Assimilation and Transport of Nitrogen in Nonnodulated (NO(3)-grown) Lupinus albus L

Abstract

The response of nonnodulated white lupin (Lupinus albus L. cv. Ultra) plants to a range of NO(3) levels in the rooting medium was studied by in vitro assays of extracts of plant parts for NO(3) reductase (EC 1.6.6.1) activity, measurements of NO(3)-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 NO(3) followed by 10 days with 1, 5, 15, or 30 millimolar NO(3). NO(3) reductase was substrate-induced in all tissues. Roots contained 76, 68, 62 and 31% of the total NO(3) reductase activity of plants fed with 1, 5, 15, and 30 millimolar NO(3), respectively. Stem, petioles, and leaflets contained virtually all of the NO(3) 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 NO(3)-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 NO(3) in xylem sap amounted to 4, 13, 12, and 17% of the total xylem N at 1, 5, 15, and 30 millimolar NO(3), 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 NO(3). NO(3) 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 NO(3). Shoot organs were ineffective in storing NO(3). Even when NO(3) 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.