EFFECTS OF SOIL-TEMPERATURE AND NITROGEN STATUS ON KINETICS OF 15NO3- UPTAKE BY ROOTS OF FIELD-GROWN AGROPYRON-DESERTORUM (FISCH EX LINK) SCHULT

Plant NO3- acquisition is largely determined by root uptake capacity. Although root uptake capacity has been shown to be sensitive to both root temperature and previous nitrogen (N) supply in hydroponic systems, the uptake capacity response to similar environmental factors under field conditions has not been investigated. Using (NO3-)-N-15, root uptake capacities were determined in excised roots of Agropyron desertorum (Fisch. ex Link) Schult grown in the field at two soil temperatures and two N fertilization treatments. Variation in soil and root temperatures was achieved by application of clear plastic film or insulating mulch to the soil immediately around the target plants. Uptake rates were measured at six different assay solution concentrations (from 1 to 1000 mum external (NO3-)-N-15 concentration range). Two months after the imposition of soil N and temperature treatments, a biphasic transport system (a high-affinity) saturable phase and a low-affinity transport phase) was apparent in low N-treated plants. Nitrate uptake capacity in the low-concentration range (1-500 mum) was significantly reduced in N-fertilized plants compared with unfertilized control plants and the effect was more pronounced at high (27-degrees-C) than low (17-degrees-C) soil and assay temperatures. Furthermore, high soil N status inhibited the expression of a low-affinity NO3- transport system which was clearly apparent at external NO3- concentration ranges between 500 and 1000 mm in plants grown at low soil N. Prior soil N and temperature history may ultimately determine root ability to exploit NO3- flushes which can result from changes in soil environmental conditions.