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164 X.-Z. Yu and J.-D. Gu

uptake by plant roots (Zhang et al. 2010). However, one conclusive result has been
proposed in that selenite uptake may not be mediated by membrane transporters and
it seems to be accumulated through passive diffusion process, whereas organic Se
compounds’ absorption by plants from the soil solutions is chiefly achieved by
active processes (Arvy 1993; Sors et al. 2005). Uptake and transport of Se by
various plants are well documented. Plants of the genus Astragalus, Neptunia,
Stanleya, Morinda, Oonopsis, and Xylorhiza have been found to be able to
hyperaccumulate Se in their shoots while they grow normally on soils with natural
Se (Virupaksha and Shrift 1965; Brown and Shrift 1982; Davis 1986). In contrast,
Se non-hyperaccumulators do not accumulate Se above 100 mg Se kg 1 DW when
grown on seleniferous soils (Brown and Shrift 1982; Terry et al. 2000).

9.4.1 Factors Affecting Uptake and Transport

The rate of botanical uptake and transport of Se depends on the concentrations and
chemical forms of Se in the soil, ionic forms in the solution, as well as rhizosphere
conditions such as pH and redox potential, and the presence of sulfate and phos-
phate, which compete with Se uptake (Bell et al. 1992; Blaylock and James 1994;
Dhillon and Dhillon 2003; Sors et al. 2005; Sun et al. 2010). Selenate is the
predominant form of Se in alkaline and well-oxidized soils (pe + pH > 15),
whereas in well-drained mineral soils with pH from acidic to neutral (7.5 < pe +
pH < 15), Se exists predominantly as selenite (Elrashidi et al. 1987; Zhu et al.
2009). Under strongly reduced soil conditions (pe + pH < 7.5), selenide becomes
the dominant form (Elrashidi et al. 1987; Zhu et al. 2009). The dependency of Se
bioavailability on redox condition, pH, and competing ions is complicated by the
dynamic environment present in the rhizosphere, when plant roots and
microorganisms can change the conditions over time (Blaylock and James 1994;
Zhang et al. 2010).
Selenite and selenate are the two most common chemical forms in the family of
Se in the environment. The former has a strong affinity of sorption, while the latter
is more water soluble (Hamilton 2003) and both can be easily taken up by plants
(Terry et al. 1992; Zhang and Moore 1997; Shardendu et al. 2003; Banuelos and Lin
2005; Banuelos et al. 2005; Yu and Gu 2007; Sun et al. 2010; Freeman and
Banuelos 2011; Quinn et al. 2011). Much more and faster Se removed can be
observed when detached roots of willows are exposed to selenate than to selenite
(De Souza et al. 1998; Zhao et al. 2005; Yu and Gu 2008), suggesting that
independent botanical uptake pathways exist in plants between the two Se species.
Additional efforts from greenhouse experiments show that the uptake rate of
selenate by willow cuttings was approximately 2.86-fold higher than that of selenite
(Yu and Gu 2007). A higher result has also been reported by Zayed et al. (1998), in
which the total Se uptake was greater in selenate-supplied plants (4–5-fold higher)
than in selenite-supplied ones. One interesting result showed that detached leaves of
willows have been found to be unable to take up either selenate or selenite from the