Selenium is present in soil in small amounts, ranging from 0.01 to 2 mg kg⁻¹. High selenium concentrations >5 mg kg⁻¹ can be encountered in seleniferous soils (Mayland et al. 1989). Chemically selenium (Se) has commonalities with sulphur (S). That is selenium can exist as selenide Se²⁻, as elemental selenium Se⁰, selenite SeO₃²⁻ and selenate SeO₄²⁻ oxidation states. Soil pH largely determines which of these chemical species is present. In alkaline and well-oxidized soils the predominant form of selenium is selenate, in well-drained mineral soils with pH from acidic to neutral its selenite, and selenide is found under reduced soil conditions (Elrashidi et al. 1987).
For animals and humans selenium is an essential micronutrient, but essentiality has not been established for higher plants (Terry et al. 2000; Sors et al. 2005). Deficiency of Se in humans is common between 0.5 and 1 billion people worldwide may benefit from increased Se intake (Combs et al. 2001). This is one of the main reseasons for plant research on Se. Nevertheless, there are some beneficial effects of Se in higher plants. Reported are higher seed set in Brasica Napa, delayed senescence in lettuce, better UV resistance and growth promotion in ryegrass. Aside from these examples there are not many reports on the effects of Se on higher plants.
As plant-based foods are an important source of Se for both humans and domestic animals, it is important to understand how plants take up and metabolize Se. Because selenite (Se) is a chemical analogue of sulphate (S); Se and S compete for the same uptake transporters. Therefore, high sulphate supply will reduce selenate uptake (Mikkelsen and Wan 1990; Zayed and Terry 1992). However, upregulated expression of sulphate transporter genes, leads to a strong increase in the capacity for selenate uptake (Li et al. 2008; Shinmachi et al. 2010).
There are large differences between plant species in Se uptake, accumulation and sensitivity to high Se levels. An example of these differences between plant species in Se accumulation is shown in Table 5.3. Based on differences in Se content plants can be classified into Se-accumulators and non-accumulators. Most agricultural and horticultural plant species are non-accumulators (Shrift 1969) in these species Se toxicity can occur even at concentrations below 100 μg Se g⁻¹ (Mikkelsen and Wan 1990). White et al. (2007) found that Se non-accumulators had a strong relationship between leaf S and leaf Se concentration (Figure 5.7), confirming that selenate and sulphate accumulation are strongly linked. For example, Brassicaceae species can accumulate more selenium because they have a greater ability to accumulate sulphur. From Figure 5.7 it seems that the transporter(s) of non-accumulators may have a higher affinity for sulphate than for selenate. Visa versa Se accumulators have a higher selectivity for selenate than for sulphate.
Se concentration (mg kg-1 dw)
Table 5.3: Se concentrations in shoots of accumulator and non-accumulator species growing on a soil with 2–4 mg Se kg-1 (Copied from Marschner 2012)
Figure 5.7: Relationship between leaf Se and S concentrations in 39 plant species grown hydroponically with 0.91 mM sulphate and 0.63 µM selenate. Closed symbols represent Brassicaceae species. (Copied from Marschner 2012).
At time of writing there is a growing interest in enhancing selenium content in plants. This interest follows from increased awareness of the importance of Se to human health. Selenium mainly enters our food chain by plant uptake from the soil. The high variation in Se soil content results in variation of Se in the human diet. The minimum Se concentration for animals and humans is about 50–100 μg Se kg-1 dw in fodder/food (Gissel-Nielsen et al. 1984). Currently there are two strategies to increase human intake of Se by eating plants: i) using Se fertilizers (agronomic biofortification) or ii) by genetic improvement in crop Se accumulation.
Agronomic biofortification with Se has already been practised in Finland since the mid 1980s. Mandatory additions of Se as Na selenate to all multi-nutrient fertilizers (6–16 mg Se kg-1 fertilizer) has been very effective. Selenium concentrations in cereals, vegetables and animal products was significantly increased and Se intake by the Finnish population more than doubled (Hartikainen 2005). Agronomic biofortification is considered to be advantageous over Se supplementation because inorganic Se is assimilated by plants into organic forms and these organic form are more bioavailable to humans. Plants can, moreover, act as an effective buffer preventing accidental excessive Se intake as may occur with direct supplementation (Hartikainen 2005). It is relatively easy to increase Se concentrations in food crops by fertilization, because selenate is highly bioavailable to plants and is readily transported from roots to shoots, where it is assimilated into different organic forms. The field studies of Broadley et al., (2010) with winter wheat show for example that Se concentration in the grain increased linearly from 30 μg kg-1 in the control to 2,600 μg kg-1 in the treatment receiving an additional 100 g Se ha-1 in the form of Na selenate (Figure 5.8).
Figure 5.8: Relationship between Se fertilization and grain Se (Copied from Marschner 2012).
Biofortification of Se in crops through classical breeding is hard as it requires genetic variation within the species in the uptake and/ or assimilation of Se, which is hardly found in many species. Inter species variation is much bigger, as illustrated by the Se accumulators (Figure 5.8 and Table 5.3). Several studies have reported a multi fold increase in Se uptake by transgenic plants overexpressing various genes involved in S/Se assimilation. These studies are however aimed to enhance Se uptake for the purpose of phytoremediation of Se-contaminated soils (Pilon-Smits and LeDuc 2009), thus soil with extremely high Se concentration. If these transgenic plants can accumulate more Se from soils with low Se availability is unkown.
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