**4.2 Bioaccumulation of Zn(II) and Cu(II)**

Heavy metals are inorganic pollutants that cannot be degraded, so the principal strategy for plants should be to immobilize them in their rhizosphere, accumulate them in the roots, or translocate them to the aerial part [72]. They enter the root either by crossing the plasma membrane of the root endodermal cells, by entering the root apoplast through the space between cells, or with the aid of membrane transporter proteins. These transporters are present in membranes of different organelles such as tonoplasts, endoplasmic reticulums, mitochondria, or chloroplasts [73]. Inside the plant, they can be chelated by glutathione (GSH), phytochelatins (PCs), or metallothioneins (MTs), chelators that have thiol (dSH) groups, which gives them a high affinity for metal cations [74]. Also, this process may work synergistically with secondary stress-defensive antioxidative systems to combat metal-induced oxidative stress [75]. Metals in roots can be stored in vacuoles, cell walls or exported to the shoot via the xylem. Vacuoles are considered the main storage site for metals in plant cells, being a part of the tolerance mechanism [76].

In general, plants can contain, in their total biomass, Zn(II) in ranges from 30 to 100 mg kg<sup>1</sup> dry weight (DW); concentrations higher than 300 mg kg<sup>1</sup> DW are considered phytotoxic [77], but for other authors, this limit is set at 100 mg kg<sup>1</sup> DW [78]. For Cu(II), normal total biomass content ranges from 2 to 50 mg kg<sup>1</sup> DW, depending on the plant species. However, 5–20 mg kg<sup>1</sup> DW seems to be optimal, as toxicity symptoms appear above and deficiency symptoms below this critical range [79]. In the present work, *C. indica* accumulated values higher than the limits considered phytotoxic, reaching up to 8723.99 694.68 mg kg<sup>1</sup> DW for Zn(II) (SD) and 1432.15 91.13 mg kg<sup>1</sup> DW for Cu(II) (SD) in the total biomass in the maximum tested concentrations. Numerous authors showed the capacity of Zn(II) and Cu(II) accumulation of *C. indica* growing on different substrates [80–82].

Indexes are calculated to determine the phytoextraction efficiency, mainly being the bioaccumulation index (BI) and the translocation index (TI) [83]. An effective phytoextraction process requires the translocation of metals to easily harvestable parts. Plants with BI values less than 1 are unsuitable for phytoextraction. In this work, *C. indica* indexes suggest that this plant could act as a phytostabilizer because it showed low translocation to the aerial part but a high accumulation of both metals in the roots. Under this type of stress, the root suffers the first exposure, limiting transmission of heavy metals to other tissues [84]. Many studies found the same for the *Canna* genus for different heavy metals [85–87].
