*3.3.2. Accumulators/hyperaccumulators*

Metal accumulators/hyperaccumulators are plants that can concentrate metals in their above-ground tissues to levels that exceed those in the soil or also to those in the non accumulating species found growing nearby with concentrations up to 100 times more than non hyperaccumulators (Salt *et al.,* 1998). Accumulators/hyperaccumulators growing on metal contaminated environments can naturally accumulate higher levels of heavy metals in their shoots than in their roots (Kachout *et al.,* 2009). Some plants can accumulate only a specific metal while others can accumulate multiple metals ((Mganga *et al.,* 2011; Almås *et al.,* 2009). Presently, at least 45 plant families comprising more than 400 species have been found to accumulate metals in their harvestable tissues, and the majority of them belong to the *Brassicaceae* family (Pal and Rai 2010). The best known genera from this family are *Alyssum* and *Thlaspi*. *Thlaspi* species can accumulate more than 3% of their shoots in Zn, 0.5% in Pb and 0.1% in Cd. *A. halleri* can also accumulate more than 1% of its aboveground biomass in Cd and Zn and *Alyssum* species can accumulate over 1% Ni in their harvestable parts (Di Baccio *et al.,* 2011). There are variations among family, species and populations in the ability to accumulate metals. For example*, Arabidipsis halleri* can accumulate Cd and Zn in their harvestable parts where as *A. thaliana* is known to be a metal excluder and restricts metals in the roots. *Betula spp.* can accumulate Zn, while other trees species of the same family (*Carpinus* and *Corylus*) are unable to do so (Ernst, 2006; Ernst, 2004).

#### *3.3.3. Indicators*

Like accumulators, metal indicators accumulate metals in their aerial tissue, but the metal levels in the above ground tissue of these plants usually reflect the metal concentration in the surrounding environment (Baker and Walker 1990). If these plants continue to uptake metals, they will eventually die-off. These plants are of biological and ecological impor‐ tance since they are pollution indicators and also, like accumulators, they absorb pollu‐ tants (Mganga *et al.,* 2011).

#### *3.3.4. Determination of excluders, indicators and hyperaccumulators plants*

A plant is classified as a hyperaccumulator when it meets four criteria including; a) when the level of heavy metal in the shoot divided by level of heavy metal in the root is greater than 1 (shoot/root quotient > 1); b) when the level of heavy metal in the shoot divided by

total level of heavy metal in the soil is greater than 1 (extraction coefficient > 1) (Rotkitti‐ khun *et al.,* 2006; Harrison and Chirgawi 1989); c) when the plant takes up between 10 – 500 times more heavy metals than normal plants (uncontaminated plants - control plants) (Fifield and Haines 2000; Allen,1989); and d) more than 100mg/kg of cadmium, 1000g/kg of copper, lead, nickel, chromium; or more than 10000mg/kg of zinc (Mganga *et al.,* 2011; Ernst, 2006; Brooks, 1998). An excluder is a plant that has high levels of heavy metals in the roots but with shoot/root quotients less than 1 (Boularbah *et al.,* 2006). Finally, Baker and Walker (1990) classified a plant as an indicator when the levels of heavy metals within their tissues reflect those in the surrounding soil.

#### **3.4. Physiological mechanisms of metal resistance**

Resistant plants are able to grow on metal contaminated soil due to avoidance and/or tolerance strategies. Plant resistance to high levels of heavy metals in soils can result from either reduced uptake or once taken up, metals have to be transformed into a physiologically tolerable form.
