Long distance translocation of sucrose, serine, leucine, lysine, and carbon dioxide assimilates: I. Soybean

To determine the selectivity of movement of amino acids from source leaves to sink tissues in soybeans (Glycine max L.] Merr. `Wells'), ¹⁴C-labeled serine, leucine, or lysine was applied to an abraded spot on a fully expanded trifoliolate leaflet, and an immature sink leaf three nodes above was monitored with a GM tube for arrival of radioactivity. Comparisons were made with ¹⁴C-sucrose and ¹⁴CO₂ assimilates. Radioactivity was detected in the sink leaf for all compounds applied to the source leaflet. A heat girdle at the source leaf petiole essentially blocked movement of applied compounds, suggesting phloem transport. Transport velocities were similar (ranged from 0.75 to 1.06 cm/min), but mass transfer rates for sucrose were much higher than those for amino acids. Hence, the quantity of amino acids entering the phloem was much smaller than that of sucrose. Extraction of source, path, and sink tissues at the conclusion of the experiments revealed that 80 to 90% of the radioactivity remained in the source leaflet. Serine was partially metabolized in the transport path, whereas lysine and leucine were not. Although serine is found in greater quantities than leucine and lysine in the source leaf and path of soybeans, applied leucine and lysine were transported at comparable velocities and in only slightly lower quantities than was applied serine. Thus, no selective barrier against entry of these amino acids into the phloem exists.