| Abstract: | We have examined the long-term effects of NO3– concentrationson NO3– (15NO3–) fluxes and cellular pool sizesin roots of intact 30-d-old wheat (Triticum aestivum cv. Courtot)grown hydroponically. Compartmental analysis was performed understeady-state conditions at five different levels of NO3–concentration (from 0.1 up to 5 mol m–3 taking into accountmetabolism and secretion into the xylem (Devienne et al., 1994).Nitrate and reduced nitrogen levels in the tissues were largelyindependent of external NO3– concentration although below1.5 mol m–3 NO3–; concentration limited plant growth.In the chamber, marked diurnal variations in net uptake occurredand, in the light, higher NO3– concentrations yieldedhigher NO3– uptake rates. After transfer of the plantsto the laboratory, the increase in net uptake linked to elevationof NO3–; concentrations was even larger (from 0.1 to 8.8µmolh–1 g–1 FW) as a result of a marked increase (x10–11) in the unidirectional influx at the plasmalemmawhile NO3– efflux was less enhanced (x 4–5). Underthese conditions, influx into the vacuole was also higher (x2–4) while efflux from the vacuole was little affected(x 1–3). NO3– concentrations within the cell compartmentswere estimated under the clas sical assumptions. The vacuolarconcentration was a little modified by NO3– availabilitywhereas that in the cytosol increased from about 10 mol m–3to about 20 mol m–3 indicating that (1) the absolute valuefor the cytosol was high and (2) it displayed only a small increasedespite very large changes in NO3– fluxes. NO3–distribution within the cells did not seem to involve an activeaccumulation of NO3– in the vacuole. Key words: Wheat, ion transport, nitrate, 15N, compartmentation |
