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 free space and walls, two intracellular compartments were involved in the tracer exchange, corresponding to fast and slow exchange compartments. These components of efflux were attributed to the cytosol and to the vacuole. Two methods were applied to analyse the N-15 efflux data, both based on a compartments-in-series model: a 'classical' method, where the kinetic parameters of the efflux curve determined by non-linear regression were used to calculate the cellular parameters; a 'numerical' method, combining non-linear fitting and numerical integration of the differential equations given in the serial model. The results obtained from both methods were fully consistent with one another and were in general agreement with other published results. The cytoplasmic nitrate pool was estimated at 1.0 mu mol g(-1) FW, corresponding to a concentration 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 supported by the good fit between the theoretical curve of (NO3-)-N-15 uptake versus time and the measured values of an independent loading experiment. However, the evolution of (NO3)-N-15, abundance in xylem sap versus time measured during a loading period was slower than predicted.