UPTAKE AND UTILIZATION OF XYLEM-BORNE AMINO-COMPOUNDS BY SHOOT ORGANS OF A LEGUME

Amino compounds representative of the major N solutes of xylem sap were pulse-fed (10-20 min) singly in 14C-labeled form to cut transpiring shoots of white lupin (Lupinus albus [cv. Ultra] L.). 14C distribution was studied by autoradiography and radioassays of phloem sap, leaflet tissues and shoots parts harvested at intervals after labeling. Primary distribution of N by xylem was simulated using a 20 min labeling pulse followed by a 30 min chase in unlabeled xylem sap. Shoots fed 14C-labeled asparagine, glutamine, valine, serine, or arginine showed intense labeling of leaflet veins and marked retention (35-78%) of 14C by stem + petioles. Shoots fed 14C-labeled aspartic acid or glutamic acid showed heaviest 14C accumulation in interveinal regions of leaflets and low uptake (11-20%) of 14C by stem + petioles. Departing leaf traces were major sites of uptake of all amino compounds, and the implications of this were evaluated. Fruits acquired only 1-5% of the fed label directly from xylem, but more than doubled their intake during the period 30-160 min after feeding through receipt of 14C transferred from xylem to phloem in stem and leaves. 14C-Labeled asparagine and valine transferred directly from xylem to phloem, but the 14C of 14C-labeled aspartic acid and arginine appeared in phloem mainly as metabolic products of the fed compound. The labeling of the soluble pool of leaflets reflected these differences. The significance of heterogeneity in distribution and metabolism of xylem amino compounds in the shoot was discussed.