Phloem Translocation from a Leaf to Its Nodal Region and Axillary Branch in Populus deltoides

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Abstract
Microautoradiography was used to follow the movement of 14C through the nodal regions and in the stems of young P. deltoides Bartr. ex Marsh plants when either entire laminae were photosynthetically fed 14CO2 or cut petioles were fed 14C-labeled amino compounds. The leaf/branch gap of the central (C) leaf trace is filled with both thick- and thin-walled, heavily pitted parenchymatous cells that retain their protoplasts. The C-trace translocates little 14C-to mature cells of the gap region because it lacks differentiated rays at this level in the stem. However, the C-trace contributes 14C to the cambium-like region that adds cells to the gap. The C-trace also transfers 14C laterally to phloem of the branch traces in the nodal region. The branch traces in turn translocate 14C according to sink demands acropetally in the branch, or basipetally in the stem, or laterally to adjacent stem traces, or inward to the gap region. The 14C translocated inward via the rays is presumably deposited in the gap cells, where it accumulates as stored starch during predormancy. No evidence was found that gap cells functioned as transfer cells. Little cross transfer occurred between unrelated leaf traces in the stem. Thus, gap cells at nodes other than the treated node received 14C only when a labeled trace lay immediately contiguous to branch traces at that node. For example, the left trace of the treated leaf contributed 14C to the branch traces and nodal region but not to the leaf trace situated 3 nodes below. Amino compounds fed through cut petioles transported 14C primarily upward in the transpiration stream. Xylem-to-phloem transfer via the rays occurred at all levels in the stem, but it was particularly pronounced at nodal junctures. No labeled amino acids were transported into the gap region.