Selenium biofortification and growth of onions as affected by Se application, biochar and irrigation

Selenium (Se) biofortification is a plant-based strategy to increase Se composition in crops by adding Se onto the crop or to the soil. Consumption of Se-enriched crops may increase Se intake needed for people living in Se-deficient regions. In this 3-year biofortification study, Se accumulation and growth were evaluated in onion (Allium cepa L.) as affected by the applied Se form (selenate, selenite, selenomethionine), Se application technique (soil vs foliar), addition of biochar originating from softwood feedstock, and irrigation rate (50, 75, 100 % ET_o). Results showed Se concentrations were highest in onions treated with foliar application of selenate, irrespective of biochar added to soil or water application rate. Selenium speciation of the onion showed that gamma-glutamyl-Se-selenomethyl-selenocysteine (γ-glu-MeSeCys) was detected at the greatest level in the onion bulb, compared to methylselenocysteine (MeSeCys) and selenomethionine (SeMet), irrespective of treatments. The combination of foliar Se application, biochar, and deficit irrigation at 50 % ET_o, reduced onion bulb fresh weight compared to biochar addition under high irrigation treatments. In conclusion, foliar application of selenate at 75–100 % ET_o, with or without biochar, was most effective for increasing Se and γ-glu-MeSeCys concentrations in onion bulbs grown in a light textured soil.