NITRATE TRANSPORT IN INTACT WHEAT ROOTS .2. LONG-TERM EFFECTS OF NO3-SOLUTION ON NO3- WITHIN THE TISSUES CONCENTRATION IN THE NUTRIENT UNIDIRECTIONAL FLUXES AND DISTRIBUTION

We have examined the long-term effects of NO3- concentrations on NO3- ((NO3-)-N-15 ) fluxes and cellular pool sizes in roots of intact 30-d- old wheat (Triticum aestivum cv. Courtot) grown hydroponically. Compar tmental analysis was performed under steady-state conditions at five d ifferent levels of NO3- concentration (from 0.1 up to 5 mol m(-3)) tak ing into account metabolism and secretion into the xylem (Devienne et al., 1994). Nitrate and reduced nitrogen levels in the tissues were la rgely independent of external NO3- concentration although below 1.5 mo l m(-3), NO3- concentration limited plant growth. In the chamber, mark ed diurnal variations in net uptake occurred and, in the light, higher NO3- concentrations yielded higher NO3- uptake rates. After transfer of the plants to the laboratory, the increase in net uptake linked to elevation of NO3- concentrations was even larger (from 0.1 to 8.8 mu m ol h(-1) g(-1) FW) as a result of a marked increase (x 10-11) in the u nidirectional influx at the plasmalemma while NO3- efflux was less enh anced (x 4-5). Under these conditions, influx into the vacuole was als o higher (x 2-4) while efflux from the vacuole was little affected (x 1-3). NO3- concentrations within the cell compartments were estimated under the classical assumptions. The vacuolar concentration was a litt le modified by NO3- availability whereas that in the cytosol increased from about 10 mol m(-3) to about 20 mol m(-3), indicating that (1) th e absolute value for the cytosol was high and (2) it displayed only a small increase despite very large changes in NO3- fluxes. NO3- distrib ution within the cells did not seem to involve an active accumulation of NO3- in the vacuole.