Nitrate transport in intact wheat roots:II. Long-term effects of NO3−concentration in the nutrient solution on NO3−unidirectional fluxes and distribution within the tissues5

Abstract

We have examined the long-term effects of NO₃⁻ concentrations on NO₃⁻ (¹⁵NO₃⁻) fluxes and cellular pool sizes in roots of intact 30-d-old wheat (Triticum aestivum cv. Courtot) grown hydroponically. Compartmental analysis was performed under steady-state conditions at five different levels of NO₃⁻ concentration (from 0.1 up to 5 mol m⁻³ taking into account metabolism and secretion into the xylem (Devienne et al., 1994). Nitrate and reduced nitrogen levels in the tissues were largely independent of external NO₃⁻ concentration although below 1.5 mol m⁻³ NO₃⁻; concentration limited plant growth. In the chamber, marked 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 8.8μmol h⁻¹ g⁻¹ FW) as a result of a marked increase (× 10–11) in the unidirectional influx at the plasmalemma while NO₃⁻ efflux was less enhanced (× 4–5). Under these conditions, influx into the vacuole was also higher (× 2–4) while efflux from the vacuole was little affected (× 1–3). NO₃⁻ concentrations within the cell compartments were estimated under the clas sical assumptions. The vacuolar concentration was a little 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) the absolute value for the cytosol was high and (2) it displayed only a small increase despite very large changes in NO₃⁻ fluxes. NO₃⁻ distribution within the cells did not seem to involve an active accumulation of NO₃⁻ in the vacuole.