Comparative Distribution and Metabolism of Xylem-Borne Amino Compounds and Sucrose in Shoots of Populus deltoides

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Abstract
The transport and metabolism of xylem-borne amino compounds and sucrose were investigated in rapidly growing shoots of cottonwood (Populus deltoides Bartr. ex Marsh.). 14C-labeled glutamine, threonine, alanine, glutamic acid, aspartic acid, and sucrose were applied to the base of severed stems for transport in xylem. Distribution and metabolism of the compounds were followed with autoradiography, microautoradiography, and radioassay. Three utilization patterns were observed: (a) little alanine and sucrose was transported to the laminae of either mature leaves or developing leaves. These compounds were taken up from xylem free-space and utilized in adjacent tissue; (b) threonine also did not move into mature leaves but was translocated to developing leaves or utilized in the stem; (c) glutamic acid and aspartic acid were transported directly into the laminae of mature leaves via the xylem. Relatively less 14C was retained in stems compared to the other compounds.