Bio-fortification and isotopic labelling of Se metabolites in onions and carrots following foliar application of Se and 77Se

► Labelling of onions and carrots by Se and 77Se was achieved using foliar spraying. ► The enrichment by 77Se was close to 100% of all Se in the plants. ► Selenite was superior to selenate as it was non-toxic to the plants. ► Carrots and onions were bio-fortified up to 200 of times relative to control. ► We identified for the first time the existence of methylselenocysteine in carrot.