*3.3.1. Excluders*

Several QTL studies on rice (*Oryza sativa* L.) have been conducted to determine the number of genes involved in metal accumulation and tolerance. Three putative QTLs involved in Cd accumulation have been found on chromosomes 3, 6 and 8 (Ishikawa *et al.,* 2010; Ishikawa *et al.,* 2005). Ueno *et al.* (2009) also identified another major QTL for Cd accumulation in *O. sativa* that was mapped on the short arm of chromosome 7. QTLs for the translocation of Cd from roots to sink regions were reported in *O. sativa* (Xu *et al.,* 2012; Tezuka *et al.,* 2010). Tezuka *et al.* (2010) revealed a major QTL (*qCdT7*), mapped to chromosome 7, which controlled the translocation of Cd from roots to shoots. This QTL explained 88% of the phenotypic variation indicating that low Cd accumulation was a dominant trait. Dufey *et al.* (2009), using recombi‐ nant inbred lines, identified in *O. sativa* 24 putative QTLs involved in Fe tolerance which were mapped to chromosomes 1, 2, 3, 4, 7 and 11. In addition, two QTLs, located on chromosomes

2 and 3, were involved in As concentration in shoots and in roots respectively.

reported (Tanhuanpää *et al.,* 2007).

60 Environmental Change and Sustainability

reduced translocation from root to shoot.

hyperaccumulators.

**3.3. Categories of plants growing on metal contaminated soils**

In durum wheat (*Triticum durum*, L.), Cd accumulation is controlled by a major gene named *Cdu1* and localized on chromosome 5BL (Knox *et al.,* 2009; Clarke *et al.,* 1997). Further, Ci *et al.* (2012) characterized 26 QTLs involved in Cd tolerance and accumulation in *T. aestivum*, where 16 were involved in Cd stress control, 8 for Cd tolerance and 2 for Cd accumulation in roots. In *A. sativa* L., a single QTL for Cd accumulation in grain has been

In wheat (*T. aestivum* L.), Mayowa and Miller (1991) reported QTLs involved in Cu tolerance and accumulation that were mapped to chromosomes 5A, 4D, 7A, 7B, 7D. Ganeva *et al.* (2003) also characterized QTLs for *T. aestivum* on chromosomes 1A, 1D, 3A, 3B, 4A and 7D. Bálint *et al.* (2003) identified QTLs associated with Cu tolerance located on *T. aestivum* chromosomes 3D, 5A, 5B, 5D, 6B and 7D. In addition, Bálint *et al.* (2007) also determined QTLs for Cu tolerance in *T. aestivum*. They reported one major QTL for Cu tolerance on chromosome 5D and minor QTLs on chromosomes 1A, 2D, 4A, 5B and 7D. A QTL affecting shoot Cu content under Cu stress conditions was mapped on chromosome 1BL and an additional QTL for Cu accumulation was found on chromosome 5AL. The role of these genes located on various chromosomes in these different studies suggests that Cu tolerance is a polygenic character, as well as the possibility of different gene expressions against distinct toxic Cu concentrations in different populations. The accumulation of Cu in the shoots is affected by different QTLs, suggesting a strong metal-specific uptake and/or translocation. Bálint *et al.* (2007) reported a negative correlation between Cu tolerance and accumulation in the shoot indicating that the key tolerance mechanism in wheat could be the restriction of Cu uptake in the roots or the

Baker and Walker (1990) categorized plants into three groups according to their strategy for coping with metal toxicity in soil; metal excluders, indicators and accumulators/ The metal excluder strategy consist in limiting the amount of metals translocated from roots to shoots thus maintaining low levels of metal concentration in their aerial parts. Large amounts of metals in the roots of excluder species have been reported (Baker and Walker 1990). Examples of excluder species include *Oenothera biennis, Commelina communis, Silene maritime*, *Agrostis stolonifera* L., woody plants such as *Salix, Populus* and *Pinus radiata* (Maestri *et al.,* 2010; Wei *et al.,* 2005).
