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Trace Elements in Abiotic and Biotic Environments
compounds in plants has not yet been documented, synthesis of Se proteins in some
plants are reported (Terry et al. vide Kabata-Pendias 2011). Several Se organic compounds were found in plants, but their metabolic functions have not yet been established. Plants may synthesize various Se organic compounds. Several selenoamino
acids, often in association with glutathione peroxidases, were found in both bacteria
and higher plants. Predominated Se forms in plants are SeMeth (selenomethionine),
in cereal grains, and legumes seeds; and Se-methyloselenocysteine (SeMSC) in vegetables (Djujić vide Kabata-Pendias 2011). SeMSC (the most-effective anticarcinogenic Se compound) is present mainly in garlic, broccoli, and brussels (Lyi et al. vide
Kabata-Pendias 2011).
Selenium taken up by plants is incorporated mainly (up to 75%) in insoluble proteins. Impact of Se on metabolic process implants is variable. It may increase glutathione peroxidase activity and decrease dismutase functions. Excesses of both selenate
(SeO
−
4 ) and selenite (SeO
−
3 ) are toxic to most plants, and can be attributed to a combination of three factors: (1) selenate and selenite are readily absorbed from the soil
by roots and translocated to other parts of the plant; (2) metabolic reactions convert
these anions into organic forms of Se; and (3) the organic Se metabolites, which act
as analogues of essential S compounds, interfere with cellular biochemical reactions.
Selenium contents of wheat grains highly differ among various countries (average
values, in μg/kg): Algeria, 920; Australia, 23; Canada, 21; Egypt, 340; Finland, 142;
France, 36; Poland, 42; Norway, 33; Sweden, 14; and the United States, 297. In general, mean Se contents of cereal grains are higher in countries with arid climate than
in countries with humid climate. There is also variation in Se content among vegetables, depending on both crop types and growth media. Mean Se contents in various plants of the United States vary as follows (in μg/kg): roots and bulbs, 407; leafy
vegetables, 110; seed vegetables, 66; vegetable fruits, 54; and tree fruits, 15 (Fordyce
2005). The highest Se amounts contain brazil nuts, 200–253,000 μg/kg fresh weight
(FW) (mean 14,700 μg/kg FW), which grows on calcareous soils derived from the
limestone enriched in Se from volcanic eruption. Also coconuts contain elevated Se
amounts, up to about 700 μg/kg FW, presumable from similar sites.
Selenium content of forage plants has also been of a great concern. Leguminous
plants contain usually more Se than grasses. The average Se contents of grass from
various countries vary from 13 to 350 μg/kg, whereas its mean contents of clover
and alfalfa are between 15 and 672 μg/kg. However, this is not a rule, and sometimes
grass may contain several times more Se than clover, from the same sites (Oldfield
vide Kabata-Pendias 2011).
In areas with low soil Se, application of Na selenites to soils or as foliage spray
are proposed for the correction of Se nutritional deficiency. However, because of
toxic properties of Se salts, these practices should be carefully controlled. The addition of Se to soils (at 10 g/ha) affects its elevated contents in barley and oats grains
from 19 to 260 μg/kg, and from 32 to 440 μg/kg, respectively. Much higher effect is
after Se application on potatoes, which resulted in its increase in tubers from 0.47 to
1068 μg/kg (Hlušet et al. vide Kabata-Pendias 2011). Industrial pollution also impact
Se level in plants. This illustrates well-increased Se content in forest crowberries
of the Kola Peninsula, from <0.5 to 11.4 mg/kg in crowberries grown close to the
Monchegorsk plant (Reimann et al. 2001).