The uptake by and subsequent translocation of selenium (Se) within the plant is dependent on its chemical from and soil properties that dictate this trace element’s bioavailability. Plant species differ in their tendency to accumulate Se. Se taken-up by plants is returned to soil in plant residues, but the bioavailability of organic Se in those residues is poorly known. We investigated the impact of inorganic (Na2SeO4), organic (Se-enriched stem and leaf residues) Se applications and also soil microbial respiration on the growth and Se concentrations of various plant organs of oilseed rape (Brassica napus L.) during its development from the rosette to the seed filling stage. Both inorganic and organic Se slightly improved plant growth and enhanced plant development. Inorganic Se was more bioavailable than the organic forms and resulted in 3-fold to 6-fold higher Se concentrations in the siliques. Inorganic Se in autoclaved soil tended to elevate the Se concentration in all plant parts and at all growth stages. The organic Se raised Se concentrations in plants much less effectively than the inorganic selenate. Therefore, the use of inorganic Se is still recommended for biofortification.

Effects of selenium (Na 2 SeO 4) was studied in two wheat genotypes under well-watered and drought conditions in greenhouse (15 µg Se L-1) and field (20-60 60 g ha-1) experiments. Application of Se improved dry matter and grain yield under both well-watered and drought conditions. Se increased leaf concentration of pigments and photosynthesis rate under both well-watered and drought conditions. Our results indicated that Se alleviates drought stress via increased photosynthesis rate, protection of leaf photochemical events, accumulation of organic osmolytes and improvement of water use efficiency. Under well-watered condition, Se-mediated growth improvement was associated with higher photosynthesis rate and water use efficiency, greater root length and diameter, and higher leaf water content.

Selenium (Se) biofortification via crops is one of the best strategies to elevate the daily Se intake in areas where soil Se levels are low. However, Se fertilizer recovery (SeFR) is low and most of the Se taken up accumulates in non‐harvested plant parts and returns to the soil with plant residues. A pot experiment with soil was undertaken to study the efficiency of inorganic Se (Na2SeO4) and Se‐enriched plant residues for biofortification, as well as to identify the bottlenecks in Se utilization by Brassica napus L. The soil was fertilized with Na2SeO4 (0 and 7 µg Se kg−1) or with Se in stem or leaf residues (0 and 7 µg Se kg−1). A treatment with autoclaved soil was included (0 and 7 µg kg−1 as Na2SeO4) to unravel the impact of microbial activity on Se uptake. The Se‐enriched plant residues produced a lower Se uptake efficiency (SeUPE) and SeFR than did inorganic Se, and soil autoclaving enhanced Se accumulation in the plants. The time required for decomposition seems to preclude crop residues as an alternative source of Se. Furthermore, B. napus had a limited capacity to accumulate Se in seeds. The study shows that the bottlenecks in Se biofortification appear to be its low bioavailability in soil and poor loading from the silique walls to seeds. Thus, improved Se translocation to seeds would be a useful breeding goal in B. napus to increase SeFR.

Se deficiency is widespread in agricultural soils; hence, agronomic Se biofortification is an important strategy to overcome its deficiency in humans and animals. In Finland, fertilizers have been amended with inorganic Se for over 20 years to reverse the negative effects of low Se content in feed and food. Plant species, climatic conditions, other nutrients and soil properties affect the efficiency of Se biofortification. The present two years' study compared the ability of oilseed rape, wheat and forage grasses to uptake fertilizer Se applied as sodium selenate in a sub-boreal environment. The effect of foliar N application on Se uptake was tested in the second year. Se concentration was determined in plant parts and in soil samples taken at the end of growth season in both years as well as from another plot where Se fertilizer had been used for 20 years. Se fertilizer recovery in harvested wheat and oilseed rape was 1-16%, and in forage grasses was 52-64% in the first harvest and 15-19% in the second harvest. Foliar N application improved Se uptake only at the higher Se fertilizer level. The efficiency of biofortification depended on weather conditions, with forage grasses being the most reliable crop. Oilseed rape as a Se semiaccumulator had no advantage in Se biofortification in field conditions due to low translocation to seeds.

The essential micronutrient selenium (Se) is retained better in animal and human tissues in its organic forms while nonprotein selenoamino acids, such as selenomethylselenocysteine (SeMetSeCys), are considered as functional organic Se species. We studied the ability of oilseed rape Brassica napus to metabolize inorganic selenate/selenite (SeVI/SeIV) into various organic Se species, including SeMetSeCys. At 14 d after the inorganic Se application, 33% of the Se had accumulated as selenomethionine (SeMet) and 60% as SeVI, whereas no SeMetSeCys was detected. SeMet was the main organic Se species (53−94%) in seeds. Brassica napus selenocysteine methyltransferase (SMT) protein sequence revealed a substitution typical of nonaccumulators explaining the low SeMetSeCys accumulation. Brassica napus absorbs rapidly inorganic Se and converts it into organic Se forms, mainly SeMet, that are suitable for augmenting animal feed and thereby supplementing the human food chain in Se-deficient countries. In contrast, Se biofortification did not result in accumulation of the more valuable SeMetSeCys.