Xylem-to-phloem transfer in young vegetative soybean (Glycine max [L.] Merr.) plants (V4 stage) was identified as the difference in the distribution of [(14)C]inulin, a xylem marker, and [(14)C]aminoisobutyric acid (AIB), a synthetic amino acid, fed via the transpiration stream. Since [(14)C]AIB was