Page 174 - Plant-Based Remediation Processes

9 Phyto-transport and Assimilation of Selenium 165

hydroponic solution (Yu and Gu 2008), implying that both Se species are unable to
pass through the cuticle of the leaves, which are the limiting barriers in foliar uptake
of a wide range of chemicals (Scho ¨nherr and Riederer 1989).
Competitive inhibition of selenate by sulfate is well documented and can be
ascribed to the chemical similarity of the two ions (Bell et al. 1992). Both selenate
and sulfate are transported across the plasma membrane of root epidermal cells
against their electrochemical gradients, with uptake being driven by the cotransport
of three protons for each ion (Lass and Ullrich-Eberius 1984; Hawkesford et al.
1993; Sors et al. 2005). It is known that Arabidopsis thaliana mutants that lack a
functional sulfate transporter are resistant to selenate (Shibagaki et al. 2002;
Yoshimoto et al. 2002; Whanger 2002; Ellis and Salt 2003). Increasing sulfate
supply in the plant growth medium resulted in a progressive inhibition in selenate
uptake, but it caused little or no effect on selenite and SeMet uptake (Zayed et al.
1998). Phosphate is not expected to be particularly inhibitory for selenate uptake
because of their chemical dissimilarities (Hopper and Parker 1999). However,
uptake of selenate by alfalfa was decreased by increasing phosphate from 32 to
129 mM (Khattak et al. 1991). The ability of phosphate to inhibit selenite uptake by
plants is also apparent; however, competitive inhibition of selenite uptake by
phosphate occurs across diverse plant genotypes (Hopper and Parker 1999). Indeed,
increasing phosphate caused a decrease by 30–50 % in Se content of ryegrass shoots
and roots exposed to selenite, while only the roots of strawberry and clover showed
comparable inhibition of selenite uptake (Hopper and Parker 1999). Another
interesting conclusion is that the inhibition of selenite uptake in non-accumulating
species is somewhat stronger than that in accumulating species (Broyer et al. 1972;
Hopper and Parker 1999).
It is known that the capability of plants to accumulate Se in their tissues highly
relies on their genetic traits. There are significant differences in the degree of
tolerance, uptake, and accumulation of Se among different species of plants (Wu
et al. 2003; Srivastava et al. 2005; Banuelos et al. 2005). According to Se
bioaccumulation capacity, plants can be divided into three groups: primary
accumulators (hyperaccumulators), secondary accumulators, and non-accumulators
(Dhillon and Dhillon 2003; White et al. 2007). A limited number of plants,
especially from the family of Fabaceae and Brassicaceae, are able to accumulate
considerably higher levels of Se in plant materials, when grown on seleniferous
soils (Pilon-Smits et al. 1999; White et al. 2004; De Fillips 2010). The ability of Se
hyperaccumulator plants to accumulate and tolerate high concentrations of Se is
thought to be associated with a distinct metabolic capacity that enables them to
divert Se away from incorporation into proteins (Brown and Shrift 1982; Pilon-
Smits and LeDuc 2009). Translocation of Se to the shoots from the roots is largely
dependent on the form of Se supplied. Completely different results have been
reported by different research groups. For examples, ryegrass translocation
percentages (percentage of total Se taken up located in shoots at harvest) ranged
from 84 to 91 % for selenate and from 44 to 46 % for selenite (Hopper and Parker
1999). Selenate was rapidly translocated to the shoots in Indian mustard, away from
the roots, whereas approximately 10 % of the selenite was translocated (De Souza