Selenium Biofortification and Increased Nutritional Quality in Alfalfa (Medicago sativa L.) Using Foliar Application of Selenium-Rich Nutrient Solution

Biofortification of selenium (Se) has emerged as an important strategy to enhance Se content in alfalfa, and sulfur (S) can be found to reduce the toxicity of Se to several crops. However, it is not clear if this interaction between Se(IV) and S enhances Se accumulation in alfalfa. In this study, alfalfa was grown on experimental pots and treated with foliar nutrient solutions containing 0–40 mg L−1 sodium selenite (Se(IV)), both with and without supplementary S. Results showed that foliar application of 10 mg L−1 Se(IV) with a S/Se(IV) ratio of 46.83 significantly increased shoot length (28.6%), shoot biomass (39.2%), content of total Se (23.4 times), and total organic Se in alfalfa leaves at the early flowering stage. Levels of selenocysteine (SeCys) and methylselenocysteine (MeSeCys) accounted for 75.2–81.8% of total Se, whereas selenomethionine (SeMet) comprised just 13.7–15.2% of total Se, confirming that SeCys and MeSeCys were the predominant form of Se in Se(IV)-treated leaves. Se-enriched leaves contained higher concentrations of minerals, including iron (33.4%), magnesium (36.3%), manganese (58.6%), zinc (53.2%), copper (34.8%), and total sulfur (22.6%). Concentrations of crude protein were increased by an average of 45.2%. Higher concentrations of S offered a protective effect against Se toxicity and indeed reduced total Se content by 10.9% (20 mg L−1 Se(IV)) and 24.6% (40 mg L−1 Se(IV)), respectively. Based on the above results, the adoption of this agronomic practice could be a promising strategy for Se biofortification and improved nutritional quality in alfalfa grown on Se-deficient soils.