# _Pteridium equilibrium_ (Dennstaedtiaceae), a Novel Hyperaccumulator Species of Hexavalent Chromium
## 4 Discussion
### Sensitivity of P. aquilinum to Cr  
According to the data, both gametophytes and sporophytes from _P. aquilinum_ showed high resistance to several concentrations of Cr VI, since neither chlorosis nor biomass reduction were observed after the exposure to this metal. This effect was enhanced in the underground parts of sporophytes since biomass increased. Although it has been documented that Cr is a micronutrient to plants that stimulates both parameters, its concentrations should not exceed 1 uM, since this metal may induce toxicity [2; 32]. In this work, the _P. aquilinum_ plants were exposed to higher concentrations of Cr and such structures showed no signs of damage from toxicity.  
Comparing the _P. aquilinum_ sensitivity, specifically the of gametophytes, with other species of seed plants showing resistance to Cr VI is difficult since the tissue structure is simple (thalloid) when contrasted with seed plants, which possess a vascular system (corn). However, the sensitivity shown by this phase may be compared to assays on germinating seeds exposed to Cr VI, since this is the most vulnerable phase in the life cycle of spermatophytes. In order to compare our results from gametophytes to other works, molar concentrations were converted to parts per million (ppm), where K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) VI 50 uM equals 4.1 ppm. Thus, K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) VI 200 uM = 24.3 ppm, 600 uM = 30.9 ppm and 800 uM = 37.1 ppm. Although the _P. aquilinum_ gametophytes lack structural complexity, they did not show any sign of damage or structural deterioration after being exposed to different concentrations of K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\). The opposite has been shown in several studies, where treatments with even lower concentrations of Cr VI during germination negatively impacted the chlorophyll _a  
Figure 4: Remotion percentage of Cr VI from substrate in gametophytes (**A**) and sporophytes (**B**). For every treatment, mean \(\pm\) SEM is presented. * indicates significant differences among treatments vs. control and in the time Interval, \(p\) < 0.05, \(n\) = 3.  
and \(b\) and biomass of _Salvia sclarea_ L. (1, 5 and 10 ppm) [33], _Echinochla colona_ (L.) Link. (1.25 and 2.5 ppm) [34], _Caesalpinia pulcherrima_ (L). Sw. (25 ppm) [35], _Zea mays_ L. (5 and 10 ppm) [36], _Lycopersicon esculentum_ Mill. (5 and 10 ppm) [36] and _Brassica olerca_ L. (5 and 10 ppm) [36]. Because of their tissular simplicity, it is expected to observe a higher sensitivity of gametophytes to Cr VI than that of a plant with more complex tissues and organs (such as roots, stems and leaves), even in young stages (e.g., germination), in contrast to our results. Thus, the homeostasis systems in gametophytes are likely to be more efficient. However, this remains to be studied in future works.  
Regarding the results from roots, rhizomes and leaves from sporophytes exposed to Cr VI, the data indicate that leaves did not show chlorosis or biomass reduction, whereas rhizomes and adventitious roots increased their size. The effect on both parameters suggests that the amount of Cr VI used in this work not only did not exceed the plant's capacity to resist the toxicity, but it also promoted the growth of underground parts. In fact, this has not been reported previously in other species exposed to Cr VI. Thus, _P. aquilinum_ is one of the most resistant species to Cr reported in the literature to date. This fern is even more resistant than _Convolvulus arreensis_ L. [37], _Helianthus annuus_ L. [38], _Prosopis_ sp. L. [39] and _Datura innoxia_ Mill. [40], all of which are reported to be highly resistant plants to Cr with superior accumulation capacity. However, it has been reported that such species show a few toxicity signs when challenged with higher concentrations of the metal.