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

Three-month old citrange Troyer (hybrid of Citrus sinensis x Poncirus trifoliata) seedlings were grown hydroponically and, after a period of NO3 starvation, 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 external (NO3-)-N-15, concentrations. Two different NO; uptake systems were found. Between 1 and 50 mM (NO3)-N-15 in the uptake solution medium, the uptake 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-treatments 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 status of the plant. Accordingly, addition of amino acids (Glu, Asp, Asn, Gln) to the uptake solution resulted in a decrease in (NO3-)-N-15 uptake rate. However, the inactivation of nitrate reductase activity after treatment of the 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 by 43.3% and 41.4% respectively at 5mM external [(NO3-)-N-15]. However, those 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 by the LATS declined with the increase of the solution pH beyond pH 4. The data presented are discussed in the context of the kinetics, energy dependence and regulation of NO3 uptake. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.