NITRATE TRANSPORT IN INTACT WHEAT ROOTS .1. ESTIMATION OF CELLULAR FLUXES AND NO3- COMPARTMENTAL ANALYSIS FROM DATA OF (NO3-)-N-15 DISTRIBUTION USING EFFLUX

A compartmental analysis of nitrate transport in intact roots of 30-d- old wheat continuously grown at a concentration of 400 mmol m(-3) NO3- was performed under steady-state conditions. The analysis was made on the whole root system and took into account the metabolism of nitrate into reduced N-compounds and the export from root to shoot during the course of the experiment. After a 90 min loading period with (NO3-)-N -15, efflux of the tracer into an unlabelled solution (400 mmol m(-3)) was monitored during 105 min. It indicated that, in addition to the f ree space and walls, two intracellular compartments were involved in t he tracer exchange, corresponding to fast and slow exchange compartmen ts. These components of efflux were attributed to the cytosol and to t he vacuole. Two methods were applied to analyse the N-15 efflux data, both based on a compartments-in-series model: a 'classical' method, wh ere the kinetic parameters of the efflux curve determined by non-linea r regression were used to calculate the cellular parameters; a 'numeri cal' method, combining non-linear fitting and numerical integration of the differential equations given in the serial model. The results obt ained from both methods were fully consistent with one another and wer e in general agreement with other published results. The cytoplasmic n itrate pool was estimated at 1.0 mu mol g(-1) FW, corresponding to a c oncentration of 10-20 mol m(-3), depending on the cytoplasmic volume. The unidirectional fluxes at the plasmalemma (2.8-2.7 mu mol g(-1) FW h(-1)) were smaller than fluxes at the tonoplast (4.7-6.5 mu mol g(-1) FW h(-1)). The half-lives of cytoplasmic and vacuolar pools were 4.6 min and 9.4 h, respectively. The consistency of our approach was suppo rted by the good fit between the theoretical curve of (NO3-)-N-15 upta ke versus time and the measured values of an independent loading exper iment. However, the evolution of (NO3)-N-15, abundance in xylem sap ve rsus time measured during a loading period was slower than predicted.