Energy-dependent Loading of Amino Acids and Sucrose into the Phloem of Soybean

Radioactive sucrose, l-leucine, l-glutamate, and gamma-aminobutyrate were applied exogenously to abraded areas of soybean leaves. The three amino acids were translocated with similar velocities and mass transfer rates on a molar basis, although they were metabolized differently in the sink tissue. The concentration dependence of leucine translocation showed a triphasic saturation response, while sucrose translocation showed a biphasic saturation response to increasing concentration. Apparent K(m) and V(max) for leucine and sucrose loading in the phloem differed. Both leucine and sucrose translocation were inhibited by uncouplers, high K(+), and p-chloromercuribenzenesulfonic acid. Treatment with 0.8 m sorbitol had little effect on sucrose translocation but stimulated leucine translocation, indicating an apoplastic route of loading for leucine. No effect on mass transfer rates was observed when sucrose and amino acids were applied exogenously together. These data provide evidence that phloem loading of amino acids and sucrose is mediated by different and separate carriers, both being dependent on an energy-requiring mechanism.