Recent advances in phytoremediation of arsenic-contaminated soils - In-Situ Remediation of Arsenic-Contaminated Sites - page 72

the bulk soils, a 2-2.8-fold greater Fe and DOC concentration was observed in P. vittata rhi-
zosphere soil, indicating that arsenic mobilization was mainly resulted from Fe solubilization
by organic compounds in P. vittata rhizosphere. In the experiment with P. vittata growing in an
arsenic-contaminated soil (total arsenic 2270 mg kg − 1 ) for 41 d, arsenic depletion and limited
resupply in the root rhizosphere was successfully illustrated by a 19.3% decrease in arsenic flux
from solid phase to soil solution using diffusive gradients in thin films (Fitz et al ., 2003). It
seems DGT technique holds promise as an effective tool to monitor the bioavailability of target
contaminant during and after phytoextraction.

4.2.2.2 Efficient root uptake system
In addition to being efficient in arsenic solubilization from the soil, P. vittata is also effective
in arsenic uptake from soils. A comparison study between P. vittata and non-hyperaccumulating
fern N. exaltata demonstrated that a more extensive root system and highly efficient root uptake
contribute to the effective arsenic extraction by P. vittata (Gonzaga et al ., 2007b). Compared to
N. exaltata , a 2.4-3.8 times greater root system in terms of biomass was developed by P. vittata
after 8 weeks of growth in both arsenic-contaminated and control soils. Furthermore, arsenic
root uptake efficiency, which is defined as arsenic accumulation in fronds or roots per unit root
biomass was 8-23 times greater in P. vittata than that of N. exaltata , indicating a more efficient
root uptake system in P. vittata . The more extensive root system coupled with much higher root
uptake efficiency of P. vittata accounts, at least partially, for the 29-fold higher arsenic depletion
from soil by P. vittata in comparison to N. exaltata (2.51 vs . 0.09 mg arsenic per plant). This is
consistent with the study of Poynton et al. (2004) who reported a significantly lower Michaelis
constant K m for arsenate [As(V)] influx into the roots of P. vittata than N. exaltata (1.1-6.8

µ M

vs . 9.9-19.9 M), indicating a higher affinity of transporter protein for As(V) in P. vittata .

As the major arsenic species in aerobic soils, As(V) shares P transport system in higher plants
including P. vittata (Zhao et al ., 2009). As(V) uptake by P. vittata roots tend to be inhibited by
increasing P in a competitive manner (Wang et al ., 2002). Nevertheless, in the presence of As(V)
at 1-10 mg L − 1 in the uptake solution, P concentration in P. vittata was increased by 6.3- and 2.2-
fold in the roots (from 0.91 to 5.76 mg g − 1 ) and fronds (from 2.33 to 5.19 mg g − 1 ), respectively
(Luongo and Ma, 2005), which is in direct contrast to other tested ferns with an average of 40%
reduction in P level. Therefore, it is suggested that the maintenance of sufficient P in fern tissues
via efficient root uptake in the presence of high arsenic may constitute an essential detoxification
mechanism in P. vittata .
On the other hand, it should be noted that due to the restriction of fern root extension, arsenic
phytoextraction using P. vittata from soil profiles beyond root zones is much slower (Fitz et al .,
2003). Therefore, it is necessary to evaluate fern rooting depth under field conditions, which will
help to determine the effective depth of phytoextraction using P. vittata .

µ

4.2.2.3 Efficient arsenic translocation to fronds
Not only does P. vittata have efficient root uptake system but also effective translocation mecha-
nisms, making it the most striking attribute of an arsenic hyperaccumulator. Translocation factor
( TF ) has been used to characterize the effectiveness of plant arsenic translocation from the roots
to fronds. As reported by Ma et al. (2001), arsenic TF in P. vittata reached 24 with frond arsenic
being up to 7,234 mg kg − 1 compared to root arsenic of 303 mg kg − 1 after 20-week growth in an
arsenic-contaminated soil (98 mg kg − 1 As). Under variable soil arsenic concentrations from 6 to
1500 mg kg − 1 , arsenic accumulation in P. vittata fronds increased rapidly to 755-15,861 mg kg − 1
after two weeks with frond BF being 10.6-126 (Ma et al ., 2001). Following the first report
unraveling the highly efficient translocation of arsenic in P. vittata , much research using hydro-
ponics, greenhouse and field studies has consistently supported this constitutive trait of P. vittata
(Natarajan et al ., 2009; Singh and Ma, 2006; Su et al ., 2008; Tu et al ., 2002). For instance, by
growing accessions of P. vittata from both contaminated and uncontaminated environments in
arsenic-contaminated soils (0-500 mg kg − 1 ), efficient arsenic translocation resulted in an average
of TF value of 6.8 with frond BF being 11.7 to 21.6 (Zhao et al ., 2002).