Characterization of the low affinity transport system for NO3- uptake by Citrus roots

Three-month old citrange Troyer (hybrid of Citrus sinensis x Poncirus trifo liata) seedlings were grown hydroponically and, after a period of NO3 starv ation, plants were transferred to solutions enriched with (KNO3)-N-15 (96% atoms N-15 excess) to measure (NO3-)-N-15 uptake rates as a function of ext ernal (NO3-)-N-15, concentrations. Two different NO; uptake systems were fo und. Between 1 and 50 mM (NO3)-N-15 in the uptake solution medium, the upta ke rate increased linearly due to the low affinity transport system (LATS). Nitrate reductase activity showed the same response to external [NO3-] and also appears to be regulated by the rate of nitrate uptake. Nitrate pre-tr eatments had a represive effect on NO3- uptake rate measured at 5 or 30 mM external [(NO3-)-N-15]. The extent of the inhibition depended on the [NO3-] during the pre-treatment and in the uptake solution. These results suggest that the LATS of Citrus seedlings is under feedback control by the N statu s of the plant. Accordingly, addition of amino acids (Glu, Asp, Asn, Gln) t o the uptake solution resulted in a decrease in (NO3-)-N-15 uptake rate. Ho wever, the inactivation of nitrate reductase activity after treatment of th e seedlings with either 100 or 500 muM WO42- did not affect the activity of the LATS. Metabolic uncouplers, 2,4-DNP and KCN, reduced the uptake rate b y 43.3% and 41.4% respectively at 5mM external [(NO3-)-N-15]. However, thos e compounds had little effect when (NO3-)-N-15 uptake was assayed at 30 mM external concentration. The ATPase inhibitors DCCD and DES reduced (NO3-)-N -15 uptake by 68.8%-35.6%, at both external [(NO3-)-N-15]. Nitrate uptake b y the LATS declined with the increase of the solution pH beyond pH 4. The d ata presented are discussed in the context of the kinetics, energy dependen ce and regulation of NO3 uptake. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.