# _Pteridium equilibrium_ (Dennstaedtiaceae), a Novel Hyperaccumulator Species of Hexavalent Chromium
## 4 Discussion
### P. aquilinum as Novel Cr Hyperaccumulator Species  
Sporophytes from _P. aquilinum_ showed the highest Cr VI absorption values. It also fulfills most of the criteria to be considered a hyperaccumulator. Both roots and rhizomes accumulated an average of 1.1973% Cr DW, and 0.2198% Cr DW was accumulated in leaves, significantly higher than the values previously reported by [PERSON] et al. [51], [PERSON] et al. [52] and [PERSON] and [PERSON] [10]. Such works report that to be considered a hyperaccumulator, a given species must absorb a minimum percentage of 0.1% DW of Cr. The roots and rhizomes from _P. aquilinum_ accumulated 11,973.93 mg Cr\(\cdot\)Kg\({}^{-1}\) DW, while the reported values for Cr-absorbing plants (under natural conditions) range from 0.2 to 5 mg Cr\(\cdot\)Kg\({}^{-1}\) DW. Our results indicate that _P. aquilinum_ has an efficiency about 50,000-fold higher. Thus, this fern certainly could be considered a hyperaccumulator.  
Moreover, the criteria reported by [PERSON] and [PERSON] [49] and [PERSON] et al. [50] establish that a hyperaccumulator plant must incorporate higher amounts of Cr in the aerial parts (stem and leaves) than the roots. With such criteria, the fern studied here could not be considered a hyperaccumulator. It is worth noting that a consensus rule to classify hyperaccumulator plants is lacking. Here, three different criteria are cited and _P. aquilinum_ fulfills two of them. The third criterion proposed by [PERSON] and [PERSON] [49] and [PERSON] et al. [50] could be discarded since there are no reports of any plant capable of absorbing higher amounts of Cr in leaves than roots, possibly because of its low translocation rate.  
The capacity of gametophytes to remove Cr VI from culture media, regardless of its tissular simplicity, surpasses that exhibited by vascular plants, such as _Nymphaea spontanea_ [PERSON], (31.6%) [53], _Spirodelia punctata_ ([PERSON] [PERSON]. (19%) [54], _Spirodelia polyrhiza_ [PERSON][PERSON] 15% [54] and _Lenna gibba_ [PERSON]. 10% [54]. On the other hand, the sporophytes by far surpassed the remotion capacity shown by the gametophytes. This could be compared to _Eichlornia crassipes_ [PERSON][PERSON] [55], which removes 84% of metal, _Spirodelia polyrhiza_ [PERSON][PERSON] 75% [56], _Salvinia molesta_ [PERSON]. [PERSON]. 90% [56] and _Miscanthus Sinensis_ 90-92% [57]. These plants have been reported to have the highest remotion values. It is worth highlighting that sporophytes from _P. aquilinum_ have terrestrial habits and showed similar or even higher remotion values than those shown by aquatic angiosperms.  
The sporophytes from _P. aquilinum_ exhibited Cr VI hyperaccumulation and robust resistance. Moreover, this species has interesting biological attributes; it is cosmopolite [58; 59; 60] and has high underground propagation, fire resistance [61] and drought resistance [62]. Altogether, these characteristics allow it to establish and grow in perturbed and polluted soils. Furthermore, interesting metabolic traits have been reported in this species; it has allelopathic effects [63] and several defenses against herbivory [64], both important parameters considered in large-scale ex situ remediation processes. The allelopathy prevents other species from colonizing the bioreactor where the remediation process is carried out. Meanwhile, the anti-predatory strategies prevent biomagnification [6], namely the entry of Cr into the trophic network when it has been accumulated in the vegetal tissues. In consequence, and taken together with previously reported biological features, _P. aquilinum_ is an excellent candidate due to its capacity for accumulation, remotion and resistance to Cr VI characterized for the first time in this work.