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Selenium [Se, 34]
Although Se is not an essential element for plants, with some exceptions, it is being
added to soils to ensure that both food and fodder plants contain its adequate amounts
for dietary needs. It should be emphasized, however, that the margin of safe Se concentration is rather narrow. Contents of Se in crop plants received recently much
attention, because of its importance in the food chain. Thus, most data are for Se in
food and fodder plants. Especially cereal grains, as the most common Se source in
diets, have been broadly analyzed. In particular, the anticarcinogenic effectiveness
of various Se compounds in plants have been recently investigated (Fordyce 2005).
The Se uptake by plants depends on several factors, however, it is always easily
taken up by plants, when is present in mobile species in soils. In most cases there is
a positive linear relationship between Se in plant tissues and Se content of soils, and
soil pH. Soil properties have a great impact on the Se phytoavailability. It is easily
taken up by plants from akaline sandy soils, in arid climate, than from acidic clay
soils, with elevated content of soluble organic matter, in humid climate.
The greatest Se uptake is from alkaline soils, at pH about 6.5. However, Se uptake
depends, to a great degree, on plant capability. Thus, plants have been divided into
three categories: (1) plants that are accumulators and contain high amounts of Se,
>1000 μg/kg, and presumably require this element; (2) plants that absorb medium
Se quantities, around 100 μg/kg; and (3) plants that are nonaccumulators, with low
Se contents, <30 μg/kg. Plants that hyperaccumulate Se, up to about 5000 μg/kg, are
mainly of Astragalus family, and also of several other herbaceous species.
The kind of Se added to soils (contamination or supplementation), and various
plant factors have an impact on the Se speciation, and thus its uptake by plants.
Especially the formation of SeMeth by both, rhizosphere microbes and roots of
some plants, have an impact on Se phytoavailability and phytovolatilization. In some
specific root–soil interactions, Rhizobium bacteria and root exudates stimulate the
oxidation of SeO 3 to SeO 4 , what increases Se availability to plants. Thus, plants inoculated with bacteria reveal a higher capability to both Se uptake and Se volatilization. On the other hand, the reduction of SeO 4 to SeO 3 appears to be a rate-limiting
step in the production of volatile Se compounds by plants. Inhibitory effects of S on
the uptake and volatilization of Se may be reduced substantially if Se is supplied as,
or converted to, SeO 3 and/or SeMeth. Interactions between Se and S, at the root–soil
surface, may also control Se phytoavailability, whereas some Se compounds stimulate S adsorption by plants (Rios et al. vide Kabata-Pendias 2011). The phytovolatilization of Se during the growing season of some plants may be significant (e.g.,
about 35 mg Se/m 2 by Salicornia species). It seems to be a promising method for the
removal of excess Se from contaminated soils.
All mobile Se species are easily taken up by plants with the water flow. Only selenates and organic Se are driven metabolically. In general, organic Se compounds are
more readily available to plants than inorganic species. On the other hand, SeMeth
compounds are easily transported within plants tissues. However, the mechanism of
Se absorption and metabolization by plants is very similar to those of S.
The Se function in plants has been broadly investigated in recent times, but there
is still little evidence that Se is essential for all plants. Recent studies on some grasses
and vegetables have indicated that at proper Se addition plant growth is stimulated
(Hartikainen vide Kabata-Pendias 2011). Although the essentiality of Se organic