Selenium biofortification in field-grown tomatoes as affected by Stanleya pinnata-derived organic Se application, biochar and irrigation

Selenium (Se) biofortification is a plant-based strategy to increase Se content in crops. Consumption of Se-enriched crops can enhance Se intake for populations experiencing mild/moderate Se deficiencies. In these two different Se biofortification studies, Se accumulation was evaluated in field-grown tomatoes (Solanum lycopersicon). In year 1, organic Se as Se-enriched plant material (Stanleya pinnata; 350 mg Se kg⁻¹ DW) was directly applied to soil at three different rates (100, 200, and 400 g Se ha⁻¹). In year 2, organic Se extracted from S. pinnata was applied to soil at two different rates (50 and 100 g Se ha⁻¹). On the field site, the light-textured soil was previously amended with biochar (softwood feedstock) 3-years earlier to enhance water retention in soil during drought conditions in California. For both experiments, irrigation water was applied at two different rates (50 and 100 % ETo). In year 1, Se concentrations were significantly (p < 0.05) increased to 0.39 μg Se g⁻¹ DW in fruit grown with application of S. pinnata to soil and in year 2, fruit Se concentrations significantly (p < 0.05) increased to 0.07 μg Se g⁻¹ DW with high rates of extractable Se from S. pinnata applied to soil, irrespective of biochar or irrigation treatment for both years. For both experiments, Se speciation of the tomato fruit showed that selenomethionine was the major selenoamino acid significantly detected, followed by SeCys₂, selenite, and selenate with all treatments. In conclusion, these two Se biofortification studies have demonstrated that organic Se applied as either Se-enriched plant material or as extractable organic Se, can result in Se biofortified fruit. Although crops like tomatoes grown in such soils are sensitive to drought and insufficient irrigation (<100 % ETo), the addition of softwood biochar did not result in significant changes in fruit Se accumulation in either biofortification experiment.