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Assimilation and Transport of Nitrogen in Nonnodulated (NO₃-grown) Lupinus albus L ¹

^a Department of Botany, University of Western Australia, Nedlands, Western Australia 6009

The response of nonnodulated white lupin (Lupinus albus L. cv. Ultra) plants to a range of NO₃ levels in the rooting medium was studied by in vitro assays of extracts of plant parts for NO₃ reductase (EC 1.6.6.1) activity, measurements of NO₃-N in plant organs, and solute analyses of root bleeding (xylem) sap and phloem sap from stems and petioles. Plants were grown for 65 days with 5 millimolar NO₃ followed by 10 days with 1, 5, 15, or 30 millimolar NO₃. NO₃ reductase was substrate-induced in all tissues. Roots contained 76, 68, 62 and 31% of the total NO₃ reductase activity of plants fed with 1, 5, 15, and 30 millimolar NO₃, respectively. Stem, petioles, and leaflets contained virtually all of the NO₃ reductase activity of a shoot, the activity in extracts of fruits amounting to less than 0.3% of the total enzyme recovered from the plant. Xylem sap from NO₃-grown nonnodulated plants contained the same organic solutes as from nodulated plants grown in the absence of combined N. Asparagine accounted for 50 to 70% and glutamine 10 to 20% of the xylem-borne N. The level of NO₃ in xylem sap amounted to 4, 13, 12, and 17% of the total xylem N at 1, 5, 15, and 30 millimolar NO₃, respectively. Xylem to phloem transfer of N appeared to be quantitatively important in supplying fruits and vegetative apices with reduced N, especially at low levels of applied NO₃. NO₃ failed to transfer in any quantity from xylem to phloem, representing less than 0.3% of the phloem-borne N at all levels of applied NO₃. Shoot organs were ineffective in storing NO₃. Even when NO₃ was supplied in great excess (30 millimolar level) it accounted for only 8% of the total N of stem and petioles, and only 2 and 1% of the N of leaflets and fruits, respectively.

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