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

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Abstract
The response of nonnodulated white lupin (L. 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 nitrate 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 mM NO3 followed by 10 days with 1, 5, 15 or 30 mM NO3. Nitrate reductase was substrate-induced in all tissues. Roots contained 76, 68, 62 and 31% of the total nitrate reductase activity of plants fed with 1, 5, 15 and 30 mM NO3, respectively. Stem, petioles and leaflets contained virtually all of the nitrate 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 50-70% and glutamine 10-20% 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 mM 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 mM 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.