# _Pteridium equilibrium_ (Dennstaedtiaceae), a Novel Hyperaccumulator Species of Hexavalent Chromium
## 4 Discussion
### P. aquilinum Cr Uptake  
The sporophytes from _P. aquilinum_ accumulate high amounts of Cr VI in their tissues with respect to other terrestrial ferns such as _Pellaea falcata_ [PERSON]. (35 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in fronds and 130 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in roots) [41] and _Pteris vittata_ L. (5717 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in rhizome-roots) [42]. In fact, its capacity is similar to the aquatic ferns [43]_Azolla filiculoides_ [PERSON]. (12,383.6 mg Cr\(\cdot\)Kg\({}^{-1}\) DW), _A. microphylla_ [PERSON]. (14,931.7) and _A. pimata_ ([PERSON] [PERSON] (9125.3 mg Cr\(\cdot\)Kg\({}^{-1}\) DW), all of which are employed in water body phytoremediation. Regarding the terrestrial species, this fern surpasses flowering plants such as _Datura inoxia_ (10 469 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in roots) [40], _Helianthus annuus_ (11,030 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in roots) [38] and _Prosopis_ sp. (10,983 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in roots) [39]. The accumulation capacity of _P. aquilinum_ is only surpassed by _Convolvulus arreensis_, which accumulates an average of >20,000 mg Cr\(\cdot\)Kg\({}^{-1}\) DW [37]. The biological relevance of Cr VI accumulation in plants remains unknown [5], however, the fern _P. vittata_ accumulates As, it is proposed that the plant uses this metal as strategy against the herbivory [42], this remains to be elucidated for the Cr VI. The sporophytes from _P. aquilinum_ accumulate large quantities of Cr VI in their tissues, surpassing plants such as _Datura inoxia_ (10,469 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in roots) [40], _Helianthus annuus_ (11,030 mg Cr\(\cdot\)Kg\({}^{-1}\) in roots) [38] and _Prosopis_ sp. (10,983 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in roots) [39]. Likewise, when compared to _Pteris vittata_ L. (5717 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in rhizomes and roots) [42], the only terrestrial fern in which the accumulation capacity has been tested, _P. aquilinum_ is more efficient. It is worth highlighting that according to the data, _P. aquilinum_ is surpassed only by _Convolvulus arreensis_, which accumulates on average >20,000 mg Cr\(\cdot\)Kg\({}^{-1}\) DW in its roots [37]. Bioassays describing the absorption of different oxidation states of Cr in fern gametophytes are lacking; thus, it is not possible to compare the accumulation capacity of gametophytes from _P. aquilinum_ to other plants from these taxa in this phase of the life cycle. Only a few studies report the accumulating capacity of toxic metals in this group of plants, i.e., _Athyrium yokosense_ ([PERSON] & Sav.) [PERSON] absorbs -5500 mg Pb\(\cdot\)Kg\({}^{-1}\) DW, -7500 mg Cd\(\cdot\)Kg\({}^{-1}\) DW [44] and 443 mg Ba\(\cdot\)Kg\({}^{-1}\) DW [45], _Pittyrogramma alomelanos_ (L.) [PERSON] accumulates 646.51 mg Pb\(\cdot\)Kg\({}^{-1}\) DW and 1885 mg Pb\(\cdot\)Kg\({}^{-1}\) DW [46] and _Pteris vittata_ 2180 mg As\(\cdot\)Kg\({}^{-1}\) DW [47]. It is worth highlighting that gametophytes with metal accumulation retain a correspondence with the sporophytes, since a higher accumulation for sporophytes versus gametophytes has been reported. Such an effect was also observed in _P. aquilinum_, where the sporophyte accumulation was approximately 13-fold higher than that of the gametophytes, suggesting the presence of similar mechanisms in both life cycle phases despite the differences between them. Thus, the gametophyte may be proposed as a metal-indicating plant, whether tolerance and accumulation bioassays in this phase were developed and show resistance and accumulation, it is probable that the sporophyte show high performance for the evaluated metals. The accumulating curve from the assays on gametophytes showed that metal absorption increases with time, reaching the asymptote of the curve, the time that determines the maximum capacity of Cr VI accumulation, at 48 h. For the sporophytes, the highest accumulation was not determined (as observed in Figure 3B,C) since in both curves from leaves, rhizomes and adventitious roots, the asymptote is not observed, suggesting that higher accumulation may be achieved.