed
hyperaccumulators. This term was first proposed by Brooks et al. (1977)
for those plants that can accumulate more than 1000 mg/kg of heavy metals. However, the most authentic definition is given by Baker and Brooks (1989), who state that the plants that can absorb and
accumulate greater than 100 mg/kg Cd; 1000 mg/kg Pb, Ni, and Cu; 1000 mg/kg of Zn and Mn as their
dry weight are called hyperaccumulators.
As mentioned previously, phytoextraction uses plants to absorb heavy metals. The term is alsoconfused with tolerance. Studies of evolutionary trajectories suggest that tolerance and metal accumulationare genetically different responses of plants against excess metal. The plants used for such purposes must have certain characteristics thatcan help them in phytoextraction of heavy metals. For example, they should have a high rate of growthand high production of above- and below-ground organs such as stems, leaves, and roots so thatefficient translocation of heavy metals to all parts can be accomplished. They should also be tolerant tohigh concentrations of heavy metals as well as adaptable to biotic and abiotic stresses so that they canbe easily cultivated and harvested. The plants that have all of these characteristics andare able to tolerate and accumulate high levels of heavy metals in their above-ground parts are termedhyperaccumulators. This term was first proposed by Brooks et al. (1977)for those plants that can accumulate more than 1000 mg/kg of heavy metals. However, the most authentic definition is given by Baker and Brooks (1989), who state that the plants that can absorb andaccumulate greater than 100 mg/kg Cd; 1000 mg/kg Pb, Ni, and Cu; 1000 mg/kg of Zn and Mn as theirdry weight are called hyperaccumulators.
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