Xylem-to-Phloem Transfer of Organic Nitrogen in Young Soybean Plants

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 retained in the stem to some extent, whereas [(14)C]inulin was not, the distribution of these marker compounds in each leaf was expressed as a percentage of the total [(14)C] radioactivity recovered in the foliage. The developing third trifoliolate was a consistent and reliable indicator of xylem-to-phloem transfer. The phloem stream provided to the developing trifoliolate up to fourfold the relative proportion of solute received from the xylem stream; this was markedly reduced by increased light intensity and consequently water flow through the xylem. Evidence from heat girdling experiments is discussed with respect to the vascular anatomy of the soybean plant, and interpreted to suggest that direct xylem-to-phloem transfer in the stem, in the region of the second node, accounted for about one-half of the AIB supplied to the developing trifoliolate, with the remainder being provided from the second trifoliolate. Since AIB is not metabolized it seems likely that rapid transfer within the second trifoliolate occurred as direct veinal transfer rather than indirect cycling through the mesophyll. This study confirmed that xylem-to-phloem transfer plays a major role in the partitioning of nitrogen for early leaf development.